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ISSN 2079-9780, Review Journal of Chemistry, 2018, Vol. 8, No. 1, pp. 1–33. © Pleiades Publishing, Ltd., 2018

Phosphors Based on Phosphates of NaZr2(PO4)3and Langbeinite Structural Families

A. E. Kanunova,* and A. I. Orlovaba Russian Federal Nuclear Center, All-Russia Research Institute of Experimental Physics,

Sarov, Nizhny Novgorod oblast, 607188 Russiab Lobachevsky State University, Nizhny Novgorod, 603950 Russia

*e-mail: [email protected]

Abstract—The review covers aspects of modeling the composition and luminescent properties of phos-phates of NaZr2(PO4)3 (NZP) and langbeinite structural families. Based on the analysis of phosphatesstructure, a plausible algorithm of the use of crystal chemistry data for modeling compositions, struc-ture, and properties of new compounds is proposed. The following properties determined by require-ments to materials for the chosen purposes are studied: behavior on heating and in water systems,luminescence, and biocompatibility. Prospects for the use of such data for solving various problems ofmaterials science, LED technologies, bioimaging, and X-ray induced photodynamic therapy of onco-logical diseases are shown.

Keywords: phosphates, lanthanides, structural type, NaZr2(PO4)3, NZP, langbeinite, phosphors,LED technologies, bioimaging, X-ray induced photodynamic therapyDOI: 10.1134/S207997801801003X

Table of contents1. Introduction1.1. Phosphate Phosphors for LED Technologies1.2. Inorganic Phosphors for Intracellular Bioimaging1.3. Inorganic Phosphors for X-ray Induced Photodynamic Therapy of oncological Diseases2. NaZr2(PO4)3 and Langbeinite Structural Families2.1. NZP Family2.2. Lb Family2.3. Crystal Chemistry Approach in the Design of New Phosphate Phosphors. Choice of Formula Composi-

tions3. Synthesis3.1. Brief Review of Methods for the Preparation of Orthophosphates of NZP and Lb Families4. Features of Phase Formation in Systems of Lanthanide-Containing Phosphates with NZP and

Langbeinite Structures4.1. Phase Formation4.2. Structural Data5. Luminescent Properties5.1. Phosphate Phosphors for LED Technologies5.2. Phosphate Phosphors for Bioimaging5.3. Phosphate Phosphors for X-PDT6. Other Properties: Behavior on Heating, Chemical Stability, Biocompatibility6.1. Behavior on Heating6.2. Chemical Stability6.3. Biocompatibility7. Conclusions

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REVIEW JOURNAL OF CHEMISTRY Vol. 8 No. 1 2018

KANUNOV, ORLOVA

1. INTRODUCTIONThe need in new “on a plan” functional materials, including those favoring the better quality and lon-

ger duration of human life [1–5] constantly increases under the conditions of continuously developingscience and high technology. Tasks of the development and improvement of methods of the preparationand study of such materials are included in the List of Russian Critical Technologies.

Within these tasks, an innovative direction of present-day inorganic chemistry and materials science isthe development of new ecologically safe and biocompatible phosphors with adjustable properties madeas nanocrystalline powders and ceramics. The control of their composition and, correspondingly, prop-erties opens wide possibilities for the use of such materials in promising industrial and biomedical tech-nologies, also as energy-efficient sources of while light, biocompatible optically active substances formonitoring pathological processes in tissues of living systems (bioimaging), and X-ray induced photody-namic therapy of oncological diseases (X-PDT).

1.1. Phosphate Phosphors for LED TechnologiesEnergy-saving technologies in lighting have a need in new compounds and materials on their basis for

economic and ecologically safe light sources. Among them of interest are LED technologies and thedevelopment of white light-emitting diodes. Two methods are known to obtain white light: mixing colorsaccording to the RGB (red, green, blue) technology and application of phosphors on industrially pro-duced light-emitting diodes (LEDs), emitting in the blue (Fig. 1a) or ultraviolet (Fig. 1b) spectral regions.White LEDs coated by phosphors are significantly cheaper than LED RGB matrix panels. Because of acombination of different phosphors, white light with coordinates close to {0.33; 0.33} in the color scale ofthe International Commission on Illumination for these devices can be obtained in a simpler way. Themost widespread design includes a InxGa1 – xN light-emitting diode (λ = 460 nm) [6–10] and a phosphorbased on YAG:Ce3+ [11–13], which converts part of radiation of the light-emitting diode to light in a widespectral band with the maximum in the yellow region due to photoluminescence (Fig. 1a). Being mixed,radiation of the phosphor and the light-emitting diode give white light. However, the white light obtainedin this case does not possess the maximum intensity. It is increased using phosphors with other color pos-sibilities.

Phosphor materials must satisfy certain requirements, such as (1) safe composition, (2) chemical andthermal stability, (3) possibility of regulation of optical properties by changing composition (is desirable),and (4) simplicity of synthesis and low cost. These requirements necessitate the improvement of the prop-erties of already known materials and the search for new compositions, possessing better characteristics,and also the sophistication of methods of their preparation.

A special place among the known phosphors is occupied by Eu2+-containing phosphors, adapted toblue or UV spectral region [14]. Emission and absorption spectra of Eu2+ contain wide bands correspond-

Fig. 1. Method of creation of while light-emitting diodes: (a) blue light-emitting diode coated with a yellow phosphor;(b) UV light-emitting diode coated with blue, green, and red phosphors.

0.8

1.0I

0.6

0.4

0.2

InGaNLED

0450 500 550 600 650 700 750

λ, nm λ, nm400 380 555 755

(a) (b)Phosphorabsorption

Phosphoremission

UV Light-emittingdiode

Combinedspectrum Phosphor

emission

UV Visible region IR


PHOSPHORS BASED ON PHOSPHATES 3

ing to transitions from the excited 4f 65d1 state to the ground 4f 7 state. As 5d orbitals are outer orbitals, thepositions of energy levels and, correspondingly, excitation and emission wavelengths, strongly depend onthe “host” crystal (matrix) [14]. Therefore, the choice of a matrix is a critical parameter in determiningoptical properties of the Eu2+ cation.

Eu3 +, Sm3+, and Mn2+ cations are also used as luminescence activators in many phosphors [15].The most intense luminescence bands characteristic for Eu3+ and Sm3+ cations correspond to 4f–4f

transitions (red luminescence) on excitation in the UV or blue spectrum region [16].The emission of the Mn2+ cation covers a wide frequency region and, as was found in [17], with an

increase in the effect of crystal field, is shifted from the green to the red region.It is known from the published data that salt compounds with tetrahedrally coordinated oxoanions—

phosphates, silicates, vanadates, molybdates, etc.—are studied as crystal matrices containing Eu2+, Eu3+,Sm3+, and Mn2+ activator cations [18–23]. It should be noted that phosphate-based phosphors offer evi-dent advantages, as they differ by stable physical and chemical properties, safety, and also by the low costof the starting components [22, 23]. Phosphate phosphors activated by Eu2+, Eu3+, Sm3+, and Mn2+ andknown from the literature and the corresponding regions of spectral band maxima are summarized inTable 1. Note that the number of publications on the study of lanthanide-containing inorganic com-pounds of oxide and salt character for optical applications constantly grows every day, because of whichcorresponding information is sketchy.

As methods for the preparation such materials, researchers use the solid-phase method and the sol–gel technology, including the Pechini citrate method. The final temperature of synthesis is in the range700–1300°C.

An important requirement for phosphors is a possibility of the regulation of optical properties bychanging their composition. In this regard, attention should be paid to substances with the structures ofNaZr2(PO4)3 (NZP, NASICON) and langbeinite (Lb, basic analogue of K2Mg2(SO4)3). Possibilities ofthese structural types, because of their wide isomorphism, open a promising direction of scientificresearch and assume the preparation of a wide range of substances (individual compounds and solid solu-tions) with adjustable specified properties, including optical ones [65–68]. In this case, the crystal chem-istry approach is the basic one. However, the data on such substances for optical applications with themain activator cations are sketchy and quite limited. However, as described above, there are extremelywide possibilities for the preparation new compounds with useful optical properties, also regulated in thedesirable direction, from these classes of substances using the crystal chemistry approach.

1.2. Inorganic Phosphors for Intracellular BioimagingThe use of inorganic compounds as phosphors for intracellular bioimaging or labeling individual sub-

cellular structures is a fundamental issue of bioengineering (genetic, cellular, tissue, etc.) and an innova-tive method of the study of mechanisms of physiological and pathological processes in living systems. Flu-orescent labels are detected as individual samples by standard f luorescence microscopy, which allows thevisualization of processes at a level of individual cell structures and molecules [69–75]. The main “target”molecules are antibodies, to which various labels can be attached by chemical methods [75].

The necessary requirements to such materials are their biological inertness, presence of photolumines-cence in the visible spectral region with an acceptable intensity, and safety of excitation sources for bio-logical samples.

Several classes of compounds are studied for these purposes, i.e., (1) organic: f luorescent proteins;(2) quantum dots; and (3) inorganic composites of complex composition.

Fluorescent proteins. Fluorescent proteins of the GFP (green fluorescent protein) family are the mostnative structures ensuring the complete elimination of the problem of biodegradation and affinity to anycell type [76, 77]. However, a small number of these compounds known by now hinder observations in anyspectral region [78–80]. The f luorescence of GFP depends on the pH and temperature of the mediumand can be completely quenched on protein denaturation [79]. In addition, f luorescent proteins, as allother proteins, possess antigen properties, so that their use for labeling antibodies is inexpedient.An important fact is the high cost of the preparation of f luorescent proteins and work with them.

Quantum dots. Semiconductor quantum dot nanocrystals consist of 103–105 atoms of elements gro-ups 2–6 or 3–5 of the Periodic System (CdHg, CdSe, CdTe, CdS, ZnS, PbS, InP, InAs, etc.), and theirsize is 2–10 nm [69]. They offer a number of advantages, such as photostability and bright emission in thewide optical region on excitation by a source of infrared radiation [69, 81–84]. However, the main prob-

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KANUNOV, ORLOVA

Table 1. Positions of maxima (or range) of luminescence bands in the visible region and spectra of luminescenceexcitation for the known phosphate phosphors

PhosphorPositions of band maxima, nm

Referenceexcitation emission

Simple phosphates with a layered structure

Li(Ca,Sr,Ba)PO4:Eu2+ 350−400 450−480 [24–27]

NaCaPO4:Eu2+ 390 506 [28]

NaCaPO4:Eu3+ 393 594–621 [29]

K(Ca,Sr,Ba)PO4:Eu2+ 292–325 425–473 [25, 30–32]

CsMgPO4:Eu2+ 420 650 [33]

LaPO4:Eu3+ 254 575–625 [34]

Phosphates with mineral whitlockite structure

Ca9(La,Eu,Lu)(PO4)7:Eu2+, Eu2+–Mn2+ 257–365 485–645 [35–37]

Ca8Mg(Gd,Lu)(PO4)7: Eu2+, Eu3+, Eu2+−Mn2+ 365–400 480–615 [14, 38, 39]

(Ca,Mg,Sr)9Y(PO4)7:Eu2+, Eu2+−Mn2+ 280–360 435–490, 632 [40]

Ca2.89Mg0.11(PO4)2:Eu2+ 350 413 [41]

Ca9Bi(PO4)7:Eu3+ 393 592, 615 [42]

Ca3–x–ySry(PO4)2: Eu2+ 365 400−700 [43]

Phosphates with mineral apatite structure

Ca5(PO4)3Cl:Eu2+ 375 459 [44, 45]

Ca5(PO4)3Cl:Eu2+−Mn2+ 375 585 [44, 45]

Sr5(PO4)3Cl:Eu2+ 345 445 [46, 47]

BiCa4(PO4)3O:Eu3+ 269, 395 570−620 [48]

Sr5Cl0.75F0.25(PO4)3:Eu2+ 277, 341 445 [49]

Pyrophosphates

(Ca,Sr)2P2O7:Eu2+, Eu2+–Mn2+ 300–330 415–600 [50, 51]

Li2BaP2O7:Eu3+ 252–467 581–622 [52]

Li2BaP2O7:Sm3+ 227–476 525–650 [52]

Other phosphates

Ca3Mg3(PO4)4:Eu2+, Mn2+ 300 450, 625 [53]

(Ba,Sr)10(PO4)4(SiO4)2:Eu2+ 315, 350 507 [54]

Ba7Zr(PO4)6:Eu2+ 370 585 [55]

K3Y(PO4)2:Sm3+ 405 600−700 [56]

Ba3Y(PO4)3:Sm3+ 401 600 [57]

Ca5(PO4)2(SiO4):Eu2+ 365 495 [58]

Na5Al(PO4)2F2:Eu2+−Eu3+ 265, 325 420, 530, 615 [59]

SrMg2(PO4)2:Eu2+,Eu2+−Mn2+ 375 420, 675 [60–62]

SrZn2(PO4)2:Eu2+,Eu2+−Mn2+ 365 416, 613 [60–62]

Na2Ba1–xSrxMg(PO4)2:Eu3+ 220, 300 585–625 [63]

K3Gd5(PO4)6:Eu3+ 394 575–625 [64]



lem limiting the use of quantum dots is connected with their toxicity and ability to initiate morphologicalchanges in the cells, internal parts of a body, and DNA [70, 84–89].

Fluoride-based inorganic biosensors. Nanodimensional particles on the basis of f luorides (LaF3, YF3,GdF3, NaYF4, GdOF) are of interest as energy converters from the near IR to the visible spectral region;correspondingly, by their optical characteristics they can be used as biotags for the visualization of cancertumors [90]. A drawback of f luorides is in the strict requirements to synthesis conditions, i.e., speciallypure oxygen- and water-free atmosphere.

Inorganic composites of complex composition. The effect of toxicity can be reduced using substancesformed by biogenous elements, for example, Ca, P, Si, etc. The published data also cover phosphorson the basis of silicates and phosphates containing alkaline-earth elements: MgSiO3, CaMgSi2O6,Sr2MgSi2O7, Ca0.2Zn0.9Mg0.5Si2O6 with Eu2+, Dy3+, and Mn2+ impurities [91, 92]; Ca9Eu(PO4)7 [37] andCa10.5–1.5(x + y)ErxYby(PO4)7 [93]. The last two compounds are chemical analogues of biogenous mineral whit-lockite β-Ca3(PO4)2 [94]. Mixed oxide compounds Gd14 – x – yErxYbyB6Ge2O34 and Gd11 – n – mYbnErmSiP3O26were described in [95]. Silicates containing Eu2+, Dy3+, and Mn2+ cations are characterized by an emis-sion band in the red region of visible spectrum at 600–750 nm with an intensity maximum at 690 nmon excitation by an UV source with λexcit ~ 350 nm. For phosphates with the structure of whitlockiteCa10.5 – 1.5(x + y)ErxYby(PO4)7, emission was observed in the visible spectral region (λem = 525, 550, 650 nm)on excitation with an IR source (λexcit = 977 nm) by an upconversion mechanism. Because of its safety(green luminescence) and high intensity, it is acceptable for the visualization of living systems. Phosphatesof this kind correspond to the criterion of safety by results of testing their toxicity [93]. The shortcomingsof silicate and phosphate compounds are the high temperature of synthesis (~1150°C) and intermediatepressing stages [91, 93]. In addition, β-whitlockite as a biogenous mineral forms the main part of bonetissue, and lanthanides found in its composition can be accumulated in it.

Compounds capable of emitting light in the visible spectral region on excitation by an IR source withλexcit ~ 980 nm (anti-Stokes shift, including the upconversion mechanism) are most acceptable for bioim-aging tasks. Such emission can be attained on the simultaneous presence Er3+ and Yb3+ lanthanides in thematrix [96–100].

As follows from the summary of the published data, the most promising direction in the creation offluorescent tags by the moment is the synthesis of such compounds combining useful properties of naturalf luorophores (f luorescent proteins) and quantum dots. Russian analogues of such nanomaterials are notknown.

The use of principles of crystal chemistry modeling allows the prediction of new compositions of com-pounds of anticipated structures. In the present work, this approach is used as a basis for the developmentof biocompatible phosphates of structural families NaZr2(PO4)3 and langbeinite with high concentrationsof calcium and also of magnesium, silicon, and lanthanide cations (Er3+, Yb3+) in their compositions.

1.3. Inorganic Phosphors for X-ray Induced Photodynamic Therapy of Oncological DiseasesInorganic compounds bearing lanthanides are studied as phosphors for specific applications in medi-

cine, i.e., X-ray induced photodynamic therapy of oncological diseases (X-PDT). This method consistsin the introduction of photosensitizers, enhancing the sensitivity of tumors to light, in an organism. Underthe effect of light, the photosensitizer generates active forms of oxygen (singlet oxygen, peroxides, etc.),which destroy pathogenic cells [101–105]. A sophistication of the X-PDT method is the development ofa pharmacological preparation, which, along with a photosensitizer, contains a colloidal solution of ananophosphor, emitting light with the wavelength necessary for the excitation of a photochemical reactionunder the influence of high-energy photons, which easily penetrate into organism tissues (Fig. 2) [105].

Such a phosphor must satisfy the following requirements: (1) be safe for an organism by its chemicalcomposition; (2) possess particle size of 60–90 nm; (3) on excitation by X- or γ-rays, emit in a very narrowwavelengths region, necessary for the implementation of a photochemical transformation of the used pho-tosensitizer [105].

Chlorin, purpurine, and benzoporphyrin derivatives are the most efficient and safe among the studiedphotosensitizers. Preparations on the basis of such compounds possess a photodynamic therapy spectrumwith absorption maxima in the region 620–700 nm: “KillerRed®” λ = 610 ± 5 nm [106, 107], “Foskan®”λ = 652 ± 5 nm [108], “Fotoditazin®” λ = 662 ± 5 nm [109, 110], and “Fotosens®” λ = 670 ± 5 nm[108]. The attention of researchers is concentrated on the development of phosphor materials with emis-sion in the above regions of optical spectrum for activating the known photosensitizers.

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KANUNOV, ORLOVA

Phosphors based on quantum dots. Nanophosphors based on ZnSe and other quantum dots are char-acterized by X-ray luminescence in the red spectral region: ZnS/CdS: Ag+, Cu2+, Ga3+, In3+, Cl– (λem =650–660 nm), ZnS/CdSe: Cu2+, Ga3+ (λem. = 655–680 nm) [111]. However, as is evident from elementalcomposition and as was stated above, such phosphors are toxic.

Oxide phosphors. The luminescence spectrum of the In Y2O3:Sm3+ phosphor exhibits intense bands in thered region of optical spectrum. Such a phosphor was synthesized by a complex combustion reaction [112].

Silicate and phosphate phosphors. Manganese-containing magnesium orthosilicate Mg2SiO4:Mn2+ ischaracterized by emission at the maximum wavelength 660 nm (photo and cathode luminescence) [113].Phosphors on the basis of magnesium orthosilicate, for example, Mg2SiO4:Tb3+, are used for thermolumi-nescence dosimetry and are sensitive to γ-rays [114]. A drawback of this phosphor is the complex technologyof the synthesis of nanopowders. Phosphate phosphors possess high biocompatibility and are stable to radi-ation. Phosphors based on zinc phosphate Zn3(PO4)2:Mn2+ and barium phosphate Ba3(PO4)2:Eu2+ possessradioluminescence in region λem. = 560–720 nm with a maximum at 630 nm [105, 115, 116]. Europium-containing calcium phosphate β-Ca3(PO4)2:Eu2+ can be used for the activation of the “Fotoditazin®”photosensitizer in the region of its “blue” absorption band (400 nm). The maximum in its luminescencespectrum is at the wavelength 412 nm [113]. The simultaneous introduction of Mn2+ and Sm3+ into thecomposition of this phosphate can ensure emission in the red spectral region.

It should be noted that a few number of the inorganic phosphors accepted for X-PDT, in particular,those containing f elements (Eu2+, Sm3+) were described in the literature. Taking into account the specificfeatures of the framework structure of compounds of NaZr2(PO4)3 and langbeinite families, which maycontain different cations and ensure their strong binding in the structure, and also the data about the radi-ation [117–119], thermal [120–125], chemical [124, 126–129] stability of the known isostructural ana-logues, it was supposed that phosphates containing biogenous elements (K, Mg, Ca) with such structuresmay be of interest for the “photosensitizer+nanoluminophor” version of X-PDT.

The generalization of approaches to the development of luminescent materials for the consideredproblems of LED equipment and biomedical applications suggests that studies based on the principles ofchemical and structural similarity can be promising and systematic. Such crystal chemistry concept weare developing on an example of compounds with tetrahedral oxoanions, first of all, crystallizing in thestructural types of NaZr2(PO4)3 (NZP) and langbeinite (Lb) with high possibilities of various heterovalentcation and/or anion substitutions. Modeling of compositions on the basis of such isostructural com-pounds, including those of necessary compositions (biogenous elements, lanthanides as activators ofpractically important optical properties, phosphorus, other) was described in [65–68].

This review summarized and systematizes data on the synthesis of new phosphates, study of regularitiesof structure formation, microstructures, and properties responsible for their application in LED technol-ogies and biomedicine.

2. NaZr2(PO4)3 AND LANGBEINITE STRUCTURAL FAMILIESFollowing the known “composition–structure–property” paradigm allows a researcher to develop

functional materials and control their properties. The prediction of the properties of inorganic materialsconsisting of octahedral–tetrahedral framework compounds is based on the dynamic crystal chemistry

Fig. 2. An X-PDT procedure using a pharmacological preparation including a photosensitizer (D) and a nanophosphor-converter of radiation: D* is excited photosensitizer; 3O2 is tissue oxygen;

1O2 is singlet oxygen [105].

D3O2 → 1O2D*

X- or γ-rays

Nanophosphor–converterof radiation

Visiblelight

Photosensitizer Singletoxygen

generation

Killing pathogeniccell



theory of mixed frameworks, developed by Academician N.V. Belov and his school [130–134], and prin-ciples of crystal chemistry modeling of formula compositions, developed by our group for ortho com-pounds with single-nucleus tetrahedral oxoanions [65, 67, 68].

The most widespread type of structure-forming frameworks is the mixed tetraedral–octahedral frame-work of topological domains , formed by [TO6] octahedra and [XO4] tetrahedra [135].The discreteness of these types of Pauling polyhedrons is determined by the ratio T : X = 2 : 3 [133].Depending on the oxidation state of cations T (from +1 to +5) and X (from +4 to +6), the charge of theframework n takes values from 0 to 4. For different X (X4+, X5+, X6+), plausible compositions are similarto those presented in [65, 66] for X = P(X5+) at a shift of framework charge n to unity (towards increasefor X4+ or decrease for X6+) [68].

Crystal chemistry analysis revealed three main polymorphic types of the framework withdifferent relative positions of [TO6] and [XO4] polyhedra: NaZr2(PO4)3 (mineral kosnarite) [136],K2Mg2(SO4)3 (mineral langbeinite) [137], and Ca3Al2(SiO4)3 (mineral garnet) [138].

Structure-forming domains in these modifications are packed differently. As a result, chan-nels and voids in the structure differ by their shapes and sizes, and the total numbers of voids for cations com-pensating the charge of the framework per one formula unit are also different. These structural types aregenetically related to one another. Frameworks of structural types NZP and Lb possess the closest relation-ship, which is confirmed by the existence of morphotropic transitions in these systems [139–142].

2.1. NZP FamilyZirconium–sodium orthophosphate NaZr2(PO4)3 is the parent compound of the structural type cov-

ering a great number of isostructural analogues and solid solutions. The first study of the crystal structureof NaZr2(PO4)3 was performed in 1968 by Swedish crystal chemists Hagman and Kierkegaard [136].The NaZr2(PO4)3 phosphate crystallizes in the trigonal system and has space group R c of rhombohedralsymmetry (a = 8.8043(2) Å, c = 22.7585(9) Å, V = 1530.5 Å3, Z = 6) [136]. It is characterized by an octa-hedral–tetrahedral framework structure with the structure-forming domain (Fig. 3a)formed by two [ZrO6] octahedra connected with three [PO4] tetrahedra via oxygen bridges. Uniformdomains form clusters extended along the axis. The places of intersection of channels penetratingthrough the structure are crystallographic voids of M1 and M2 types (Fig. 3b), differing in size, shape, andcoordination environment of the cations arranged in the voids. The M1 positions are between [ZrO6] octa-hedra and have distorted octahedral coordination. Positions of the other type, M2 with CN 8 are betweenstructure-forming {[Zr2(PO4)3]–}3∞ domains. Their number is three times higher, than the number of M1positions [143, 144]. M1 and M2 positions can be occupied completely, partially, or remain unoccupied.

In general, the crystal chemistry formula of compounds with the structure of NZP is as following:

,

where is the framework of the structure (topological domain), L is the position of theframework; XO4 is tetrahedral radical anion; M1 and M2 are types of extra framework cation positions;and [4, 6, 8] are coordination numbers [144, 145].

The NZP structure was most completely investigated in phosphate systems: transition from one com-pound to another occurred by iso- and heterovalent isomorphous substitutions of cations in the frameworkand/or cavity at the preservation of the electroneutrality of the whole structure. The possible formula com-positions of such phosphates were calculated and summarized in the review [65] and the known representa-tives were summarized in the later review [146] (though incompletely) and other works [2, 68, 124, 147].

In the known zirconium orthophosphates of the type AZr2(PO4)3, where A = Li, Na, K, Rb, and Cs,M1 positions are completely occupied by atoms of alkali elements [148]. At isomorphous substitutionsinvolving elements in oxidation states +2 (alkaline-earth elements B) and +3 (lanthanides Ln), the sym-metry of the cell is reduced:

1. R c → R : atoms B occupy positions 3b {0; 0; 1/2}, positions 3a {0; 0; 0} are vacant, glide planes cdisappear [145, 149–151];

2. R c → P c: Ln atoms are ordered in positions 2b {0; 0; 0 }, translation symmetry changes R → P[152–154].

−2 4 3{T (XO ) }

n

−2 4 3{T (XO ) }

n

−2 4 3{T (XO ) }

n

3

−2 4 3{Zr (PO ) }

3

−∞

[6] [8] [6] [4]3 2 4 3 3( 1) ( 2) {[ (X O ) ] }

nM M L−

∞2 4 3{[ (XO )] }nL

3 3

3 3

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KANUNOV, ORLOVA

The data on the inclusion of lanthanum and lanthanides in the structure of NZP are of special interestin developing luminescent materials.

The analysis of published data indicates that lanthanum and f element (3+, 4+) cations can enter intothe composition of phosphates with NZP structure both in in-framework and extra-framework positions[155–164].

The population of L positions by Ln3+ cations depends on the ionic radius of the cation and decreaseswith its growth [155]. This was found on an example of the Na1 – xLnx(PO4)3 system, where Ln = Yb–Gdon decreasing x from 1 for Yb to 0.2 for Gd. Lanthanide cations of bigger size do not occupy the L position[155–157].

Complex zirconium phosphates in which trivalent element cations are compensating cations are pre-sented by compounds of the type R0.33Zr2(PO4)3, where R = Sc, Y, Bi and Ln = La, Pr, Nd, Sm, Eu, Gd,Tb, Er, Tm, Yb, Lu [154–163].

It was found that, with an increase in the serial number of the lanthanide, the parameter c of the unitcell trends to reduce, which is due to the phenomenon of lanthanide contraction; at the same time, theparameter a increases and the volume V decreases [154].

The refinement of the structures of Ln0.33Zr2(PO4)3 (Ln = Ce, Eu, Yb) phosphates using the Rietveldmethod allowed us to find that the symmetry of the structures of the studied phosphates reduced to spacegroup P c [154].

2.2. Lb FamilyThe crystal structure of the double potassium–magnesium sulfate K2Mg2(SO4)3, an analogue of nat-

ural mineral langbeinite, was first described by Zemann in 1957. This compound crystallizes in the cubicsystem, space group P213 (a = 9.920 (4) Å, V = 976.2 (1) Å3, Z = 4) [137, 165]. According to the data [166,167], sulfur atoms are in the center of slightly distorted oxygen tetrahedra and occupy in the lattice com-mon positions of C1 symmetry. K and Mg atoms are at threefold rotation axes in the centers of polyhedraof C3 symmetry. The [SO4] tetrahedra are located in the lattice so as to form slightly distorted [MgO6]octahedra of C3 symmetry around the Mg atom (Fig. 3c).

K atoms in points of local symmetry C3 are located in structure voids in two positions, surrounded by9 or 12 oxygen atoms (Fig. 3c). The values of O–S–O angles lie in the range 107.1° –111.0° and the lengthsof S–O bonds, within 1.458–1.468 Ǻ. Mg atoms are located so that two types of distorted octahedra,[Mg(1)O6] (dbond. = 2.057 Ǻ) and [Mg(2)O6] (dbond. = 2.066 Ǻ) can be distinguished in the structure. K(1)atoms are coordinated by twelve and K(2) atoms, by nine oxygen atoms: the average lengths of K(1)–O andK(2)–O bonds are equal to 3.045 and 2.992 Å, respectively [167].

The [SO4] tetrahedra and [MgO6] octahedra, bound with each other via vertexes, form the framework{Mg2(SO4)3}2– (Fig. 3c). In the structure of K2Mg2(SO4)3, columns of octahedra are oriented along fournonintersecting directions, parallel to the main diagonals of the cube.

3

Fig. 3. (a) A fragment of a crystal structure of NaZr2(PO4)3 (b) with designations of types of extra-framework positionsM1, CN 6 and M2, CN 8 and (c) a fragment of langbeinite structure.

NaZrPO

M1 M2

KMgSO

(a) (b) (c)



Based on structural data, the crystal chemistry formula of langbeinite-like phases can be presented as:

,

where L1, L2 are in-framework positions and M1, M2 are extra-framework positions [67].The most numerous group of the known compounds with langbeinite-like topology is exemplified by

sulfates. The results of the generalized analysis of compounds with Lb structure we performed in [67]point to wide possibilities of manifestation of this crystal structure also in phosphates.

The charges n of langbeinite frameworks can take values from 2 to 4 if both M1 and M2 positions arefully occupied. The majority of the known phosphates are characterized by frameworks with n = 2:A2[LnM(PO4)3], where A = K, Rb, Cs; M = Ti, Zr; Ln = Pr–Lu [140, 168–173]. For n = 3, the formulatypes are as follows: A1.5Ln0.5[MgZr(PO4)3], where A = K, Rb, Cs; Ln = Pr, Sm, Yb [124, 174];AB[Ln2(PO4)3], where A = KBa, RbBa, CaBa, K, Cs; Ln = Dy, Ho, Er, Tm, Yb [124, 174–187];A1.5Ln0.5[Fe2(PO4)3], where A = K, Rb, Cs; Ln = Pr, Sm, Yb [124, 174]. For n = 4, the following phos-phates are known: Ba2Mg1 + 0.5(1 – x)[LnxZr0.5(1 – x)(PO4)3], where 0 ≤ x ≤ 1; Ln = Sm, Yb [178, 179].

Some phosphates with partially vacant M positions were also described: K1.822Nd0.822Zr1.178(PO4)3 andRb2.05Ti0.81Yb1.19(PO4)3 [180].

The analysis of the published data indicates that the possibilities of the Lb structure on the inclusionof lanthanides cations in-framework and extra-framework positions are significantly wider than those ofthe NZP structure. The most favorable crystallographic positions occupied by f elements in the Lb struc-ture are framework-forming L positions, and the atomic concentration of lanthanides can be increased totwo in the formula composition AB[Ln2(PO4)3] [124, 174–177]. The data on the occupation of void posi-tions by small f element cations are scanty.

It should be also noted that, along with the formation of framework ortho compounds {T2(XO4)3}n–including differently charged cations and iso- and heterovalent substitutions of cations in the formation ofsolid solutions, there is also a possibility of the formation of both NZP and Lb isostructural phases withdifferent oxoanions XO4 (X are cations forming tetrahedral oxoanions with oxidation numbers from +3 to+7) [68].

In [68], calculations of formula compositions of compounds with tetrahedral oxoanions in the frame-work structure with noninteger frameworks charges were performed (on an example of an NZP structure)and their graphical representation was given. This opens additional possibilities for “modeling” complexcompositions of solid solutions on the basis of iso- and heterovalent isomorphism, also with the simulta-neous participation of cations both in the cationic and anionic parts of the framework. This crystal chem-istry principle can be used in the design of new materials with properties varied in the desirable directionand also with anticipated new properties, including optical ones. At the same time, the above reviews andmonographs contain no systematic presentation of optical properties of NZP and Lb compounds andtheir application as luminescent materials. As follows from the analysis of the published data, compounds(in particular, phosphates) with the structures of NZP and Lb offer promise for the creation of “on a plan”materials because of specific features of their structures: presence of several crystallographic positions thatcan be occupied by various cations and anions in various combinations and ratios. Studies of a possibilityof the introduction of biogenous elements and lanthanides into such crystal structures seem to be a prom-ising direction of inorganic chemistry and materials science, favoring the solution of a number of scientificand applied problems, in particular, the design of phosphors materials for LED and biomedical technol-ogies.

In general, the analysis of the literature, as was shown above, allowed us to see the critical problems ofpresent-day inorganic chemistry and materials science as the need in the development, preparation, andstudy of luminescent materials for LED and biomedical technologies. It should be also noted that theavailable data on inorganic phosphors for biomedical applications are fragmentary, which complicates thepurposeful search and the development of new compounds. The practical implementation of the princi-ples of crystal chemistry modeling on the basis of compounds with tetrahedrally coordinated oxoanionscan successfully contribute to solving the listed problems. The “modeled” compounds containing activeluminescence centers can be a basis for phosphors materials for the above technologies, in particular, forenergy saving white light sources, in vitro and in vivo monitoring of physiological processes at the celllevel, and for therapeutic purposes in the treatment of oncological diseases by the X-PDT method.

The development of studies in this direction retains a need in new experimental data and thus opens awide field for research.

−∞

[12] [9] [6] [6] [4]4 3 3( 1) ( 2) {[( 1) ( 2) (X O ) ] }

nM M L L

10


KANUNOV, ORLOVA

2.3. Crystal Chemistry Approach in the Design of New Phosphate Phosphors.Choice of Formula Compositions

The analysis of a wide range of experimental data on the isomorphism in phosphates of NZP and Lbfamilies performed in [65, 67] allows a conclusion that, in these systems, the empirical Goldschmidt andSobolev–Povarennykh rules are met not in all cases. The difference in the radii of the cations simultane-ously located in the framework can reach 68%. The charges of cations may differ by two units in voids andthree units in the framework. The difference in the electronegativity of cations can be as high as 0.8 invoids and 0.6 in the framework. Cations with various electron configurations, s–s, s–d, and s–f canoccupy identical positions. Cations of s, p, d, and f elements in different combinations may simultaneouslybe framework-forming cations [65].

The “design” of chemical compositions of compounds for the above-stated purposes the authors ofthis review performed on the basis of the previously calculated scheme [65, 68] of possible formula com-positions of compounds with tetraedral–octahedral frameworks, using different combinations and ratiosof A+, B2+, R3+, and M4+ cations responsible for the stabilization of the structure and the expected prop-erties. This scheme with the selected formula compositions for carrying out studies in this work is shownin Fig. 4 [181]. In the choice of chemical compositions, we took into account that the compounds mustbe safe and biocompatible. Taking into account these properties, we proved compositions containing, firstof all, sodium, potassium, magnesium, calcium, and phosphorus. For the deeper scientific understandingof the effect of compound composition on the studied functional characteristics of compounds, we alsointroduced atoms of chemical analogues into the structures. As activators we used manganese (Mn2+) andf elements (Eu2+, Eu3+, Sm3+, active centers in phosphates for LED technologies; Er3+, Yb3+ for bioimag-ing; and Eu3+, Sm3+ for X-PDT). In general, the chosen compositions of compounds contained the fol-lowing cations: A+ = Na, K, Rb, Cs; B2+ = Mg, Ca, Sr, Mn, Eu2+; R3+ = Ln (Sm, Eu, Er, Yb); M4+ = Zr.

In modeling the chemical formulas of compounds, we took into account both formal and crystalchemistry criteria.

The chosen compounds formed series presented below.Compounds with the structure of NZP. Zirconium and alkali (Na, K, Rb, Cs) and alkaline-earth (Ca,

Sr) element phosphates containing lanthanides (Eu3+, Eu2+, Sm3+, Er3+, Yb3 +) and Mn2+

A1 – 3xEuxZr2(PO4)3 (Eu3+), A = Na, K, Rb, Cs; x = 0, 0.001, 0.01, 0.05, 0.1, 0.25, 0.33;

Ca0.5 – xEuxZr2(PO4)3 (Eu2+), x = 0, 0.001, 0.01, 0.05, 0.1, 0.2, 0.5;

Ca0.2EuxSmyZr2(PO4)3 (Eu2+, Sm3+), (x, y) = (0.067, 0.2); (0.133, 0.1); (0.167, 0.05); (0.2, 0);

Ca0.5 – (x + y)MnxEuyZr2(PO4)3 (Eu2+, Mn2+), (x, y) = (0.1, 0); (0.1, 0.2), (0.2, 0.1);

Sr0.5 – xEuxZr2(PO4)3 (Eu2+), x = 0, 0.1, 0.2;

Ca0.5 – 1.5(x + y)ErxYbyZr2(PO4)3 (Er3+, Yb3+), 0.02 ≤ x + y ≤ 0.33.

Zirconium and calcium phosphate silicates containing lanthanides (Er3+, Yb3+)Ca0.75 – 1.5(x + y)ErxYbyZr2(PO4)2.5(SiO4)0.5, 0.1 ≤ x + y ≤ 0.4.

Phosphates with the structure of langbeinite. Zirconium, magnesium, and potassium phosphates con-taining lanthanides (Sm3+, Er3+, Yb3+)

K2 – xSmxMg0.5 + xZr1.5 – x(PO4)3 (–4 ≤ n ≤ –2), 0 ≤ x ≤ 1;K2Mg0.5 – 0.5(x + y)ErxYbyZr1.5 – 0.5(x + y)(PO4)3 (n = –2), 0 ≤ x ≤ 1;

The following isomorphous substitutions were found in the series of the studied compounds:(1) isovalent in voids

B2+ ↔ Eu2 + (B = Ca, Sr);(2) heterovalent in voids with the formation of cationic vacancies (h)

A+ ↔ 1/3Eu3+ + 2/3 h (A = Na, K, Rb, Cs),3Ca2+ ↔ Er3+ + Yb3+ + h;

with interstitial cations4 P5+ + h ↔ 5Si4+;



with the substitution of cationic couples with equal total charges (coupled isomorphism)

K+ + M4+ ↔ Sm3+ + Mg2+, M = Ti4+, Zr4+,

1/2Mg2+ + 1/2M4+ ↔ R3+ (R = Er3+, Yb3+).

3. SYNTHESIS

3.1. Brief review of Methods for the Preparation Orthophosphates of NZP and Lb Families

The majority of anhydrous framework orthophosphates of NZP and Lb families are obtained by reac-tions proceeding in the solid phase and aqueous solutions (sol–gel and hydrothermal technologies) and,more rarely, in melts (alkali metal chlorides and nitrates, boron oxide).

In the preparation of phosphates by the solid-phase method, the initial reagents are salts easily decom-posing on heating (mainly nitrates, carbonates), also chlorides or oxides of elements forming the cationicpart of the compound, and ammonium hydrogen and dihydrogen phosphates. A mixture of stoichiomet-ric amounts of the initial reagents is subjected to consecutive thermal treatment at a number of tempera-tures with intermediate dispersing [146, 182–184]. The drawback of the solid-phase method is the needin high calcination temperatures (T > 950°C) for the formation of the final monophase product and thecareful dispersing of powders at each stage of heating.

In recent years, much attention has been paid to the improvement of the sol–gel method of synthesisbased on gel-formation in water sols [146, 185–188]. The method consists in mixing stoichiometricamounts of aqueous salt solutions of elements forming the cationic part with a phosphorus-containingreagent followed by the coagulation of the formed sol.

This gives a gel of zirconium hydrogen phosphate according to the scheme:

[189].

In the formation of colloidal particles of zirconium hydrogen phosphates, metal ions are uniformly dis-tributed between the formed micelles, which is assisted by the continuous stirring of the solution.

⎯⎯⎯⎯→ ↓3 4H PO2 4 2ZrOCl Zr(HPO )

Fig. 4. Calculated formula compositions of compounds with NaZr2(PO4)3 and langbeinite structures.

MMMM1 A B1/21/2 R1/31/3

B

0

1

2

3

4

NaZr2(PO4)3 structural typeLangbeinite structural type

Interstitial cations(n) (T)

A B1/2 R1/3

A2B

AB

AR

RB

RR

MA

MBCA

CB

MR

MM

CM

M1/2B3/2

M4/3A2/3

M5/3A2/3B1/2

C3/4A5/4C1/4B5/3

C2/3B4/3

C5/4A3/4

C1/2R3/2

C3/2A1/2

C7/4A1/4C5/3B1/3C3/2R1/2

C4/3B1/3CR

M3/23/2M3/2

Frameworkcharge

Frameworkcations

A, B, R, M, C are elements in oxidation states +1, +2, +3, +4, +5, respectively

12


KANUNOV, ORLOVA

The obtained gel is dried at T = 90–100°C and subjected to consecutive heat treatment at a number oftemperatures under the conditions of free air access. The scheme [190] of the formation of NaZr2(PO4)3is presented on the following example:

[190].The advantages of solution technologies are in the simplicity of the preparation and control of the com-

position of solutions and the size and morphology of powder particles, in the achievement of high degreeof homogeneity and purity of products, and in the reduction of the time and temperature of their synthesisin comparison with the conditions of interaction in the solid-phase.

Except for the conditions of sol–gel of synthesis indicated above, which have become classical, thereare various procedures to a certain degree replacing or supplementing some stages of the process: intro-duction of salting-out agents (for example, alcohols) [191, 192], organic complexants, and the Pechinimethod [193–196]. The Pechini method is based on the ability of tricarboxylic acids (in particular, citricacid) to form chelate complexes with many cations and to enter polycondensation (esterification) reac-tions with polyatomic alcohols (ethylene glycol) (Fig. 5) [193, 196].

The hydrothermal method allows the researchers to synthesize crystal phosphates at low T = 200–350°C and often opens a possibility of obtaining well-shaped crystals suitable for X-ray diffraction analy-sis. Two versions of this method are known [197–202]. In the first version, the initial reagents (hydrox-ides, oxides, metal salts, phosphoric acid) in certain molar ratios are mixed as aqueous solutions andstored at 200–350°C in f luoroplastic-lined autoclaves for a certain time. An important factor for obtainingphases of a specified composition is the initial pH of the solution (1 < pH < 2). In the second version, stoi-chiometric amounts of aqueous metal salt solutions are mixed at room temperature, and then a phos-phoric acid solution is added. The obtained gel is dried at 80°C, filled with water again, and kept underhydrothermal conditions at T = 200–350°C. This method of synthesis has a number of shortcomings.It can hardly or even cannot give phosphates of complex composition. In addition, the conditions ofpreparation strongly affect the possibility of formation of the target product. Therefore, the application ofthe hydrothermal method requires substantial time and resources for revealing the specific features of theprocesses of the formation of solid products, i.e., the choice of the optimum reagent ratio, temperature,and pressure.

3.2. Synthesis Methods Used in the Synthesis of PhosphorsIn this section and below we present a summary of data on the synthesis of phosphors collected by the

authors.In general, for the preparation phosphor materials, we widely use the sol–gel method, including the

version with the introduction of additional organic reagents into the reaction system, as was noted above.In the synthesis version with a salting-out agent (Fig. 5, Scheme 1), we used stoichiometric ratios for

the initial solutions of salts and initial solutions of metal salts and precipitants. The obtained gel washeated at 90°C for dehydration. The dry residue was dispersed in an agate mortar (till 30 min) and thenconsecutively heated at 600, 800, and 900°C. In some experiments, temperature was raised to 1100°C.After each step of isothermal storage, the samples also carefully dispersed in ethyl alcohol (till 30 min).

In the version of the Pechini citrate method (Fig. 5, Scheme 2), initial salt solutions were mixed withcitric acid (CA) in a stoichiometric ratio, and then a 1 M NH4H2PO4 solution and ethylene glycol (EG)were added dropwise to the obtained mixture under continuous stirring. The molar ratio of CA to the

metal was CA : M = 15 : 1, where M = , and the ratio CA : EG was 1 : 4. The obtained gelwas heated to 130–350°C to remove water; the formed dry residue was dispersed in an agate mortar within30 min and then successively heated to 600–1100°C at a step of 100 K.

As phosphors must meet the requirements of microstructure uniformity and optical “purity,” the con-formity with these requirements must be taken into account in the course of synthesis.

Uniform powder material can be introduced at the synthesis stage. For example, the effects of the tem-perature of gel formation and ultrasonic treatment on the microstructure of Ca0.5–1.5(x + y)ErxYbyZr2(PO4)3phosphates, 0.02 ≤ x + y ≤ 0.33 were studied in [203]. The size distribution of particles found for phosphatesamples was close to the normal distribution and depended on the synthesis conditions (Table 2). Tem-perature increase from 0 to 80°C at the stage of gel formation led to an increase in the average size of crys-tallites, while the use of dispersing between the stages of heating and ultrasonic treatment favored theirreduction. The minimum average size of particles, 40 ± 10 nm (Tgel form = 20°C using dispersing and ultra-

° °⎯⎯⎯→ ⎯⎯⎯⎯⎯→, C NaCl,700 C4 2 2 7 2 4 3Zr(HPO ) ZrP O NaZr (PO )T

=ν∑ 1 (Met)

nii



sound) and their maximum size, 110 ± 50 nm (Tgel form = 80°C, without dispersing) differed more than2.5-fold (Table 2).

To meet the second requirement in the preparation substances, one should completely exclude theircontact with other substances containing activators. In [181], powder precursors containing europium 2+were subjected to preliminary dehydration and decontamination at T = 400°C for 20 h. Then they wereplaced in a quartz container, which was placed in a quartz tube and heated in situ in a tubular furnace ina reducing argon–hydrogen atmosphere (Ar + 5% H2) at T = 800°C for 3–20 h. The qualitative analysisof substances for the presence of Eu2+ and Eu3+ cations was performed by their irradiation with an UVWood lamp. Under UV irradiation, the samples containing Eu3+ emitted pink light and those containingEu2+, blue light. Samarium-containing phosphates had red–pink colors of different intensity. After theexperiment, samples were stored in the atmosphere of Ar + 5% H2 in vials with paraffinized stoppers.

4. FEATURES OF PHASE FORMATION IN SYSTEMS OF LANTHANIDE-CONTAINING PHOSPHATES WITH NZP AND LANGBEINITE STRUCTURES

4.1. Phase FormationThe obtained substances were fine white powders. Erbium-containing phosphates were colored in light

pink or pink, depending on the concentration of erbium in the samples.The temperature conditions of synthesis were chosen based on the data of differential thermal analysis

(DTA) and X-ray powder diffraction on an example of batch mixtures in the synthesis ofNa0.25Eu0.25Zr2(PO4)3, Ca0.3Eu0.2Zr2(PO4)3 phosphates with the expected structure of NZP and ofK1.5Sm0.5MgZr(PO4)3 phosphate with the expected structure of Lb.

For representatives of both structural types, irreversible processes accompanied by thermal effects atT = 90–160°, 230–260°, 610–730°C were found in the temperature range 20–900°C. These effects canbe due to the removal of crystallization water, ethanol, and gaseous products of reaction and chemicalinteraction, respectively [181].

The conditions of formation of highly crystalline phases (temperature, duration of heat treatment)were chosen and optimized in special experiments and were as follows: T = 800–900°C, τ = 20 h in air,τ = 3 – 20 h in an atmosphere of Ar + 5% H2.

The chemical composition and agreement with the calculated composition were confirmed by energy-dispersive X-ray spectroscopy (EDX).

X-ray diffraction studies. X-ray diffraction patterns of samples were characterized by reflexes typical forcompounds of NZP (hexagonal system) and Lb (cubic system) structural families, no asymmetry of dif-

Fig. 5. Schemes of colloidal chemical synthesis of powders.

Scheme 1

Addition of precipitantAddition of

Mixture of salt solutions +

NH4H2PO4 and HOCH2CH2OH

(HOOCCH2)2C(OH)COOHMixture of salt solutions +

Gel formation, stirring,T = 20°C

Complex formation, stirring,T = 80°C

T = 90°C, τ = 5−20 h T = 90−350°C, τ = 20 h

T = 90−350°C, τ = 20 h

C2H5OH

Scheme 2

solutions

Dispersing in C2H5OH, 30 min

Control by X-raypowder diffraction

14


KANUNOV, ORLOVA

fraction line profiles and diffusion halo of amorphous phases and, in general, pointed to the preparationthe monophase products. As examples, Fig. 6 presents X-ray diffraction patterns of phosphates from dif-ferent space groups.

X-ray diffraction patterns were indexed by comparison with the known analogues for different struc-tural types and space groups:

(1) phosphates with the structure of NZP analogues of NaZr2(PO4)3 (space group R c) [136],KZr2(PO4)3 (space group R c) [204, 205], RbZr2(PO4)3 (space group R c) [206], CsZr2(PO4)3 (spacegroup R c) [207], Eu0.33Zr2(PO4)3 (space group P c) [154], Eu0.5Zr2(PO4)3 (space group R3) [208],Ca0.5Zr2(PO4)3 (space group R ) [209, 210];

(2) phosphates with the structure of Lb analogues of K2Mg2(SO4)3 [137, 165], K2YZr(PO4)3 [175],K2Mg0.5Zr1.5(PO4)3 [211], (space group P213).

The analysis of X-ray diffraction patterns of phosphates with the structure of NZP revealed a series ofmorphotropic transitions due to the ordering of rare-earth element cations of in the structure positions.Thus, in the series A1–3xEuxZr2(PO4)3 (A = Na, K, Rb) phosphates at x ∈ (0; 0.1) crystallized in the spacegroup R c, while representatives of the same series with higher concentrations of europium (x = 0.25)were superstructures of the space group P c. Morphotropic transitions also occurred in the seriesB0.5 ‒ xEuxZr2(PO4)3 (B = Ca, Sr): R → R3 at x = 0.2; Ca0.5–1.5(x + y)ErxYbyZr2(PO4)3: R → P c at x =0.2 (Table 3).

For phosphates with the structure of Lb, the morphotropism phenomenon was not observed.Limited isomorphous mixibility was noted for representatives of both structural families, namely,

for the series Cs1 – 3xEuxZr2(PO4)3 (NZP) and K2 – xSmxMg0.5 + xZr1.5 – x(PO4)3 (Lb). For the seriesCs1–3xEuxZr2(PO4)3 with cesium, it was found that solid solutions occurred in the range 0 ≤ x ≤ 0.1 [212].A sample of Cs0.25Eu0.25Zr2(PO4)3 was a mixture of phases, which was confirmed in repeated experiments.The formation of monophase products in the series K2 – xSmxMg0.5 + xZr1.5 – x(PO4)3 was limited by thecompositions 0 ≤ x ≤ 0.5 [213] (Table 3).

The parameters of unit cells for all of the obtained phosphates were calculated by the results of index-ation of X-ray diffraction patterns by the least-squares technique (Table 3); they changed with the growthof x. In the series of phases with the structure of NZP, parameters c and V increased and parameter adecreased; in the series of phases of the Lb family, parameters a and V increased.

The considered isomorphism illustrates an important property of frameworks, their“elasticity” at high strengths, which determined a wide range of possible substitutions in different groupsof cations without the rupture or cardinal reorganization of the initial motif.

Data of IR spectral analysis. For A1 – 3xEuxZr2(PO4)3 systems (A = Na, K, Rb, Cs), two types of IRspectra were observed depending on the concentration of europium (x) [212]; their typical views are pre-sented in Fig. 7 as examples. Factor group analysis for AZr2(PO4)3 phosphates (A = Na, K, Rb, Cs) with

the space group R c predicts six active regions of valence vibrations of groups in IR spectra: ν1 – Eu;ν3 – 2A2u + 3Eu and 7 bending vibrations: ν2 – 2Eu; ν3 – 2A2u + 3Eu [214, 215]. The ordered distributionof lanthanide atoms, in particular, of europium, in M1 positions of the NaZr2(PO4)3 structure results inthe reduction of symmetry to the space group P c [154]. The results of factor group analysis for the com-pounds Ln0.33Zr2(PO4)3 where Ln = Ce–Lu indicated 14 active IR modes of ν3 vibrations of groups:

33 3

3 33

33

3 3 3

−2 4 3{T (XO ) }

n

3 −34PO

3−3

4PO

Table 2. AFM data. Samples of Ca0.35Er0.05Yb0.05Zr2(PO4)3 obtained under different conditions

USD is ultrasonic dispersing.

Tgel, °CAverage particle size, nm

without dispersing with dispersing

0 90 ± 30 60 ± 2020 100 ± 60 50 ± 20

20 (USD) 90 ± 10 40 ± 1080 110 ± 50 90 ± 20



5A2u + 9Eu; and 4 vibrational ν1 modes: A2u + 3Eu [216]. The IR spectra of Na0.25Eu0.25Zr2(PO4)3 phos-phates (as of analogous potassium and rubidium phosphates) were similar to the spectra ofLn0.33Zr2(PO4)3 and can also be assigned to space group P c. Note that alkali metal atoms can occupy notonly M1 but also/or M2 positions of the NZP structure, or reaction products can be mixtures of com-pounds with space groups R c and P c. In this case, vibrational bands of the rhombohedral phase willoverlap with vibrational bands of compound with the space group P c. Based on the fact that the band ofinteraction P–O/Zr–O at 1205 cm–1, characteristic for IR spectra of phosphates of the space group R c,was not observed in IR spectra of A0.25Eu0.25Zr2(PO4)3 phosphates, we can suppose that the obtainedproducts are NZP phases in which alkali metal cations occupy M2 positions.

Similar data were obtained for the morphotropic series Ca0.5 – 1.5(x + y)ErxYbyZr2(PO4)3 (Er3+, Yb3+) [181, 203].In general, the data of IR spectroscopy confirmed the functional composition of phosphates, and

pointed to their crystallization in space groups R c, R , P c (NZP phosphates), and P213 (Lb phos-phates) and also to the absence of X-ray amorphous impurities in the samples.

4.2. Structural Data

To reveal specific features of the structures of the synthesized and characterized compounds, and alsoto study the distribution of cations between the structural positions, structural studies for some com-pounds were performed by methods of full-profile analysis by the data of X-ray powder diffraction (Riet-veld method) and EXAFS.

Rietveld method. Crystal structures of two representatives of the NZP family containing europium, i.e.,Na0.7Eu0.1Zr2(PO4)3 and Na0.25Eu0.25Zr2(PO4)3, were studied as an example. These phosphates are mem-bers of the same series, in which the ratio of Na to Eu was changed, as a result of which, a morphotropictransition with a change of space group from R c to P c was observed. As basic models for the refinement

3

3 33

3

3 3 3

3 3

Fig. 6. X-ray diffraction data of (a) K1.5Sm0.5MgZr(PO4)3, phosphates, Lb type, cubic system, space group P213;(b) Na0.7Eu0.1Zr2(PO4)3, NZP type, trigonal system, space group R c; (c) Na0.25Eu0.25Zr2(PO4)3, NZP type, trigon.system, space group P c; (d) Ca0.3Eu0.2Zr2(PO4)3, NZP type, trigon. system, space group R .

15 20 25

I, re

l. un

its

30 35 40 45 5010

003

101 0

12

104 11

0

015

113

024 1

16

018 2

14 300

208

119

306

128

134 22

640

113

412

830

621

711

920

8300

214

01822

111

6

024

016

114

113

112

110

012

104 11

0

113

024 1

16

018 21

430

0

208

119

306

128

134 22

643

2

431

422

332

331

410

321

320

311

310

22121

121

0111

104

00401

210

100

3

02.1

0

2θ, deg

a

b

c

d

33 3

16


KANUNOV, ORLOVA

of structures of the above phosphates, coordinates of atoms of NaZr2(PO4)3 (space group R c) [136] andEu0.33Zr2(PO4)3 (space group P c) [154] phosphates, respectively, were used. More complete crystallo-graphic information, including the found coordinates, isotropic thermal parameters of atoms, and bondlengths and bond angles was presented in [181, 212]. The obtained data indicate that the crystal structuresof Na0.7Eu0.1Zr2(PO4)3 and Na0.25Eu0.25Zr2(PO4)3 phosphates are isotypic. They formed by topologicalinvariants [Zr2(PO4)3], which consisted of isolated Zr octahedra and PO4 tetrahedra bound via commonvertexes. Each two Zr octahedra were connected with three PO4 tetrahedra. In the structure of theNa0.7Eu0.1Zr2(PO4)3 phosphate, the bond length in tetrahedra and octahedra varies in narrow ranges:1.53(5)–1.55(5) Å (P–O) and 2.01(4)–2.06(4) Å (Zr–O). The bond angles O–P–O and O–Zr–O alsocorresponded to tetrahedral and octahedral coordination, respectively [212]. In the structure of theNa0.25Eu0.25Zr2(PO4)3 phosphate, the framework-forming polyhedra were characterized by a wider dis-persion of interatomic distances: 1.92(2)–2.22(3) Å (Zr–O) and 1.50(3)–1.60(3) Å (P–O). Sodium andeuropium atoms occupied extra-framework positions. Thus, in the structure of the Na0.7Eu0.1Zr2(PO4)3phosphate (space group R c), these atoms occupied M1 positions of the symmetry 6b, and, in theNa0.25Eu0.25Zr2(PO4)3 phosphate (space group P c), they were distributed between two positions, M1 andM2, of the symmetry 2b and 6f, respectively [212].

EXAFS method. Phosphates of the series Ca0.5 – xEuxZr2(PO4)3 were investigated by the EXAFSmethod [217]. An approximation of an EXAFS spectrum was obtained based on the data on theCa0.5Zr2(PO4)3 structure [209]. It was taken into account that Eu2+ substitutes for Ca2+, forming a Eu–Obond in the first coordination sphere and a Eu–Zr bond in the second coordination sphere. Figure 8 pres-

33

33

Table 3. Crystallographic data for compounds

Compound series x Space group a, Å c, Å V, Å3

Phosphates with the structure of NZPNa1 – 3xEuxZr2(PO4)3 0 ≤ x ≤ 0.1

0.25 ≤ x ≤ 0.33R сP с

8.757(5)–8.819(1)

22.832(2)–23.00(2)

1522(1)–1542(2)

K1 – 3xEuxZr2(PO4)3 8.710(1)–8.739(8)

23.34(7)–24.04(4)

1534(2)–1588(2)

Rb1 – 3xEuxZr2(PO4)3 8.661(2)–8.692(4)

23.76(4)–24.48(2)

1554(1)–1594(1)

Cs1 – 3xEuxZr2(PO4)3 0 ≤ x ≤ 0.1 R с 8.580(4)–8.604(9)

24.91(2)–25.02(7)

1588(2)–1604(3)

Ca0.5 – xEuxZr2(PO4)3 0 ≤ x ≤ 0.20.2 ≤ x ≤ 0.5

RR3

8.696(5)–8.772(2)

22.767(3)–23.418(2)

1517(1)–1534(1)

Sr0.5 – xEuxZr2(PO4)3 8.681(4)–8.694(3)

23.352(3)–26.281(5)

1529(1)–1715(1)

Ca0.2EuxSmyZr2(PO4)3 0 ≤ x ≤ 0.20.067 ≤ y ≤ 0.2

R 8.737(1)–8.786(7)

22.42(2)–22.97(6)

1499(2)–1519(8)

Ca0.2EuxMnyZr2(PO4)3 0.1 ≤ x ≤ 0.20.1 ≤ y ≤ 0.2

R 8.811(6)–8.832(6)

22.87(4)–22.91(6)

1540(1)–1545(1)

Ca0.5 – 1.5(x + y)ErxYbyZr2(PO4)3 0 ≤ x ≤ 0.20.2 ≤ x ≤ 0.33

RP с

8.770(5)–8.840(5)

22.48(3)–22.783(4)

1513(0)–1525(1)

Ca0.75 – 1.5(x + y)ErxYbyZr2(PO4)2.5(SiO4)0.5 0.02 ≤ х + y ≤ 0.4 R 8.773(5)–8.791(12)

22.70(2)–22.76(4)

1512(2)–1523(3)

Phosphates with the structure of LbK2 – xSmxMg0.5 + xZr1.5 – x(PO4)3 0 ≤ x ≤ 0.5 P213 10.259(9)–

10.325(2)– 1079(7)–

1100(2)K2Mg0.5 – 0.5(x + y)ErxYbyZr1.5 – 0.5(x + y)(PO4)3 0 ≤ x ≤ 1 P213 10.161(7)–

10.320(5)– 1049(6)–

1099(6)

33

3

3

3

3

33

3



ents an example of a k2-weighed LIII absorption edge of europium and average interatomic distances Eu–O forCa0.5 – xEuxZr2(PO4)3. According to the results of EXAFS, with the growth of europium concentration,the bond lengths in M1 polyhedra increased because of the larger ionic radius of Eu2+ in comparison withthat of Ca2+ (1.17 and 1.00 Å according to Shannon [218], respectively). This agrees with crystallographicparameters found by us and those presented above (an increase in cell volume with the growth of x), andalso with the results of work [219], where the structure was refined by the Rietveld method.

Summarizing the experimental data presented in Sections 3 and 4, let us note that we have demon-strated the application of crystal chemistry principles to the formation of crystal materials with theexpected NZP and Lb structures. All of the predicted compounds with the expected structures were syn-thesized. The analysis of their composition and structure validated the chosen approach. The majority ofindividual compounds and solid solutions were obtained for the first time.

A possibility of the formation of substitution solid solutions shown in this work for a series of frame-work phosphates points to the stability of the total structural motif of the framework octahedral–tetrahe-dral construction and opens ways to the control of useful properties of these substances in the design ofmonophase phosphate materials.

5. LUMINESCENT PROPERTIESThe luminescent properties of the obtained phosphates were studied within three tasks stated above:

LED technologies, intracellular bioimaging, and X-PDT of oncological diseases.

5.1. Phosphate Phosphors for LED TechnologiesAZr2(PO4)3:Eu3+ system (A = Na, K, Rb, Cs). Eu3+ luminescence in phosphates of the series

A1 – 3xEuxZr2(PO4)3, A = Na, K, Rb, Cs, 0.001 ≤ x ≤ 0.25 at λexcit = 210 nm was observed in the regionsλem = 370–410 nm and λem = 580–620 nm [212]. The optical properties of europium in the oxidation state+3 are due to 4f–4f transitions. According to the Laporte rule, these transitions are forbidden, but,because of vibrations of the polyhedron, substantial peaks of these transitions can be observed even if theirintensities are relatively diffuse. According to the data presented in [220], the 4f–4f transitions of euro-pium (3+) in the emission mode in the visible spectral region consist of the following components:

Fig. 7. IR spectra of A1 – 3xEuxZr2(PO4)3 phosphates, where A = (a) Na, (b) K; x = 0 (1), 0.05 (2), 0.1 (3), space group R c;0.25 (4) space group P c.

1400 1200 1000 800 600 400

Tran

smitt

ance

, %1

2

3

4

ν, cm−1 ν, cm−11400 1200 1000 800 600 400

Tran

smitt

ance

, %

1

2

3

4

(a) (b)

1210

1114

1063

1032 1018

866

648

578

556

463

553574

646

1021

1042

106511

03

1207

1204

1097

1064

1033 10

1593

9

644

577

555

466

428

1233

1131

1079

1058 10

2499

1 970

640

583

567

554

428

424

517

553

573

639

947

10451

139

1207

1205

1103

1028 10

17

640 57

5

424

423

573

640

1017

1030

1112

1205

946

554

565

582

640

938

973

1025

1049

1066

109211

33

1227

33

18


KANUNOV, ORLOVA

(1) 5D0 → 7F0 at apprx. 570–580 nm. This transition is singlet, which means that peak intensity isdirectly related to the number of crystallographic positions occupied by Eu3+;

(2) 5D0 → 7F1 at apprx. 585–600 nm. This transition belongs to a magnetic dipole. The number of thepeaks relating to one crystallographic position is equal to three. Splitting is proportional to crystal fieldstrength. This transition is due to the localization of europium in an environment with centers of inver-sion;

(3) 5D0 → 7F2, also named “hypersensitive transition,” at apprx. 600–625 nm, relates to an electricdipole. This transition is due to the localization of europium in an environment without a center of inver-

sion. The ratio is used to characterize the centrosymmetric character of europium position.

Figures 9a–9d present emission spectra of phosphates of the series with sodium, Na1 – 3xEuxZr2(PO4)3,x = 0.001, 0.01, 0.1, 0.25 [212].

The intensity of bands in the more short-wavelength region increased with the growth of x and reachedthe maximum value at x = 0.25. The spectra also contained wide low-intensity bands at apprx. 400–450 nm,pointing to the presence of trace amounts of Eu2+. The emission bands corresponded to the expected ones.

Such changes in the spectra can be due to the transition of europium from an M1 to an M2 position inthe structure with CN values of 6 and 8, respectively (Fig. 3). This transition results in the weakening ofthe crystal field and the shift of emission to the short-wavelength region.

For systems with potassium and rubidium, changes in the spectra were similar (Figs. 9e–9g) [212].B0.5Zr2(PO4)3:Eu2+ system (B = Ca, Sr). For compounds of the series B0.5 – xEuxZr2(PO4)3, B = Ca,

Sr, x = 0.001, 0.01, 0.05, 0.1, 0.2, 0.5, photoluminescence was measured on excitation by laser sources withλ = 350 and 400 nm (UV and blue regions, respectively). Figure 10 presents normalized emission spectraof phosphates [217]. They are characterized by wide excitation bands in the region 250–420 nm and emissionin the region 400–700 nm with wide shoulders in the long-wavelength region and correspond to europiumtransitions from the excited state 4f 65d1 to the ground state 4f 7. Low intensity of emission at 610 nm, presentin some spectra, is due to europium in the oxidation state +3 (trace amounts after storage of samples inthe reducing atmosphere).

The analysis of emission spectra of phosphates of the series B0.5 – xEuxZr2(PO4)3, B = Ca, Sr, 0.001 ≤x ≤ 0.5 at different x (λexcit = 350, 400 nm) indicates that the intensity of emission increased with thegrowth of x in range 0.001 ≤ x ≤ 0.2 and decreased at x ≥ 0.2. The last effect may be due to concentrationquenching. With the growth of x, the bands shifted to the region of higher energies because of the reduc-tion of crystal field strength on the replacement of the Ca2+ cation by the larger Eu2+ cation, as in the sub-stitution of Ca2+ by Sr2 + ( = 1.00 Å, = 1.17 Å, = 1.18 Å, CN 6, according to Shannon [218]).

The shape of spectra also indicates that emission is not characterized by only one band. As a result ofthe mathematical processing of spectra, it was found that each emission band was presented by two Gauss-ians. The spectrum of the Ca0.3Eu0.2Zr2(PO4)3 phosphate as an expansion of an emission band into two

−

−

5 70 2

5 70 1

D F

D F

I

I

+2Car +2Eur +2Srr

Fig. 8. Data of EXAFS spectroscopy of Ca0.5 – xEuxZr2(PO4)3 phosphates: (a) normalized EXAFS function extractedfrom the experimental absorption spectrum and the result of its approximation; (b) interatomic distance Eu−O as a func-tion of Eu concentration.

0

0.6

−0.62 4 6 8 10 12 140

k2χ(

k)

2.6

2.7

2.8

2.50.1 0.2 0.3 0.4 0.50

Eu−

O, Å

(a) (b)k2

k, Å−1 x



Gaussians, high-energy g2 (Fig. 11, curve 3) and low-energy g1 (Fig. 11, curve 4), is shown in Fig. 11 as anexample. As the smaller Stokes shift is characteristic for crystallographic positions of a larger volume, onecan suppose that the g2 component corresponds to europium occupying the M2 position (octagon, CN 8),and the g1 component, to the M1 position (trigonal antiprism, CN 6) (Fig. 3) [217].

The contribution of the high-energy Gaussian increased with the growth of x. Its intensity increased,which points to an increase in the percentage of occupied M2 positions (Fig. 12a).

Because of the growth of population of some or other position with an increase in x of the cation withthe larger radius (europium), emission must shift to the short-wavelength region (for both components g1and g2), as was found from the obtained experimental data (Fig. 12b).

For the deeper understanding and description of Gaussian components, Glorieux et al. investigatedphotoluminescence of the Ca0.3Eu0.2Zr2(PO4)3 phosphate at 80 K [217]. The spectra at 80 K and roomtemperature were similar. As one would expect, the width of emission band decreased with decreasingtemperature. At 80 K, the width of the Gaussian decreased, which was due to the weakening of the vibra-tion processes. The decrease in temperature favored the strengthening of high-energy components, which,probably, reflects the weakening of the channel of nonradiative relaxation between the cations, whereasconfiguration quenching did not play an important role.

The observed very weak red shift was due to the contraction of the lattice at low temperatures, and,therefore, the strengthening of the crystal field of the environment of the europium cation because of thereduction of the Eu–O bond length. This strengthening led to the splitting of the 5d level [221, 222] and,as a result, to the lowering of the level of the excited state responsible for the red shift. With an increase inexcitation wavelength, a small red shift due to a transition to a low-energy state was observed.

Fig. 9. Photoluminescence spectra of phosphates of the series Na1 – 3xEuxZr2(PO4)3, x = (а) 0.001, 0.01 (b), 0.1 (c),0.25 (d) and A1 – 3xEuxZr2(PO4)3 phosphates, where A = K (e), Rb (f), Cs (g).

55D0−7F1

5D0−7F1

5D0−7F1

10000

20000

30000

40000

50000

60000

0300 400 500 600 700200

10000

20000

30000

40000

50000

60000

0300 400 500 600 700200

200000

400000

600000

800000

1000000

1200000

0300 400 500 600 700200

300 400Wavelength, nm Wavelength, nm

Wavelength, nm Wavelength, nm

Wavelength, nm Wavelength, nm

Wavelength, nm500 600 700200 300 400 500 600 700200 300 400 500 600 700200

20000

40000

60000

80000

100000

0

20000

40000

60000

80000

100000

0

20000

40000

60000

80000

100000

0

20000

40000

60000

80000

100000

0

300 400 500 600 700200

Inte

nsity

, rel

.u.

Inte

nsity

, rel

.u.

Inte

nsity

, rel

.u.

Inte

nsity

, rel

.u.

λexc = 210 nm λexc = 210 nm

λexc = 230 nmλexc = 300 nm



λexc = 210 nm


λem = 592 nmλem = 610 nm

λem = 584 nmλem = 592 nm

λem = 585 nmλem = 591 nmλem = 585 nmλem = 590 nm

λem = 610 nm

λem = 585 nmλem = 592 nmλem = 610 nm λem = 610 nm

λem = 610 nm

λem = 588 nmλem = 592 nmλem = 610 nm

Eu2+

Eu2+

Eu2+

Eu3+

5D0−7F0

5D0−7F1

5D0−7F2

5D0−7F2

5D0−7F1

5D0−7F0

5D0−7F2

5D0−7F1

5D0−7F0

5D0−7F15D0−7F0

5D0−7F2

K0.25Eu0.25Zr2(PO4)3

K0.97Eu0.01Zr2(PO4)3Rb0.97Eu0.01Zr2(PO4)3

Rb0.7Eu0.1Zr2(PO4)3 C0.85Eu0.05Zr2(PO4)3

(a) (b)

(c) (d)

(e) (f) (g)

20


KANUNOV, ORLOVA

The process of luminescence decay was described by two components: the first one with the longer(>71 ns) and the second one with the shorted (from 16 to 39 ns) decay time [217]. With the growth of emis-sion wavelength, the values of τ1 and τ2 decreased; their ratio τ1/τ2 increased. The intensity of the short-wavelength components in this case decreased and that of the long-wavelength component increased.A similar effect was observed with an increase in excitation wavelength at a constant emission wavelength.Therefore, the short-lived component can be assigned to low-energy emission. However, both decay pro-cesses always appeared simultaneously in all experiments; therefore, it was impossible to accurately extractone process. It was only clear that each process was not associated with only one crystallographic position.In this case, as was also noted above, we could distinguish environments of two types: the first of themdetermined high-energy emission with the long lifetime and the second, low-energy emission with theshort lifetime because of the influence of the process of nonradiative relaxation.

The calculated ICE (International Commission on Energy) color coordinates for all of the studiedcompounds B0.5 – xEuxZr2(PO4)3 (B = Ca, Sr) changed almost linearly from {0.32; 0.41} to {0.18; 0.17} withan increase in the amount of europium irrespective of the excitation wavelength [217].

To search for the compositions of phosphates within the studied family with color coordinates mostclose to standard NTSC (National Television Standard Committee) coordinates of white luminescence

Fig. 10. Normalized emission spectra of phosphates of the series Ca0.5 – xEuxZr2(PO4)3, x = (1) 0.001, (2) 0.01, (3) 0.05,(4) 0.1, (5) 0.2, (6) 0.5, and (7) Sr0.3Eu0.2Zr2(PO4)3.

0.5 1

234 5

6

7

1.0I

0400 450 500 550 600 650 700

λ, nm350

Fig. 11. Photoluminescence of Ca0.3Eu0.2Zr2(PO4)3 phosphate: (1), excitation spectrum, λexcit = 350 nm; (2), photolu-minescence spectrum, λem = 480 nm; (3) and (4), Gaussian components (g1 and g2, respectively); (5), total component.

300 350 400

λexcit = 350 nm λem = 480 nm

450 500 550 600 650 700λ, nm

250

I

1 25

3

4



{0.33; 0.33}, we studied compounds simultaneously containing europium and manganese and europiumand samarium [181, 217].

Ca0.5Zr2(PO4)3:Eu2+, Mn2+ system. For the phosphate system containing manganese and europiumCa0.5 – (x + y)MnxEuyZr2(PO4)3 (x, y) = (0.1, 0); (0.1, 0.2); (0.2, 0.1), emission was observed in the blue spec-tral region (λem. = 475 nm) on excitation with an UV source (λexcit = 350 nm). The Ca0.4Mn0.1Zr2(PO4)3phosphate was characterized by red luminescence of low intensity. However, at the simultaneous presenceof Mn2+ and Eu2+, a sharp increase in emission intensity was noted in comparison with the emission ofthe sample containing no manganese (Fig. 13) [223].

Ca0.5Zr2(PO4)3:Eu2+, Sm3+ system. For the phosphate system simultaneously containing europiumand samarium, CaxSmyEuzZr2(PO4)3 (x, y) = (0.067, 0.2); (0.133, 0.1); (0.167, 0.05); (0.2, 0), photolumi-nescence spectra were registered at λexcit = 400 nm (Fig. 14), which corresponds to the most sensitive tran-sition of Sm3+, 6H5/2 → 4K11/2, and also the wavelength of an UV excitation source InxGa1 – xN [8]. Theobserved emission bands were characteristic for these cations, λem. = 400–700 nm (Eu2+) and λem = 550–670 nm (Sm3+), and were also due to 4f–4f transitions (Fig. 14). The best color coordinates (close to whitelight) were obtained for the Ca0.2Sm0.133Eu0.1Zr2(PO4)3 phosphate at λexcit = 400 nm. They were {0.27,0.34} [217].

The data on luminescent properties for phosphates with NZP structure, known from the recent revieware presented in Table 4. Among these phosphates, special attention should be paid to the seriesB0.5Zr2(PO4)3:Eu2+ (B = Ca, Sr, Ba) [219]. At the isomorphous transition Ca → Sr → Ba, a shift of emis-sion maxima to the left was observed, which was also due to the influence of the crystal environment onthe activator cation.

5.2. Phosphate Phosphors for Bioimaging

Erbium and ytterbium phosphates and phosphates silicates with NZP and Lb structures of the typesCa0.5 – 1.5(x + y)ErxYbyZr2(PO4)3, Ca0.75 – 1.5(x + y)ErxYbyZr2(SiO4)0.5(PO4)2.5, and K2Mg0.5 – 0.5(x + y)ErxYbyZr1.5 – 0.5(x + y)(PO4)3were studied as phosphors for bioimaging [203, 231]. Erbium and ytterbium in the composition of thestudied compounds were present in different concentrations and ratios.

The spectra of compounds of both structural families exhibited two emission regions, in the visible andnear infrared (NIR) spectral regions on excitation with an IR source, λexcit = 977 nm.

Luminescence spectra were identical for all compounds. As an example, Fig. 15 presents spectra forCa0.5 – 1.5(x + y)ErxYbyZr2(PO4)3, 0.02 ≤ x + y ≤ 0.33; x : y = 1 : 1 (NZP) and K2Mg0.5 – 0.5(x + y)ErxYbyZr1.5 – 0.5(x + y)(PO4)3,0.25 ≤ x + y ≤ 1.0, x : y = 1 : 9 (Lb) phosphates.

The spectra in the visible region (Fig. 15) demonstrated two bands at the wavelength apprx. 525 andapprx. 625 nm. The first band had a higher intensity. The positions and shapes of these bands differedfrom the known ones for the case of anticipated upconversion of Er3+.

Fig. 12. Dependences of (a) relative intensity and (b) emission wavelengths on composition x for phosphates of the seriesCa0.5 – xEuxZr2(PO4)3.

0.25

0.50

0.75

1.00

I rel

λ em

, nm

0.1 0.2 0.3 0.4 0.5x x

0

500

450

550

600

0.1 0.2 0.3 0.4 0.50

(a) (b)

g1

g1

g2 g2

22


KANUNOV, ORLOVA

For the Er0.165Yb0.165Zr2(PO4)3 phosphate, a shift of emission maximum to the region of shorter waves wasobserved (Fig. 15), with was also due to a morphotropic transition in the series Ca0.5 – 1.5(x + y)ErxYbyZr2(PO4)3(similarly to the cases described above).

The highest emission intensity was observed for compounds of the composition NaZr2(PO4)3 at x : y =1 : 4.

The spectra in the NIR region (Fig. 15) exhibited a luminescence band at 1550 nm, characteristic forEr3+ and corresponding to the transition 4I13/2 → 4I15/2, and a set of narrow bands due to the Stark splittingof the 4I15/2 level in this transition. Lanthanide concentration x + y affected luminescence intensity; how-ever, no monotonous dependence was observed.

The highest intensity was obtained for phosphates of the series Ca0.5 – 1.5(x + y)ErxYbyZr2(PO4)3 at x : y =1 : 10 for all x + y values.

Fig. 13. Luminescence excitation and luminescence spectra of phosphates (1) Ca0.4Mn0.1Zr2(PO4)3, (2)Ca0.2Mn0.2Eu0.1Zr2(PO4)3, and (3) Ca0.3Eu0.2Zr2(PO4)3.

300 350 400 450 500 550 600 650 700λ, nm

250

3

I

2

1

1

2

3

Excitationλ = 350 nm

Emissionλ = 475 nm

Fig. 14. Photoluminescence of CaxSmyEuzZr2(PO4)3 phosphates; x, y, z = (1) 0.2, 0.2, 0; (2) 0.2, 0.167, 0.05; (3) 0.2,0.133, 0.1; (4) 0.2, 0.067, 0.2; (5) 0.3, 0, 0.2.

300 350 400 450 500 550 600 650 700λ, nm

250

IExcitation

Emission

12345

4 G5/

2 →

6 H5/

2

4 G5/

2 →

6 H7/

2

4 G5/

2 →

6 H9/

2



The time of luminescence decay was ~3 ms (for λem = 1550 nm) and decreased with an increase in thetotal concentration of Er3+ and Yb3+ in all samples. No correlation was observed between the intensity anddecay time and the growth of the value of x + y. This means that concentration quenching does not playan important role in luminescence relaxation mechanisms for the studied materials.

As a whole, obtained data on the luminescence of the studied phosphates of the types Ca0.5 – 1.5(x + y)ErxYbyZr2(PO4)3and K2Mg0.5 – 0.5(x + y)ErxYbyZr1.5 – 0.5(x + y)(PO4)3 and phosphate silicate of the type Ca0.75 – 1.5(x + y)ErxYbyZr2(PO4)2.5(SiO4)0.5suggest that they are characterized by luminescence in the visible spectrum region with intensity sufficientfor detection by the naked eye, on excitation with a source of IR radiation (in the region of “therapeutictransparency window”). Therefore, the studied substances meet the requirements to materials for bioim-aging by their optical properties.

5.3. Phosphate Phosphors for X-PDT

To activate a number of the known photosensitizers “KillerRed®”, 610 nm; “Gematoporfirin®”,620 ± 5 nm; “Foskan®”, 652 ± 5 nm; “Fotoditazin®”, 662 ± 5 nm; and “Fotosens®”, 670 ± 5 nm inthe region of their “red” absorption bands, we investigated phosphors on the basis of phosphates contain-ing Eu3+ and Sm3+ cations [181, 232].

The test samples were phosphates forming the series Na1 – 3xEuxZr2(PO4)3, x = 0.001, 0.01, 0.05,0.1 (NZP structure) (described above and investigated as phosphors on excitation with an UV source) andK2 – xSmxMg0.5 + xZr1.5 – x(PO4)3, x = 0.25, 0.5 (Lb structure).

For europium-containing phosphates of the series Na1 – 3xEuxZr2(PO4)3, 0.001 ≤ x ≤ 0.1, we observedemission in the region 575–700 nm on excitation with Eexcit = 40–50 keV (Fig. 16a). The positions of emis-sion bands remained at the same wave numbers as on excitation with an UV source. These spectra werecharacterized by the presence of only bands corresponding to 4f–4f intraconfiguration transitions of theEu3+ ion.

For Sm-containing phosphates, a series of characteristic bands was observed in the region λem. = 550–675 nm (Fig. 16b).

It is clear that the observed emission was in the region of absorption of the known photosensitizers,which is a necessary condition for the initiation of their photochemical transformations [181, 232].

In general, the obtained data on the luminescence of Na1 – 3xEuxZr2(PO4)3 (x = 0.001, 0.01, 0.05, 0.1)and K2 – xSmxMg0.5 + xZr1.5 – x(PO4)3 (x = 0.25, 0.5) phosphates suggest that they are characterized by emis-sion in the visible spectral region on excitation with an UV source. It was also found for the first time thatthe simultaneous presence of lanthanide Eu2 + and Sm3+ cations in the Ca0.5Zr2(PO4)3 matrix ensures

Table 4. Positions of maxima (or range) of luminescence bands in the visible region and spectra of luminescenceexcitation for the known phosphate phosphors with the structure of NZP

Phosphor Space groupPositions of band maxima, nm

Referenceexcitation emission

Na3Al2(PO4)3:Eu3+

Na3Al2(PO4)3:Mn2+R c 243

288593; 615

515 [224]

Mn0.5Zr2(PO4)3 R 400 550−615 [225]

Eu0.5Zr2(PO4)3 R3 254 460 [226]

Ca0.5Zr2(PO4)3:Eu2+

Sr0.5Zr2(PO4)3:Eu2+

Ba0.5Zr2(PO4)3:Eu2+

R 350341335

484459435

[219]

CaZr4(PO4)6:Eu2+ R 305 485 [227]

Ca0.5Fe1 – xEuxSb(PO4)3:Eu3+ R 395 590−650 [228]

La1/6Pb1/3Zr2(PO4)17/6(SiO4)1/6 R 254 580−625 [229]

Eu0.33Zr2(PO4)3 P с1 360; 393 611 [230]

3

3

3

3

3

3

3

24


KANUNOV, ORLOVA

emission close to white light by color coordinates; Er3+ and Yb3+ cations in Ca0.5 – 1.5(x + y)ErxYbyZr2(PO4)3and Ca0.75 – 1.5(x + y)ErxYbyZr(PO4)2.5(SiO4)0.5 favor emission in the green region at λ = 525 nm with anacceptable intensity on excitation with an IR source.

Ca0.2EuxSmyZr2(PO4)3 phosphates bearing cations being sources of “red” emission (Eu3+, Sm3+) pos-sess emission in the region 550–700 nm (Eexcit = 40–50 keV), necessary for the initiation of photochemicaltransformations of the known photosensitizers on excitation with an X-ray source.

Therefore, the studied substances meet the requirements imposed on phosphor materials for LEDtechnologies, bioimaging, and X-PDT by their optical properties.

The regulation of optical properties in some of the studied systems was attained by changing the influ-ence of the crystal field force of the ligand on changing the coordination environment of the emitting ion.

As was shown in Sections 4 and 5, an increase in the concentration of lanthanide ions in NZP phos-phates led to morphotropic transitions, which manifested themselves in luminescence spectra and werespectroscopic confirmations of crystal lattice distortions.

Therefore, we can believe with confidence that the experimental results presented in this review willfavor a success in the preparation luminescent materials with optimum and controlled optical character-istics.

Fig. 15. Luminescence spectra in the visible and NIR ranges of phosphates of the series: (a), Ca0.5 – 1.5(x + y)ErxYbyZr2(PO4)3,x : y = 1 : 1, x + y = (1) 0.02, (2) 0.1, (3) 0.2, (4) 0.33; (b) K2Mg0.5 – 0.5(x + y)ErxYbyZr1.5 – 0.5(x + y)(PO4)3, x : y = 1 : 9;x + y = (5) 0.25, (6) 0.5, (7) 0.75, (8) 1.0.

λ, nm

λ, nm λ, nm

λ, nm

I

1234

500 550 600 650 700 1400 1450 1500 1550 1600 1650450

3

I

2

5678

(b)

500 550 600 650 700 1400 1450 1500 1550 1600 1650450

I (a)



6. OTHER PROPERTIES: BEHAVIOR ON HEATING,CHEMICAL STABILITY, BIOCOMPATIBILITY

In the development of the crystal chemistry concept of the formation of crystalline phosphor materials,a

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