Optical Materials 34 (2012) 1477–1481

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Optical Materials

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Role of charge transfer state and host matrix in Eu3+-doped alkali and earth alkalifluoro-aluminoborate glasses

N.T. Thanh a, V.X. Quang a, V.P. Tuyen a, N.V. Tam b, T. Hayakawa c, B.T. Huy b,d,⇑a Institute of Materials Science, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet, Hanoi, Viet Namb Nha Trang Institute of Technology Research and Application, 2 Hung Vuong, Nhatrang, Viet Namc Department of Materials Science and Engineering, Nagoya Institute of Technology Showa-ku, Nagoya 466-8555, Japand Department of Chemistry, Changwon National University, Changwon 641-773, South Korea

a r t i c l e i n f o a b s t r a c t

Article history:Received 9 December 2011Received in revised form 16 February 2012Accepted 5 March 2012Available online 28 March 2012

Keywords:PSHBCTSFluoroborate glassJudd–Ofelt

0925-3467/$ - see front matter Crown Copyright � 2http://dx.doi.org/10.1016/j.optmat.2012.03.006

⇑ Corresponding author at: Nha Trang Institute oApplication, 2 Hung Vuong, Nhatrang, Viet Nam.

E-mail address: buithehuy.nt@gmail.com (B.T. Huy

Judd–Ofelt parameters of Eu3+-doped alkali and earth alkali fluoro-aluminoborate glasses are determinedfrom the fluorescence spectra. The dependence of these parameters on the compositional changes of thematerials is discussed. The glass containing sulphate component shows the larger values ofO compared withthe fluoro-aluminoborate glasses because of higher DCT (Eu3+–F�) values. The difference of ionic radius andelectronegativity between Na and Ca lead to higher O2 parameter of CaF2–Al2O3–B2O3:Eu3+ glass.

Persistent spectral hole burning has been observed in Eu3+ ions doped alkali-and earth alkali fluoro-alum-inoborate glasses with the compositions of 16NaF�73B2O3�8Al2O3�3Eu2O3 and 16CaF2�73B2O3�8Al2O3�3Eu2O3, which have been prepared using conventional melting technique.

Crown Copyright � 2012 Published by Elsevier B.V. All rights reserved.

1. Introduction

The Judd–Ofelt (JO) theory was shown to be useful to character-ize radiative transitions for RE-doped solids, as well as aqueoussolutions, and to estimate the transitions intensity for rare earthions [1–5]. The theory defines set of three intensity parameters,Ok (k = 2,4,6), that are sensitive to the local environment of therare-earth ions. These parameters can be determined experimen-tally from the measurements of the optical absorption spectra,emission spectra and the refractive index of the matrices. The JOparameters have been successfully used in estimating the sponta-neous radiative transition probability for the emission, the radiativelifetime of the excited states, the branching ratios and used furtherto examine the spectroscopic parameters of the host materials, thestructural properties of RE-ion sites in the host. Therefore under-standing the compositional dependence of Judd–Ofelt parametersof RE-ions in glass is very useful for designing the optical materialswith the desired properties. Of course, in many cases the localstructures and the site selectivity of ions cannot be predicted inthe multi-component glasses. However, the Judd–Ofelt analysiswas found to be a powerful tool for the systematic investigationsand selection of glasses with specific purposes.

Among different glass host matrices, fluoro-aluminoborateshave distinct advantages over the other glasses. The interest in

012 Published by Elsevier B.V. All

f Technology Research and

).

these host matrices is due to their high ionic conductivity, hightransparency, low melting point, high thermal stability and goodrare-earth ions solubility [6]. Recently, some optical studies on rareearth ions in alkali and earth-alkali fluoro-aluminoborate glassesRF–Al2O3–B2O3 (R = alkali or earth-alkali) have been published[6,7]. The comparative analyzing for important spectroscopic prop-erties of some reported glass matrices in [8] shows that the quan-tum efficiency and stimulated emission cross section of someglasses could be enhanced by adding some more sulphate contentin the components of some glasses. The aim of the present work isto characterize and to compare the spectroscopic properties of Eu3+

ion in the alkali and earth-alkali fluoro-aluminoborate glass matrixwith and without sulphate group.

It is well-known that trivalent europium Eu3+ ion is the mostsuitable candidate to be used as a optical probe to investigate thelocal structure around Eu3+ ion in a condensed matter. The low lyingenergy level scheme of Eu3+ is simple. The non-degenerate levels ofelectronic transition between 7Fo and 5Do is convenient to apply thefluorescence line narrowing (FLN) and persistent spectral holeburning (PSHB) techniques to analyze changes from site to sitethrough energy level scheme analysis [9–11]. In this paper, persis-tent spectral hole burning PSHB of Eu3+ in alkali and earth-alkali flu-oro-aluminoborate glass matrices are also observed. Luminescenceproperties of rare-earth (RE)-doped inorganic materials have beenextensively studied for their unique optical characteristicsincluding their frequency high resolution spectra (PSHB and FLNphenomena) [11–16]. PSHB phenomenon is interesting not only

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1478 N.T. Thanh et al. / Optical Materials 34 (2012) 1477–1481

as a tool for high-resolution spectroscopy but also application pros-pect in optical memories technology [17]. The PSHB phenomenonhas been extensively studied in various materials because of its po-tential application to high-density optical storage using frequency-domain recording. Many materials doped rare earth ions display thePSHB properties [11,15,16,18]. However, most of them exhibit holeburning effect at low temperature. The key requirement for suc-cessful application of PSHB technique is the fabrication of holeburning materials with high operation temperature [19]. In recentyears, PSHB of Eu3+ and Sm3+ ions-doped glasses has been observedat room temperature [20–23]. To the best of our knowledge, only afew of reports on the PSHB of Eu3+ ions in alkali fluoro-aluminob-orate glasses has been published [7,24]. According to these papers,the photoreduction of Eu3+ ions plays the dominant role for theholes formation and the released holes are subsequently trappedby the neighboring Eu2+, which is produced during the preparationprocess in the reducing atmosphere. In these experiments, the PSHBis formed by the photo-chemical reaction of these RE-ions under la-ser irradiation.

The aim of this study is to observe the new mechanism of PSHBwhich is formed by the photo-physical reaction. In ourexperiments, the glasses having the same composition but wereprepared in the air and irradiated by X-rays, it becomes possibleto extend the study beyond the limitation of Eu2+ ion-dopedglasses. Instead of this, the Eu3+ ions, the defect centers and theirinteraction play the more important role in the PSBH formation.

2. Experimental

The 1% wt Eu3+ ions doped 16NaF�73B2O3�8Al2O3, 16CaF2�73B2

O3�8Al2O3, and 20CaF2�54B2O3�15CaSO4�10Al2O3 glasses were pre-pared by melting method. The raw materials used were B2O3,Al2O3, CaF2, NaF, CaSO4 with high purity and Eu2O3 (Sigma,99.9%). First, the mixture of raw of each glass was melted using agraphite crucible at 1150 �C for 1 h, and rapidly cooled down toroom temperature (RT). After that, the obtained glasses were an-nealed at 350 �C for 5 h, which have transparent and light yellowin color. The glass was cut and polished to a thickness of1.00 mm to measure its optical properties.

Fluorescence spectra were recorded using JOBINYVON-SPEXFluorolog-3 spectrometer under excitation at 395 nm. In order toinvestigate the hole burning effect, the samples were exposed withX-ray (Cu Ka 30 kV-5 mA) for 24 h. PSHB experiments were per-formed at low temperature from 7 K up to 65 K as followed: (i)the irradiated samples were burned on absorption band of7F0–5D0 transition by 17,288 cm�1; 17,362 cm�1 of a Rhodamine-6G dye laser, 1 cm�1 FWHM with 300 mW for 1 h. (ii) the excita-tion spectra were recorded by scanning a laser from 17,500 to17,200 cm�1 of the 7F0–5D0 transition while monitoring at613 nm-emission of the 5D0–7F2 transition of the Eu3+ ion. The la-ser power for exciting is about 0.3% of that for burning.

Fig. 1. Fluorescence spectra of 1 wt.% Eu3+-doped (a) 16NaF�73B2O3�8Al2O3, (b)16CaF2�73B2O3�8Al2O3, and (c) 20CaF2�54B2O3�15CaSO4�10Al2O3 glasses.

3. Results and discussion

3.1. Judd–Ofelt analysis

In the present work, the phenomenological Judd–Ofelt (JO)parameters, Xk (k = 2,4,6) [7,25] have been calculated from thefluorescence spectra (5D0 ?

7F2,4,6) of Eu3+ ion-doped RF–Al2O3–B2O3 (R = Na, Ca) glasses with and without Suphate component.

3.1.1. Fluorescence spectraThe fluorescence spectra of 1 wt.% Eu3+-doped Eu3+ ion-doped

RF–Al2O3–B2O3 (R = Na, Ca) glasses has been measured at RT.Fig. 1 shows the fluorescence spectra of Eu3+-doped 16NaF�

73B2O3�8Al2O3, 16CaF2�73B2O3�8Al2O3, and 20CaF2�54B2O3�15CaSO4�10Al2O3 glasses. As seen from Fig. 1, it is clear that the emissionbands, 579, 591, 614, 650 and 698 nm, due to the f–f transitions5D0 ?

7FJ (J = 0,1,2,3,4) of the Eu3+ ion appeared in the spectra.,respectively. The intensities of all the emission bands in the glasscontaining sulphate component were strongest. The 5D0 ?

7F2 tran-sition is induced by electric dipole allowed and strongly depend onthe local symmetry surround Eu3+ ions, whereas the 5D0 ?

7F1 tran-sition is magnetic dipole allowed and independent to the local sym-metry. Therefore, the integrated radiative intensity ratio R, of the5D0 ?

7F2 transition to that of the 5D0 ?7F1 transition is the spec-

troscopic key to estimate the deviation from the site symmetry ofEu3+ ions. Sometime, R is defined as asymmetry ratio. In our work,the R values are at about of 3.2–3.5, these results correspond to Ku-mar’s group [26].

3.1.2. Judd–Ofelt parametersCommonly, Judd–Ofelt (JO) intensity parameters are usually de-

rived from absorption spectrum. However, owing to the special en-ergy level structure of Eu3+ ion, these JO parameters could beestimated from the fluorescence spectra. Three main emissionpeaks 5D0 ?

7F1, 5D0 ?7F2 and 5D0 ?

7F4 are used to calculate JOparameters. In this study, all parameters were calculated usingJudd- Ofelt theory base on the fluorescence spectra [1,3,7,27].

Magnetic-dipole spontaneous radiative transition probabilityAmd are evaluated from the following expression

Amd ¼64p4m3n3Smd

3hð2J þ 1Þ ; ð1Þ

where h is the Planck constant, m is the wave number of the radia-tive transition, J is the total angular momentum of the excited state,and n is the refractive index.

Smd is the magnetic-dipole line strength, which is a constant andindependent from the host matrix

Smd ¼e2�h2

4m2c2 jhðS; LÞJkLþ 2SkðS0; L0ÞJ0ij2; ð2Þ

where S is a spin operator and L is an angular moment operator.Amd can be estimated by using the reference value of A0md pub-

lished somewhere, and using the relationship Amd ¼ ðn=n0Þ3A0md,where A0md and n’ are spontaneous radiative transition probabilityand refractive index of the reference material. In our calculations,the A0md ¼ 51:9 s�1; n0 ¼ 1:539 values of 1 mol% Eu3+-doped lithiumfluoroborate glass of Kumar’s group [27] was used.

Table 2Holes-depth and hole-width dependence on temperatures.

T (K) Hole depth (%) Hole width (cm�1)

16CaF2�73B2O3�8Al2O3:Eu3+

7 9.4 1.922 6.0 2.435 5.7 2.945 4.0 3.055 3.7 3.565 3.0 3.875 – –

16NaF�73B2O3�8Al2O3:Eu3+

7 12.8 1.823 10.7 1.940 8.2 2.248 4.4 2.353 2.8 2.566 2.1 3.675 – –

N.T. Thanh et al. / Optical Materials 34 (2012) 1477–1481 1479

The 5D0 ?7FJ=2,4,6 transitions are electric dipole partially al-

lowed and their spontaneous radiative transition probabilities,Aed, are evaluated using expression as follow:

Aed ¼64p4m3

J

3hð2J þ 1Þnðn2 þ 2Þ2

9

" #Sed; ð3Þ

where mJ is the wave number of 5D0 ? 7FJ transitions, e is the elec-tron charge, J is the total angle momentum of excited state. Sed is theelectric dipole line strength of the transition and given by

Sed ¼ e2 Pk¼2;4;6

XkjhWJjUkjW0J0ij2; ð4Þ

where jhWJjUkjW0J0ij is called the reduced matrix element for a tran-sition WJj i ! W0J0

�� �. jhWJjUkjW0J0ij2 � kUkk2 is the square of the ma-

trix elements of the tensor operator, which connects <wJ| to finalstate <w0J0| and is considered to be independent of a host matrix.The value of kUkk2 for every transition of trivalent lanthanide ions(Ln3+) could be found in the published tables [26,28].

The emission intensity I of a given transition is proportional tothe area S under the emission curve:

I ¼ hmArNaS; ð5Þ

where hm is transition energy, Ar is radiative transition rate, N ispopulation of emitting level. Thus the Xk parameters could be eval-uated simply by the ratio of the radiatve intensity of the5D0 ?

7FJ=2,4,6 transitions to the intensity of 5D0?7F1 transition as

follow:RIJ dmRIl dm¼ AJ

Al¼ e2

Smdl

� �mJ

mL

� �3 nðn2 þ 2Þ2

9n3

" # Pk¼2;4;6

XkkUkk2; ð6Þ

where Smd1 refers to the magnetic dipole strength of the 5D0?7F1

transition, which can be estimated from Amd1 = 52 s�1 for the pres-ent samples, n = 1.54, the ratios Aed2/Amd1 and Aed4/Amd1 have beenevaluated from the emission curve. As seen from Eq. (6), the ratiosAJ/A1 depend on the kUkk2 value between |WJi and jW0J0i manifold.Due to the selection rules and the unique nature of transition inten-sities for Eu3+ ion, only one of the kUkk2 parameter decides theintensities of the transitions, while the remaining two parametersare zero. For example, the X2 and X4 can be evaluated indepen-dently from the emission transitions of 5D0 ?

7F2 and 5D0 ? 7F4.The parameter X6 cannot be evaluated because of the limitationof our experimental condition. This method has been used to eval-uate the Xk parameters from individual transitions without least-squares fit for Eu3+ in glass. The results of calculated X2 and X4

parameters are presented in Table 2.Because the value of the corresponding reduced matrix kU6k2 is

smaller than kU2k2, kU4k2 at an order of magnitude [26], the5D0 ?

7F6 emission intensity is usually very weak compared withthe intensities of the 5D0 ?

7F2 and 5D0 ?7F4 transitions.

It is well-known that the Xk parameters, especially the struc-ture-sensitive X2 parameter, depend on the covalence and asym-metry of the Ln3+-site. According to the theoreticalconsiderations, Xk parameters are given by [7,25].

Xk ¼ ð2t þ 1ÞPs;pjAs;pj2N2ðs; tÞð2sþ 1Þ�1ðt ¼ 2;4;6Þ; ð7Þ

where As,p are the set of odd-parity terms of the crystal fields, re-lated to the structure around rare earth ions and N(s, t) represents

Table 1Judd–Ofelt parameters Xk of present glass samples.

Glass composition X2 (�10�20 cm2)

16CaF2�73B2O3�8Al2O�3Eu2O3 5.0716NaF�73B2O3�8Al2O�3Eu2O3 3.4020CaF2�54B2O3�15CaSO4�10Al2O�1Eu2O3 5.33

a complicated sum over all excited configurations of terms, contain-ing factors like

h4f jrjnlihnljrsj4f i=DEðnlÞ; ð8Þ

where DE(nl) are the energy difference and hnl|rs|4fi are the radialintegrals between the 4fN and the excited 4fN�1nl1 configurations,s = k ± 1. The most important contribution to the mixing of wavefunctions with odd-parity are the 5d orbital h4f|rs|5di [29].

Taking these theoretical results into consideration of the phys-ical meaning of the Xk parameters, it has been found that X6 ismore affected by changes in the radial integral h4f|rs|5di than X2

and X4. Therefore, it is more sensitive to changes in the electrondensity of the 4f and 5d orbital. Otherwise, X2 is more affectedby the asymmetry of the crystal fields and by changes of the energydifference between the 4fN and 4fN�15d1 configuration. This energydifference generally decreases with increasing covalence betweenLn3+ ions and ligand.

3.1.3. The role of the anionsThe lowest energy of the excited 4f55d configuration of Eu3+ is

approximately 70,000 cm�1 higher than the ground level of the4f6 configuration. Specially, among the trivalent lanthanide serial,Eu3+ and Yb3+ have relative large optical electronegativity thusmuch lower charge transfer state (CTS) due to the higher electronaffinity of f6 and f13 electronic configurations. Therefore, the CTS ofEu3+ and Yb3+ are much lower than the lowest energy level of 4f55dconfiguration. Jorgensen et al. [30] have shown that, the CTS posi-tion relative to the 4f6 ground state can be empirically estimatedby

DCT ¼ ½aðXÞ � aðEu3þÞ� � 3:104cm�1 ð9Þ

where DCT is energy position of CTS, a(Eu3+) and a(X) are opticalelectronegativity of Eu3+ ion and the ligand anion, respectively.The a values are given in the complete electronegativity scale pub-lished by Allen and other [31,32]. The values given in the table referto the common oxidation states of the elements. For instance, the avalues have been determined to be about 1.75, 2.8, 3.2 and 3.9 forEu3+, S2�, O2� and F�, respectively. Therefore, the CTS position de-pends primarily on the nearest ligand in the host, and it may locate

X4 (�10�20 cm2) X6 (�10�20 cm2)

2.24 –3.06 –4.48 –

Fig. 3. Excitation spectra of Eu3+-doped 16NaF�73B2O3�8Al2O glass before and afterburned at different temperatures from 7 K up to 66 K.

1480 N.T. Thanh et al. / Optical Materials 34 (2012) 1477–1481

as high as 64,500 cm�1, 43,500 cm�1, 31,500 cm�1 for Eu3+–F�,Eu3+–O2�, Eu3+–S2�, respectively. Due to the much lower CTS,according to the Judd–Ofelt theory, instead of 4f65d, the CTS energylevel could be used to calculate and explain the compositionaldependence of the O parameters. Calculated results of O2,4 of theseglasses showed in Table 1. This replace could be achieved by mixingthe 4f6 configuration with the CTS having the opposite parity via theinteraction of odd crystal field component or vibration modes. Theglass containing sulphate component shows the larger values of Ocompared with the fluoro-aluminoborate glasses.

3.1.4. The role of the cationsThe higher values of O parameters in CaF2–Al2O3–B2O3:Eu3+

glass in comparison with NaF–Al2O3–B2O3:Eu3+ glass could be as-cribed to the difference of ionic radius and electronegativity be-tween Na and Ca. The values of O2 strongly depend on theasymmetry of the ligand field around the RE ions, and on the elec-tron densities of oxygen ions, which related to the electronegativ-ities of the alkali and earth-alkali ions.

3.2. PHSB properties

The excitation spectra of 7F0–5D0 transitions of Eu3+ ions doped16CaF2�73B2O3�8Al2O3 and 16NaF2�73B2O3�8Al2O3 are shown inFigs. 2 and 3, respectively. These spectra were obtained beforeand after burned by 17,362 cm�1 for 16CaF2�73B2O3�8Al2O3, and17,288 cm�1 for 16NaF2�73B2O3�8Al2O3 glasses at 7 K up to 75 K.The holes were clearly observed on excitation spectra and disap-peared when the temperature is at 75 K. The excitation curve of7F0–5D0 before hole-burning was fitted by a Gaussian functionand after hole-burning fitted by difference a Gaussian andLorentzian function, which is against theoretical consideration thatthe homogeneous spectrum (hole) is a Lorentzian shape [33].The hole-depth is defined as the intensity ratio to the total fluores-cence intensity and is normalized to the intensity before burning atthe burning wavenumber. Results of the hole-width at half-maxi-mum and hole-depth depending on temperature are given in Table2, in which the hole-depth decreased while the hole-width in-creased when increasing of temperature.

Recently, under investigation into the optical hole properties ofeuropium-doped calcium bromide-based aluminoborate glasses,H. Liang et al. [24] suggested that the self-reduction from Eu3+ toEu2+ ions partially took place during the process of glass formation.

Fig. 2. Excitation spectra of Eu3+-doped 16CaF2�73B2O3�8Al2O glass before and afterburned at different temperatures from 7 K up to 75 K.

The existence of considerable Eu2+ ions self-reduced in the prepa-ration process plays role as the hole-trappers. In the hole burningprocess, the Eu3+ ions are selectively excited to the Do level andare transformed to Eu2+ one by interacting with the local environ-ment. Therefore, the number of Eu3+ ions that were previously ab-sorbed by laser light decrease after hole-burning and then a holedip are observed in the excitation spectrum.

Although our samples have nearly similar components to thatof the Liang’s samples, the existence of the Eu2+ ions cannot beobserved (by fluorescence measurement) even with irradiatedX-ray samples. Therefore, according to our opinion, the suggestedmechanisms of Liang’s et al. [24] play not the dominated role inthe PSHB process in this case. This PSHB process could be ex-plained reasonably by the mechanism suggested by Park et al.[34]. According this suggestion, in our samples the role of thestructural disorder defects is more important than that of theEu2+ ions. These disorder defects associated with Al3+ site, whichoriginated from the existence of Eu3+ acting as the network mod-ifier ion in the glass. The positive charge of Eu3+ ions is compen-sated by the non-bridging oxygen bonded to both B3+ and Al3+

ions, in which the Al3+ ions form AlO6. The hole burning was pre-sumably related to the electron transfer between the Eu3+ andthis defect.

4. Conclusion

The luminescence spectra of Eu3+ doped RF–Al2O3–B2O3 (R = Na,Ca) and Eu3+ doped CaF2–Al2O3–B2O3–CaSO4 glasses were mea-sured and the JO parameters O2,4 were determined. The glass con-taining sulphate component shows the larger values of Ocompared to the fluoro-aluminoborate glasses. The reason can beexplained with DCT values, which depends on the nearest ligand.The O2 parameter of CaF2–Al2O3–B2O3:Eu3+ glass is higher than thatof NaF–Al2O3–B2O3:Eu3+ glass because the difference of ionic radiusand electronegativity between Na and Ca. We have analyzed thecharge transfer state (CTS) of Eu3+ ion in these glasses to explainthe compositional dependence of the J.O parameters of studiedsamples.

Eu3+ doped RF–Al2O3–B2O3 (R = Na, Ca) glasses were irradiatedwith X-rays before hole burning process, the holes could beburned and observed up to 65 K. It is suggested that the holes wereformed by the electron transfer between the Eu3+ ions and the acti-vated Eu3+ ions or the defect centers.

N.T. Thanh et al. / Optical Materials 34 (2012) 1477–1481 1481

Acknowledgment

The authors gratefully acknowledge support for this researchfrom VietNam’s National Foundation and for Science and Technol-ogy Development (NAFOSTED) with project code: 103.06-2011.58and from Priority Research Centers Program through the NationalResearch Foundation of Korea fund (NRF 2010-0029634).

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