Luminescence of Ce3+ in Different Lattice Sites of La2CaB10O19

Lan Li,† Hongbin Liang,*,† Zifeng Tian,† Huihong Lin,† Qiang Su,*,† and Guobin Zhang‡

MOE Laboratory of Bioinorganic and Synthetic Chemistry, State Key Laboratory of Optoelectronic Materialsand Technologies, School of Chemistry and Chemical Engineering, Sun Yat-sen UniVersity, Guangzhou 510275,People’s Republic of China, and National Synchrotron Radiation Laboratory, UniVersity of Science andTechnology of China, Hefei 230026, People’s Republic of China

ReceiVed: May 11, 2008; ReVised Manuscript ReceiVed: June 25, 2008

A series of samples with nominal chemical formulas La2-xCexCaB10O19 and La2Ca1-2xCexNaxB10O19 wereprepared by a solid state reaction route at high temperature. Their luminescence properties were investigatedby the steady state excitation and emission spectra in the VUV-vis range, the luminescence decays, and thetime-resolved emission spectra (TRES). The results demonstrate that Ce3+ ions occupy two lattice sites in allsamples. The lowest 5d absorption bands for two sites are at about 272 (La3+ site) and 312 nm (Ca2+ site),respectively. The emission for Ce3+ in the La3+ site shows a shorter decay time of 12 ns, and the doubletemission bands have maxima at about 291 and 310 nm. The emission for the Ce3+ in Ca2+ site has a longerlifetime of 26 ns with band maxima at about 329 and 355 nm. Efficient energy transfer between both sitesoccurs in the samples.

1. Introduction

Luminescence of Ce3+ in complex oxides has demonstratedits importance for lighting, display, and scintillation applications.Three well-known examples worth being mentioned will includethe following: Y3Al5O12:Ce3+ as a yellow-emitting componentin InGaN/GaN-based LEDs (light emitting diodes);1 the potentialapplication of Y2SiO5:Ce3+ as a blue-emitting material in FEDs(field emission displays);2 and Lu2SiO5:Ce3+ as a commerciallyavailable scintillator in medical imaging detectors for PET(positron emission tomography) systems.3

On the other hand, growing interest is focused on the 4f-5dtransitions of Ce3+ in various hosts due to the importance forbasic research. The investigation on the 4f-5d transitions ofCe3+ can provide key information on the 5d energies for otherlanthanide ions. The 5d states are outer orbital, and thecoordination around a lanthanide ion has remarkable influenceon their energies. As a result the 4f-5d transitions appear in awavelength range that depends strongly on both the kind oflanthanide ion and the host lattice. The 5d centroid, the lowest5d level, and the 5d crystal field splitting for other lanthanideions in same lattice site can be evaluated by means of theexcitation spectrum of Ce3+.4-8 In addition, with the help ofthe emission spectrum, the number of lattice sites for Ce3+ ina specific host can be understood, because Ce3+ in one definitelattice site often exhibits doublet 5d-2FJ (J ) 5/2, 7/2) emissionbands with the energy separation about 2000 cm-1. When thesite occupancy for Ce3+ in a definite host is known, that forother lanthanide ions in the same host lattice might be estimated,as trivalent lanthanide ions are with similar ionic radii.

The compound La2CaB10O19 is chemically stable and nothygroscopic, which has attracted considerable interest as apotential material for nonlinear optical (NLO) applicationsbecause it exhibits an optical second-harmonic generation (SHG)effect about twice as large as that of KDP (KH2PO4) and the

single crystal is easily grown.9-11 In the present paper, theVUV-vis luminescence properties of Ce3+ in La2CaB10O19 wereinvestigated. In particular, the spectroscopic properties in relationto the different lattice site occupancies were reported in detailby steady-state spectra at different temperature, luminescencedecay and time-resolved emission spectra at room temperature(RT).

2. Experimental Section

A series of polycrystalline samples with two types of nominalchemical formulas La2-xCexCaB10O19 and La2Ca1-2xCex-

NaxB10O19 (x ) 0.005, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07,0.08, 0.09, 0.10) were prepared by a solid-state reaction routeat high temperature by using the following chemical reactions.

* Authors to whom all correspondence should be addressed. Phone: 86-20-84111038. Fax: 86-20-84111038. E-mail: cesbin@mail.sysu.edu.cn.

† Sun Yat-sen University.‡ University of Science and Technology of China.

Figure 1. XRD patterns of samples La2-xCexCaB10O19 andLa2Ca1-2xCexNaxB10O19.

J. Phys. Chem. C 2008, 112, 13763–13768 13763

10.1021/jp804149k CCC: $40.75 2008 American Chemical SocietyPublished on Web 08/07/2008

(2- x)La2O3 + 2xCeO2 + 2CaCO3 + 20H3BO3 +

xCO98723 K/10 h981273 K/48 h

2La2-xCexCaB10O19 +

30H2O + (2+ x)CO2 (1)

2La2O3 + 2(1- 2x)CaCO3 + 2xCeO2 + xNa2CO3 +

20H3BO3 + xCO98723 K/10 h981273 K/48 h

2La2Ca1-2xCexNaxB10O19 + 30H2O + 2(1-x)CO2 (2)

The reactants include analytical grade pure CaCO3, Na2CO3,H3BO3 (excess 3 mol % to compensate for the evaporation lose)and 99.95% pure lanthanide oxides La2O3 and CeO2. Forreaction 2, Na2CO3 was added as a charge compensator becausethe substitution of a Ce3+ ion for a Ca2+ ion requires a chargecompensator to maintain overall charge neutrality of the samplesLa2Ca1-2xCexNaxB10O19. The stoichiometric mixtures were firstwell ground in an agate mortar and prefired at 723 K in airatmosphere for 10 h, and then calcined at 1273 K under COreducing ambience for 48 h.

The structure of the final products was examined by powderX-ray diffraction (PXRD), using Cu KR radiation (λ ) 1.5046Å) on a BRUKER D8 ADVANCE type powder X-raydiffractometer.

The steady-state UV excitation and corresponding emissionspectra, the luminescence decay curves, as well as the time-resolved emission spectra (TRES) were measured on an Edin-burgh FLS 920 combined fluorescence lifetime and steady statespectrometer, which was equipped with a CTI-Cryogenicstemperature controlling system. At measurements, a Xe 900lamp (450 W) was used as the excitation source for the steady-state spectra, while an nF900 ns flash lamp (150 W, with a pulsewidth of 1 ns and pulse repetition rate of 40-100 kHz) wasused as the excitation source for the luminescence decay curvesand the TRES. The time-resolved emission spectra wereconstructed by measurements of numerous decay curves andthese data were converted with TRES software attached to theinstrument.

The VUV excitation and corresponding emission spectra weremeasured at the time-resolved spectroscopy experimental stationof the National Synchrotron Radiation Laboratory (NSRL, Hefei,China) under normal operating conditions. The measurementdetails can be found elsewhere.12

3. Results and Discussion

3.1. XRD Patterns. The XRD patterns of all samples thatinclude undoped sample La2CaB10O19 and doped samplesLa2-xCexCaB10O19 and La2Ca1-2xCexNaxB10O19 with differentCe3+ concentration (x value) were first measured. As examples,the patterns of samples La2-xCexCaB10O19 for x ) 0, 0.005,0.04, 0.10 and phosphors La2Ca1-2xCexNaxB10O19 for x ) 0.005,0.02, 0.04, 0.08 are shown in Figure 1. Diffractogram (a) isconsistent with that in ref 9, showing that the undoped samplehas a single phase. The other seven curves are in agreementwith curve (a), indicating that they are of the iso-structure withthe undoped sample, and the dopant Ce3+ and Na+ ions do notshow evident influence on the position of the diffraction lines.However, the diffraction peak intensities in XRD patternsslightly decrease with the increase of the doping concentration,suggesting that the crystallinity (degree of crystallization) seemsto decrease with the increase of the doping concentration. Thisis probably related to the decreasing of the melting point forsolid solutions La2-xCexCaB10O19 and La2Ca1-2xCexNaxB10O19

with the increase of the x value.3.2. Emission Spectra of Ce3+ in La2CaB10O19. The emis-

sion spectra for samples La2-xCexCaB10O19 (x ) 0.005, 0.01,0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.10) under 254,260, 274, and 318 nm excitation were first measured at roomtemperature, respectively. The spectra upon 254 nm excitationwere chosen as typical results and displayed in Figure 2, inwhich some characteristics were found as follows.

(1) Three evident bands A (∼298 nm), B (∼333 nm), and C(∼356 nm) appear in the emission spectra of Figure 2. In termof the spectra at lower temperature (see Figure 4), these bandsare attributable to the emission of Ce3+ and they do not relateto the host emission. In general, Ce3+ ions in a single definitelattice site (especially at lower temperature) usually present twoemission bands due to the transitions from the lowest 5d excitedstate to the 2F5/2 and 2F7/2 spin-orbit split 4f ground states. Theenergy separation of the two bands corresponds to the spin-orbitsplitting and amounts to about 2000 cm-1. Figrue 2 directlyindicates that Ce3+ ions occupy two different sites in the hostlattice. The energy difference between band B and band C isnear 2000 cm-1, so they will be unambiguously assigned tothe emission of Ce3+ in the a same lattice site, and here we callthis site II. Meanwhile, the emission band A must come fromCe3+ in another site in the host lattice, and here we mark thisas site I.

Figure 2. The normalized emission (λex ) 254 nm) spectra of samples La2-xCexCaB10O19 with different x values at room temperature.

13764 J. Phys. Chem. C, Vol. 112, No. 35, 2008 Li et al.

(2) To evaluate the influence of dopant concentration (x value)on the emission intensity of two different lattice sites, wenormalized all emission spectra in the wavelength range274-480 nm in Figure 2. It can be found that the relativeemission intensity from site II is always higher than that fromsite I. Furthermore, the emission from site II gradually increaseswith the increase of the x value, whereas the emission from

site I decreases regularly. We think that this may be due to twomain factors: one is the different occupancy probability betweensite I and site II, the other is the energy transfer from site I tosite II.

First, provided that the occupancy probability of site II ishigher than that of site I, the emission intensity from site IImay be higher than that from site I. When the occupancyprobability in site II gradually increases with the dopingconcentration, the emission from site II relative to that fromsite I will regularly increase.

Second, the lowest 5d absorption for Ce3+ in site II wasobserved at about 312 nm (see the band K in Figure 5), whichhas obvious overlap to the emission bands from site I in Figure2. This spectroscopic superposition will result in a Ce3+ ion insite II absorbing the emission from site I efficiently. As aconsequence, an effective resonance-type energy transfer mayoccur from site I to site II, and this energy transfer will makethe emission intensity of site I look weakened. The higher thedoping concentration is, the higher the probability that the energytransfer occurs. Accordingly the emission intensity from site Iis expected to become weaker and weaker.

These two reasons together make the emission from site IIhigher than that from site I, and we cannot affirm which one isa dominant factor for the time being; the TRES are expected togive more information on this issue, see section 3.5. In addition,there is no doubt that the concentration quenching will also showan influence on the emission intensity, but this reason does notseem to be the main factor here, as the concentration quenchingphenomenon was not observed in the doping concentration.

Figure 3. The emission (λex ) 254 nm) spectra of samplesLa2Ca1-2xCexNaxB10O19 with different x values at room temperature.

Figure 4. The emission (λex ) 258 nm) spectra of samplesLa1.995Ce0.005CaB10O19 at different temperatures.

Figure 5. VUV-UV excitation spectra of La2-xCexCaB10O19 andLa2Ca1-2xCexNaxB10O19 at room temperature.

Figure 6. The emission spectra of La2CaB10O19:Ce3+ under differentwavelength excitation.

TABLE 1: The CFS of Ce3+ 5d Levels in Some ComplexOxides

compd coord no.ave bond

distance (Å) CFS (cm-1) ref

GdB3O6 10 2.52 11700 7LaPO4 10 2.64 11900 7LaB3O6 10 2.61 12000 7YMgB5O10 10 2.47 12600 15LaP5O14 8 2.50 16600 16CeP5O14 8 2.48 17000 7LaP3O9 8 2.53 17100 7Li6Y(BO3)3 8 2.40 17200 15YBO3 8 2.37 17600 7CaSO4 8 2.47 17900 17YPO4 8 2.34 18010 18LuBO3 (vaterite) 8 2.33 18500 7GdBO3 8 2.40 18700 7LuPO4 8 2.30 19600 19

Luminescence of Ce3+ J. Phys. Chem. C, Vol. 112, No. 35, 2008 13765

(3) For Ce3+ emission in site II, the band B corresponds tothe transition from the lowest 5d state to the 2F5/2 level, and theband C corresponds to the transition from the lowest 5d to 2F7/

2. It can be found that the intensity of band C relative to that ofband B regularly increases with the increase of the dopingconcentration. Because of the partial superposition between theabsorption band K in Figure 5 and the emission band B in Figure2, this resonance energy transfer within site II will decreasethe intensity of band B. When the doping concentrationincreases, with the increase of the energy transfer probability,the relative intensity of band B will decrease regularly.

(4) The above results can also be found in another series ofsamples La2Ca1-2xCexNaxB10O19, as shown in Figure 3. Thespectral characteristics are similar for samples La2Ca1-2x-

CexNaxB10O19 and La2-xCexCaB10O19, which seems to indicatethat the nominal chemical formulas have almost no influenceon the spectral patterns. See also the excitation spectra of Figure5 in section 3.3.

To evaluate the influence of the temperature on the lumines-cence, the emission spectra at different temperatures weremeasured. The emission spectra of sample La1.995Ce0.005-

CaB10O19 in the 280-380 nm range as a function of temperatureare shown in Figure 4. At a lower temperature, four well-resolved emission bands A1 (∼291 nm), A2 (∼310 nm), B(∼329 nm), C (∼355 nm) were clearly observed. The emissionof Ce3+ in site I corresponds to bands A1 and A2, while that insite II corresponds to bands B and C. With the increase of thetemperature, the total emission intensity decreases because ofthe thermal quenching, as shown in the right curve of Figure 4(in which an exceptional datum at 40 K may be an experimentalerror).

Although the emission intensity from both sites decreases withthe increase of temperature, further scrutinizing the spectra, wefind that the decrease magnitude of the emission from site I(bands A1 and A2) is more obvious than that from site II (bandsB and C). This also may be related to the resonance-type energytransfer between site I and site II, as described before. Theenergy transfer probability increases with the increase oftemperature, so the emission intensities of bands A1 and A2show an obvious decrease.

In addition, it was observed that the bands A1 and A2combined gradually with the increase of temperature, which isdue to the increase of the electron-lattice phonon interaction.The combined band corresponds to band A in Figures 2 and 3.

3.3. Excitation Spectra of Ce3+ in La2CaB10O19. TheVUV-UV excitation spectra for samples La2-xCexCaB10O19 andLa2Ca1-2xCexNaxB10O19 with x ) 0.005, 0.04, 0.10 at roomtemperature were measured as shown in Figure 5.

The ground state of Ce3+ contains one single optically activeelectron in the well-shielded 4f shell (4f1). It can be excited tothe 5d configuration, and depending on the site symmetry, atmost five distinct 4f-5d transitions can be observed. Sevenbands were found in the spectra, which are labeled as H (∼160nm), D (∼192 nm), E (∼219 nm), F (∼238 nm), G (∼256 nm),J (∼272 nm), and K (∼312 nm) in Figure 5. Band H isattributable to the host related absorption,13,14 and the other sixbands are assignable to the f-d absorption for Ce3+ in the host.

Figure 7. Luminescence decay of La2CaB10O19:Ce3+ at room temperature.

Figure 8. TRES of La1.995Ce0.005CaB10O19 at room tempearture.

13766 J. Phys. Chem. C, Vol. 112, No. 35, 2008 Li et al.

The occurrence of host-related absorption band H revealed theenergy transfer from the host to Ce3+ ions.

Bands E, F, G, and J overlap conspicuously with one another.Bands D and K have no obvious superposition with the otherfour bands, respectively. The occurrence of more than fiveexcitation bands in the spectra immediately indicates that Ce3+

ions occupy over one site in the lattice, which is consistent withthe emission spectra. Evidently, band K is the lowest 5dabsorption of Ce3+ in site II. Meanwhile, we think that band Jcorresponds to the lowest 5d state of Ce3+ in site I, and thiscan be further confirmed by the emission spectra in Figure 6.

Because band J in Figure 5 has no conspicuous spectraloverlap to band K, the relaxation from band J to band K isexpected to have a relatively lower probability than that betweenbands E, F, G, and J. That is, when we assume band J at 272nm corresponds to the lowest 5d absorption of Ce3+ in site I,and bands E, F, and G may contain the 5d orbit componentsfrom both sites, the emission from site I is expected to be witha higher intensity upon direct excitation of band J at 272 nmthan upon that of the other three bands E, F, and G. This is justthe result that we found in Figure 6, in which the emissionspectra of sample La1.995Ce0.005CaB10O19 under 220, 239, 258,and 272 nm excitation are displayed. It can be found that theemission band A from site I is remarkably intense upon 272nm excitation. As the occurrence of the energy transfer betweenthe two sites, the emission bands B and C from site II aredominant at other wavelength excitations.

Bands D, E, F, and G in Figure 5 cannot be unambiguouslyassigned at present, due to the clear spectral overlap and theenergy transfer between two sites.

3.4. The Nature of Site I and Site II in La2CaB10O19. Thecompound La2CaB10O19 crystallizes in the monoclinic system,a noncentrosymmetric space group C2, with a ) 11.043(3) Å,b ) 6.563(2) Å, c ) 9.129(2) Å, R ) γ ) 90°, � ) 91.47°,and two formula units per cell. The crystal structure containsB5O12 double-ring pentaborate anionic groups, in which a B5O12

group is formed by three BO4 tetrahedra and two BO3 triangleswith shared O atoms. The B5O12 groups are linked together toform an infinite two-dimensional double layer. The layer runsalmost perpendicular to the c axis of the crystal. The La atomswith 10-fold oxygen coordination are located in layers, in whichone La-O bond is at a significantly shorter distance of 2.297(3)Å and other nine La-O bonds are at longer distances from2.520(4) to 2.821(4) Å. The Ca atoms are 8-fold coordinatedby oxygen atoms and located between two layers. The distancesbetween Ca and O are in the range of 2.349(5)-2.678(5) Å.

The nature of site I and site II in La2CaB10O19 as well as thesite occupancy for Ce3+ in different lattice sites can beunderstood and assigned by the spectra. In principle, knowledgeof the centroid and the crystal-field splitting of 5d states canprovide important information for this purpose.

(1) The Centroid of Ce3+ 5d Levels in Site I and Site II.The centroid or the barycenter of Ce3+ 5d levels is defined asthe average position of the split 5d levels. Clearly, the 5dcentroid for Ce3+ in a specific host lattice is lower than that inthe free (gaseous) ion state (51230 cm-1) due to the interactionwith the crystal field. The position of the 5d centroid for Ce3+

in a specific host lattice is influenced by some factors, forexample, the site size of Ce3+, the anion coordination numberaround Ce3+, as well as the ionic radius and the electric chargeof a substituted cation. These factors affect the attractive forcesof the cations on the anion charge clouds of ligands, and thusshow an influence on the 5d centroid of Ce3+.

To interpret the 5d centroid for Ce3+ in a specific host lattice,Dorenbos5 improved the ligand polarization model and proposedfollowing semiempirical formula by some reasonable hypoth-eses.

εc ) (1.79 × 1013)∑i)1

N Rspi

(Ri - 0.6∆R)6(3)

where εc is the shift of the centroid energy (in eV) relative tothe free ion value of 6.35 eV, Ri is the distance (pm) betweenCe3+ and anion i in the undistorted lattice, ∆R is the radiusdifference between Ce3+ and the substituted cations (for thepresent case La3+ and Ca2+ ions), 0.6∆R is a correction forlattice relaxation around Ce3+, and Rsp

i (10-30 m-3) is thespectroscopic polarizability of anion i, which is closely con-nected with the polarizability of the anion. The summation isover all N anions that coordinate Ce3+. The values of Rsp (inunits of 10-30 m3) for oxygen are against the inverse square ofthe weighted average of the electronegativity of the cations inthe oxides, and can be defined as follows

Rsp0 ) 0.33+ 4.80

�av2

(4)

where �av is the weighted average of the electronegativity ofthe cations.

By this method, the red shift of the centroid energy isestimated to be about 1.03 eV for Ce3+ in La3+ sites and about1.22 eV for Ce3+ in Ca2+ sites, respectively. The results directlyshow that the decrease of the Ce3+ 5d centroid is larger in Ca2+

sites than that in La3+ sites. When we consider the centroid offree Ce3+ 5d levels is about 51230 cm-1, the 5d centroid forCe3+ in La3+ sites can be simply calculated to be ∼42.9 × 103

cm-1, and that in Ca2+ sites ∼41.4 × 103 cm-1. That is, the 5dcentroid is higher in La3+ sites than that in Ca2+ sites.

(2) The CFS of Ce3+ 5d Levels. The 5d crystal field splitting(CFS) for Ce3+ in a complex oxide host lattice is mainlydetermined by the type of coordination polyhedron and the sitesize around Ce3+. When the bond lengths remain constant, theCFS depends on the polyhedral shape. On the other hand, theCFS increases with the decrease of site size (or Ce-O bonddistance) for an exact polyhedral type.

For the present case, Ca2+ sites are 8-fold coordination withan average Ca-O bond distance of ∼2.465 Å, while La3+ sitesare 10-fold coordination with an average La-O bond distanceof ∼2.640 Å. The polyhedral types for Ca2+ sites and La3+ sitesare different, and the site sizes around Ce3+ are also different.However, we still can roughly estimate the magnitude of CFSfor Ce3+ in these two lattice sites.

From available data, one can see that though in someoccasional cases the site size (i.e., the average bond distance)may be larger for 8-fold coordination than for 10-fold coordina-tion, the CFS shows an obvious difference between 8-fold and10-fold coordination sites (see Table 1). The magnitude of CFSfor Ce3+ is always larger in 8-fold coordination sites (17.0 ×103 to 19.6 × 103 cm-1) than in 10-fold coordination sites (11.9× 103 to 12.6 × 103 cm-1). In terms of the data in Table 1, weassume that the CFS magnitude for the Ce3+ in Ca2+ sites willbe larger than that in La3+ sites.

Until now, we have deduced that the 5d centroid is higher inLa3+ sites than that in Ca2+ sites, and the CFS magnitude forCe3+ in Ca2+ sites is larger than that in La3+ sites. Combiningthe two factors, we can conclude that site I will relate to theCe3+ in La3+ site, and site II the Ce3+ in Ca2+ site. Band J inFigure 5 will be the absorption from the 2F5/2 ground state to

Luminescence of Ce3+ J. Phys. Chem. C, Vol. 112, No. 35, 2008 13767

the lowest 5d state for the Ce3+ in La3+ site, and band K fromthe 2F5/2 ground state to the lowest 5d state for the Ce3+ in Ca2+

site.3.5. Luminescence Decay and Time-Resolved Emission

Spectra of La2CaB10O19:Ce3+. We measured the emissiondecay for Ce3+ in La3+ sites (with emission at 293 and 306nm, upon excitation at 272 nm) and Ca2+ sites (with emissionat 356 nm, upon excitation at 272 and 316 nm) for samplesLa2-xCexCaB10O19 and La2Ca1-2xCexNaxB10O19 (with x ) 0.005,0.04, 0.10) at room temperature, respectively. The typical resultsare displayed in Figure 7.

In cases a, b, and d, the luminescence decay occurs as a firstorder exponential decay. For case c, upon excitation at 272 nm(the lowest 5d absorption for Ce3+ in La3+ sites) and emissionat 356 nm (Ce3+ in Ca2+ sites), a slow increase process happenedin initial time (marked as δ in the curve), which is due to theenergy transfer from La3+ sites to Ca2+ sites.

The nominal compositions La2-xCexCaB10O19 or La2Ca1-2x-

CexNaxB10O19 nearly have no influence on the decay charac-teristics. With the increase of doping concentration, the decaytime is nearly constant and only a very slight decrease wasfound. The emission decay can be well fitted with a singleexponential equation: It ) I0exp(-t/τ), where It and I0 are theluminescence intensity, t is time, and τ is the decay time forthe exponential components, respectively. The value of τ is fittedto be around 12 ns for Ce3+ in La3+ sites and about 26 ns forCe3+ in Ca2+ sites.

The time-resolved emission spectra (TRES) of sampleLa1.995Ce0.005CaB10O19 were measured at room temperature asshown in Figure 8. Ce3+ emission bands from both sites wereobserved in the spectra, and the emission intensity decreasesregularly with the time increase as displayed in the left curvesa-e. Furthermore, it is clearly found that the decay rate of twosites is different.

To compare the decay rate for Ce3+ ions in two sites, wenormalized the whole emission in the 285-400 nm range anddrew the normalized curves in right curves a′-e′. The curvesimmediately demonstrate that Ce3+ ions in site I (La3+ site)have a rather higher decay rate, which is in agreement with theresults of lifetime measurements. In addition, the emission forCe3+ ions in site I shows a higher intensity when we measurethe TRES with a shorter time (for example, 7 and 11 ns). Thisresult seems to suggest that Ce3+ ions indeed occupy site I witha higher probability. The efficient energy transfer leads to loweremission intensity from site I in Figures 2-4 and 6.

4. Conclusion

Phosphors La2CaB10O19:Ce3+ were prepared by a solid statereaction route at high temperature. The investigations on the

steady state excitation and emission spectra in the VUV-visrange revealed that Ce3+ ions occupy two distinct sites, andthe nominal chemical formula appears to have no evidentinfluence on the spectral patterns. The lowest 5d absorption bandfor Ce3+ in site I has a maximum at about 272 nm, and that insite II at about 312 nm. The emission bands from site I arearound 291 and 310 nm, while those from site II are around329 and 355 nm. In terms of the structure of the host lattice,site I is assigned to the La3+ site, and site II the Ca2+ site. Themeasurements on luminescence decay showed that Ce3+ emis-sion from the La3+ site has a relatively shorter decay time ofabout 12 ns, and that from the Ca2+ site has a relatively longerdecay time of about 26 ns. TRES measurements demonstratedthe high occupancy probability for the Ce3+ in La3+ site.

Acknowledgment. The work is financially supported by theNational Basic Research Program of China (973 Program)(Grant No. 2007CB935502), by the National Natural ScienceFoundation of China (Grant No. 20571088), and by the Scienceand Technology Project of Guangdong Province (Grant No.2005A10609001).

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