J. Phys. Chem. C 2008, 112, 13763–13768
13763
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 Materials and 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 and Technology 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 were prepared by a solid state reaction route at high temperature. Their luminescence properties were investigated by the steady state excitation and emission spectra in the VUV-vis range, the luminescence decays, and the time-resolved emission spectra (TRES). The results demonstrate that Ce3+ ions occupy two lattice sites in all samples. 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 doublet emission bands have maxima at about 291 and 310 nm. The emission for the Ce3+ in Ca2+ site has a longer lifetime of 26 ns with band maxima at about 329 and 355 nm. Efficient energy transfer between both sites occurs in the samples. 1. Introduction Luminescence of Ce3+ in complex oxides has demonstrated its importance for lighting, display, and scintillation applications. Three well-known examples worth being mentioned will include the following: Y3Al5O12:Ce3+ as a yellow-emitting component in InGaN/GaN-based LEDs (light emitting diodes);1 the potential application of Y2SiO5:Ce3+ as a blue-emitting material in FEDs (field emission displays);2 and Lu2SiO5:Ce3+ as a commercially available scintillator in medical imaging detectors for PET (positron emission tomography) systems.3 On the other hand, growing interest is focused on the 4f-5d transitions of Ce3+ in various hosts due to the importance for basic research. The investigation on the 4f-5d transitions of Ce3+ can provide key information on the 5d energies for other lanthanide ions. The 5d states are outer orbital, and the coordination around a lanthanide ion has remarkable influence on their energies. As a result the 4f-5d transitions appear in a wavelength range that depends strongly on both the kind of lanthanide ion and the host lattice. The 5d centroid, the lowest 5d level, and the 5d crystal field splitting for other lanthanide ions in same lattice site can be evaluated by means of the excitation spectrum of Ce3+.4-8 In addition, with the help of the emission spectrum, the number of lattice sites for Ce3+ in a specific host can be understood, because Ce3+ in one definite lattice site often exhibits doublet 5d-2FJ (J ) 5/2, 7/2) emission bands with the energy separation about 2000 cm-1. When the site occupancy for Ce3+ in a definite host is known, that for other 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 not hygroscopic, which has attracted considerable interest as a potential material for nonlinear optical (NLO) applications because it exhibits an optical second-harmonic generation (SHG) effect about twice as large as that of KDP (KH2PO4) and the * Authors to whom all correspondence should be addressed. Phone: 8620-84111038. Fax: 86-20-84111038. E-mail:
[email protected]. † Sun Yat-sen University. ‡ University of Science and Technology of China.
Figure 1. XRD patterns of samples La2-xCexCaB10O19 and La2Ca1-2xCexNaxB10O19.
single crystal is easily grown.9-11 In the present paper, the VUV-vis luminescence properties of Ce3+ in La2CaB10O19 were investigated. In particular, the spectroscopic properties in relation to the different lattice site occupancies were reported in detail by steady-state spectra at different temperature, luminescence decay and time-resolved emission spectra at room temperature (RT).
2. Experimental Section A series of polycrystalline samples with two types of nominal chemical formulas La2-xCexCaB10O19 and La2Ca1-2xCexNaxB10O19 (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 route at high temperature by using the following chemical reactions.
10.1021/jp804149k CCC: $40.75 2008 American Chemical Society Published on Web 08/07/2008
13764 J. Phys. Chem. C, Vol. 112, No. 35, 2008
(2 - x)La2O3 + 2xCeO2 + 2CaCO3 + 20H3BO3 + 723 K/10 h
1273 K/48 h
xCO 98 98 2La2-xCexCaB10O19 + 30H2O + (2 + x)CO2 (1) 2La2O3 + 2(1 - 2x)CaCO3 + 2xCeO2 + xNa2CO3 + 723 K/10 h
1273 K/48 h
20H3BO3 + xCO 98 98 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. For reaction 2, Na2CO3 was added as a charge compensator because the substitution of a Ce3+ ion for a Ca2+ ion requires a charge compensator to maintain overall charge neutrality of the samples La2Ca1-2xCexNaxB10O19. The stoichiometric mixtures were first well ground in an agate mortar and prefired at 723 K in air atmosphere for 10 h, and then calcined at 1273 K under CO reducing ambience for 48 h. The structure of the final products was examined by powder X-ray diffraction (PXRD), using Cu KR radiation (λ ) 1.5046 Å) on a BRUKER D8 ADVANCE type powder X-ray diffractometer. The steady-state UV excitation and corresponding emission spectra, the luminescence decay curves, as well as the timeresolved emission spectra (TRES) were measured on an Edinburgh FLS 920 combined fluorescence lifetime and steady state spectrometer, which was equipped with a CTI-Cryogenics temperature controlling system. At measurements, a Xe 900 lamp (450 W) was used as the excitation source for the steadystate spectra, while an nF900 ns flash lamp (150 W, with a pulse width of 1 ns and pulse repetition rate of 40-100 kHz) was used as the excitation source for the luminescence decay curves and the TRES. The time-resolved emission spectra were constructed by measurements of numerous decay curves and these data were converted with TRES software attached to the instrument. The VUV excitation and corresponding emission spectra were measured at the time-resolved spectroscopy experimental station of the National Synchrotron Radiation Laboratory (NSRL, Hefei, China) under normal operating conditions. The measurement details can be found elsewhere.12
Li et al. 3. Results and Discussion 3.1. XRD Patterns. The XRD patterns of all samples that include undoped sample La2CaB10O19 and doped samples La2-xCexCaB10O19 and La2Ca1-2xCexNaxB10O19 with different Ce3+ 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) is consistent with that in ref 9, showing that the undoped sample has a single phase. The other seven curves are in agreement with curve (a), indicating that they are of the iso-structure with the undoped sample, and the dopant Ce3+ and Na+ ions do not show evident influence on the position of the diffraction lines. However, the diffraction peak intensities in XRD patterns slightly decrease with the increase of the doping concentration, suggesting that the crystallinity (degree of crystallization) seems to decrease with the increase of the doping concentration. This is probably related to the decreasing of the melting point for solid solutions La2-xCexCaB10O19 and La2Ca1-2xCexNaxB10O19 with the increase of the x value. 3.2. Emission Spectra of Ce3+ in La2CaB10O19. The emission 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 room temperature, respectively. The spectra upon 254 nm excitation were chosen as typical results and displayed in Figure 2, in which 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 term of the spectra at lower temperature (see Figure 4), these bands are attributable to the emission of Ce3+ and they do not relate to the host emission. In general, Ce3+ ions in a single definite lattice site (especially at lower temperature) usually present two emission bands due to the transitions from the lowest 5d excited state to the 2F5/2 and 2F7/2 spin-orbit split 4f ground states. The energy separation of the two bands corresponds to the spin-orbit splitting and amounts to about 2000 cm-1. Figrue 2 directly indicates that Ce3+ ions occupy two different sites in the host lattice. The energy difference between band B and band C is near 2000 cm-1, so they will be unambiguously assigned to the emission of Ce3+ in the a same lattice site, and here we call this site II. Meanwhile, the emission band A must come from Ce3+ in another site in the host lattice, and here we mark this as site I.
Figure 2. The normalized emission (λex ) 254 nm) spectra of samples La2-xCexCaB10O19 with different x values at room temperature.
Luminescence of Ce3+
Figure 3. The emission (λex ) 254 nm) spectra of samples La2Ca1-2xCexNaxB10O19 with different x values at room temperature.
J. Phys. Chem. C, Vol. 112, No. 35, 2008 13765
Figure 6. The emission spectra of La2CaB10O19:Ce3+ under different wavelength excitation.
TABLE 1: The CFS of Ce3+ 5d Levels in Some Complex Oxides
Figure 4. The emission (λex ) 258 nm) spectra of samples La1.995Ce0.005CaB10O19 at different temperatures.
Figure 5. VUV-UV excitation spectra of La2-xCexCaB10O19 and La2Ca1-2xCexNaxB10O19 at room temperature.
(2) To evaluate the influence of dopant concentration (x value) on the emission intensity of two different lattice sites, we normalized all emission spectra in the wavelength range 274-480 nm in Figure 2. It can be found that the relative emission intensity from site II is always higher than that from site I. Furthermore, the emission from site II gradually increases with the increase of the x value, whereas the emission from
compd
coord no.
ave bond distance (Å)
CFS (cm-1)
ref
GdB3O6 LaPO4 LaB3O6 YMgB5O10 LaP5O14 CeP5O14 LaP3O9 Li6Y(BO3)3 YBO3 CaSO4 YPO4 LuBO3 (vaterite) GdBO3 LuPO4
10 10 10 10 8 8 8 8 8 8 8 8 8 8
2.52 2.64 2.61 2.47 2.50 2.48 2.53 2.40 2.37 2.47 2.34 2.33 2.40 2.30
11700 11900 12000 12600 16600 17000 17100 17200 17600 17900 18010 18500 18700 19600
7 7 7 15 16 7 7 15 7 17 18 7 7 19
site I decreases regularly. We think that this may be due to two main factors: one is the different occupancy probability between site I and site II, the other is the energy transfer from site I to site II. First, provided that the occupancy probability of site II is higher than that of site I, the emission intensity from site II may be higher than that from site I. When the occupancy probability in site II gradually increases with the doping concentration, the emission from site II relative to that from site I will regularly increase. Second, the lowest 5d absorption for Ce3+ in site II was observed at about 312 nm (see the band K in Figure 5), which has obvious overlap to the emission bands from site I in Figure 2. This spectroscopic superposition will result in a Ce3+ ion in site II absorbing the emission from site I efficiently. As a consequence, an effective resonance-type energy transfer may occur from site I to site II, and this energy transfer will make the emission intensity of site I look weakened. The higher the doping concentration is, the higher the probability that the energy transfer occurs. Accordingly the emission intensity from site I is expected to become weaker and weaker. These two reasons together make the emission from site II higher than that from site I, and we cannot affirm which one is a dominant factor for the time being; the TRES are expected to give more information on this issue, see section 3.5. In addition, there is no doubt that the concentration quenching will also show an influence on the emission intensity, but this reason does not seem to be the main factor here, as the concentration quenching phenomenon was not observed in the doping concentration.
13766 J. Phys. Chem. C, Vol. 112, No. 35, 2008
Li et al.
Figure 7. Luminescence decay of La2CaB10O19:Ce3+ at room temperature.
Figure 8. TRES of La1.995Ce0.005CaB10O19 at room tempearture.
(3) For Ce3+ emission in site II, the band B corresponds to the transition from the lowest 5d state to the 2F5/2 level, and the band 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 of band B regularly increases with the increase of the doping concentration. Because of the partial superposition between the absorption band K in Figure 5 and the emission band B in Figure 2, this resonance energy transfer within site II will decrease the intensity of band B. When the doping concentration increases, 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 of samples La2Ca1-2xCexNaxB10O19, as shown in Figure 3. The spectral characteristics are similar for samples La2Ca1-2xCexNaxB10O19 and La2-xCexCaB10O19, which seems to indicate that the nominal chemical formulas have almost no influence on the spectral patterns. See also the excitation spectra of Figure 5 in section 3.3. To evaluate the influence of the temperature on the luminescence, the emission spectra at different temperatures were measured. The emission spectra of sample La1.995Ce0.005-
CaB10O19 in the 280-380 nm range as a function of temperature are shown in Figure 4. At a lower temperature, four wellresolved emission bands A1 (∼291 nm), A2 (∼310 nm), B (∼329 nm), C (∼355 nm) were clearly observed. The emission of Ce3+ in site I corresponds to bands A1 and A2, while that in site II corresponds to bands B and C. With the increase of the temperature, the total emission intensity decreases because of the thermal quenching, as shown in the right curve of Figure 4 (in which an exceptional datum at 40 K may be an experimental error). Although the emission intensity from both sites decreases with the increase of temperature, further scrutinizing the spectra, we find that the decrease magnitude of the emission from site I (bands A1 and A2) is more obvious than that from site II (bands B and C). This also may be related to the resonance-type energy transfer between site I and site II, as described before. The energy transfer probability increases with the increase of temperature, so the emission intensities of bands A1 and A2 show an obvious decrease. In addition, it was observed that the bands A1 and A2 combined gradually with the increase of temperature, which is due 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. The VUV-UV excitation spectra for samples La2-xCexCaB10O19 and La2Ca1-2xCexNaxB10O19 with x ) 0.005, 0.04, 0.10 at room temperature were measured as shown in Figure 5. The ground state of Ce3+ contains one single optically active electron in the well-shielded 4f shell (4f1). It can be excited to the 5d configuration, and depending on the site symmetry, at most five distinct 4f-5d transitions can be observed. Seven bands were found in the spectra, which are labeled as H (∼160 nm), D (∼192 nm), E (∼219 nm), F (∼238 nm), G (∼256 nm), J (∼272 nm), and K (∼312 nm) in Figure 5. Band H is attributable to the host related absorption,13,14 and the other six bands are assignable to the f-d absorption for Ce3+ in the host.
Luminescence of Ce3+ The occurrence of host-related absorption band H revealed the energy 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 other four bands, respectively. The occurrence of more than five excitation bands in the spectra immediately indicates that Ce3+ ions occupy over one site in the lattice, which is consistent with the emission spectra. Evidently, band K is the lowest 5d absorption of Ce3+ in site II. Meanwhile, we think that band J corresponds to the lowest 5d state of Ce3+ in site I, and this can be further confirmed by the emission spectra in Figure 6. Because band J in Figure 5 has no conspicuous spectral overlap to band K, the relaxation from band J to band K is expected to have a relatively lower probability than that between bands E, F, G, and J. That is, when we assume band J at 272 nm corresponds to the lowest 5d absorption of Ce3+ in site I, and bands E, F, and G may contain the 5d orbit components from both sites, the emission from site I is expected to be with a higher intensity upon direct excitation of band J at 272 nm than upon that of the other three bands E, F, and G. This is just the result that we found in Figure 6, in which the emission spectra of sample La1.995Ce0.005CaB10O19 under 220, 239, 258, and 272 nm excitation are displayed. It can be found that the emission band A from site I is remarkably intense upon 272 nm excitation. As the occurrence of the energy transfer between the two sites, the emission bands B and C from site II are dominant at other wavelength excitations. Bands D, E, F, and G in Figure 5 cannot be unambiguously assigned at present, due to the clear spectral overlap and the energy transfer between two sites. 3.4. The Nature of Site I and Site II in La2CaB10O19. The compound 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°, β ) 91.47°, and two formula units per cell. The crystal structure contains B5O12 double-ring pentaborate anionic groups, in which a B5O12 group is formed by three BO4 tetrahedra and two BO3 triangles with shared O atoms. The B5O12 groups are linked together to form an infinite two-dimensional double layer. The layer runs almost perpendicular to the c axis of the crystal. The La atoms with 10-fold oxygen coordination are located in layers, in which one La-O bond is at a significantly shorter distance of 2.297(3) Å and other nine La-O bonds are at longer distances from 2.520(4) to 2.821(4) Å. The Ca atoms are 8-fold coordinated by oxygen atoms and located between two layers. The distances between 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 the site occupancy for Ce3+ in different lattice sites can be understood and assigned by the spectra. In principle, knowledge of the centroid and the crystal-field splitting of 5d states can provide 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 as the average position of the split 5d levels. Clearly, the 5d centroid for Ce3+ in a specific host lattice is lower than that in the free (gaseous) ion state (51230 cm-1) due to the interaction with the crystal field. The position of the 5d centroid for Ce3+ in a specific host lattice is influenced by some factors, for example, the site size of Ce3+, the anion coordination number around Ce3+, as well as the ionic radius and the electric charge of a substituted cation. These factors affect the attractive forces of the cations on the anion charge clouds of ligands, and thus show an influence on the 5d centroid of Ce3+.
J. Phys. Chem. C, Vol. 112, No. 35, 2008 13767 To interpret the 5d centroid for Ce3+ in a specific host lattice, Dorenbos5 improved the ligand polarization model and proposed following semiempirical formula by some reasonable hypotheses.
Risp
N
εc ) (1.79 × 10 ) 13
∑ (R - 0.6∆R)6 i)1
(3)
i
where εc is the shift of the centroid energy (in eV) relative to the free ion value of 6.35 eV, Ri is the distance (pm) between Ce3+ and anion i in the undistorted lattice, ∆R is the radius difference between Ce3+ and the substituted cations (for the present case La3+ and Ca2+ ions), 0.6∆R is a correction for i lattice relaxation around Ce3+, and Rsp (10-30 m-3) is the spectroscopic polarizability of anion i, which is closely connected with the polarizability of the anion. The summation is over all N anions that coordinate Ce3+. The values of Rsp (in units of 10-30 m3) for oxygen are against the inverse square of the weighted average of the electronegativity of the cations in the oxides, and can be defined as follows
R0sp ) 0.33 +
4.80 χav2
(4)
where χav is the weighted average of the electronegativity of the cations. By this method, the red shift of the centroid energy is estimated to be about 1.03 eV for Ce3+ in La3+ sites and about 1.22 eV for Ce3+ in Ca2+ sites, respectively. The results directly show that the decrease of the Ce3+ 5d centroid is larger in Ca2+ sites than that in La3+ sites. When we consider the centroid of free Ce3+ 5d levels is about 51230 cm-1, the 5d centroid for Ce3+ 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 5d centroid 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 mainly determined by the type of coordination polyhedron and the site size around Ce3+. When the bond lengths remain constant, the CFS depends on the polyhedral shape. On the other hand, the CFS increases with the decrease of site size (or Ce-O bond distance) for an exact polyhedral type. For the present case, Ca2+ sites are 8-fold coordination with an average Ca-O bond distance of ∼2.465 Å, while La3+ sites are 10-fold coordination with an average La-O bond distance of ∼2.640 Å. The polyhedral types for Ca2+ sites and La3+ sites are different, and the site sizes around Ce3+ are also different. However, we still can roughly estimate the magnitude of CFS for Ce3+ in these two lattice sites. From available data, one can see that though in some occasional cases the site size (i.e., the average bond distance) may be larger for 8-fold coordination than for 10-fold coordination, the CFS shows an obvious difference between 8-fold and 10-fold coordination sites (see Table 1). The magnitude of CFS for 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, we assume that the CFS magnitude for the Ce3+ in Ca2+ sites will be larger than that in La3+ sites. Until now, we have deduced that the 5d centroid is higher in La3+ sites than that in Ca2+ sites, and the CFS magnitude for Ce3+ in Ca2+ sites is larger than that in La3+ sites. Combining the two factors, we can conclude that site I will relate to the Ce3+ in La3+ site, and site II the Ce3+ in Ca2+ site. Band J in Figure 5 will be the absorption from the 2F5/2 ground state to
13768 J. Phys. Chem. C, Vol. 112, No. 35, 2008 the lowest 5d state for the Ce3+ in La3+ site, and band K from the 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 emission decay for Ce3+ in La3+ sites (with emission at 293 and 306 nm, upon excitation at 272 nm) and Ca2+ sites (with emission at 356 nm, upon excitation at 272 and 316 nm) for samples La2-xCexCaB10O19 and La2Ca1-2xCexNaxB10O19 (with x ) 0.005, 0.04, 0.10) at room temperature, respectively. The typical results are displayed in Figure 7. In cases a, b, and d, the luminescence decay occurs as a first order exponential decay. For case c, upon excitation at 272 nm (the lowest 5d absorption for Ce3+ in La3+ sites) and emission at 356 nm (Ce3+ in Ca2+ sites), a slow increase process happened in initial time (marked as δ in the curve), which is due to the energy transfer from La3+ sites to Ca2+ sites. The nominal compositions La2-xCexCaB10O19 or La2Ca1-2xCexNaxB10O19 nearly have no influence on the decay characteristics. With the increase of doping concentration, the decay time is nearly constant and only a very slight decrease was found. The emission decay can be well fitted with a single exponential equation: It ) I0exp(-t/τ), where It and I0 are the luminescence intensity, t is time, and τ is the decay time for the exponential components, respectively. The value of τ is fitted to be around 12 ns for Ce3+ in La3+ sites and about 26 ns for Ce3+ in Ca2+ sites. The time-resolved emission spectra (TRES) of sample La1.995Ce0.005CaB10O19 were measured at room temperature as shown in Figure 8. Ce3+ emission bands from both sites were observed in the spectra, and the emission intensity decreases regularly with the time increase as displayed in the left curves a-e. Furthermore, it is clearly found that the decay rate of two sites is different. To compare the decay rate for Ce3+ ions in two sites, we normalized the whole emission in the 285-400 nm range and drew the normalized curves in right curves a′-e′. The curves immediately demonstrate that Ce3+ ions in site I (La3+ site) have a rather higher decay rate, which is in agreement with the results of lifetime measurements. In addition, the emission for Ce3+ ions in site I shows a higher intensity when we measure the TRES with a shorter time (for example, 7 and 11 ns). This result seems to suggest that Ce3+ ions indeed occupy site I with a higher probability. The efficient energy transfer leads to lower emission intensity from site I in Figures 2-4 and 6. 4. Conclusion Phosphors La2CaB10O19:Ce3+ were prepared by a solid state reaction route at high temperature. The investigations on the
Li et al. steady state excitation and emission spectra in the VUV-vis range revealed that Ce3+ ions occupy two distinct sites, and the nominal chemical formula appears to have no evident influence on the spectral patterns. The lowest 5d absorption band for Ce3+ in site I has a maximum at about 272 nm, and that in site II at about 312 nm. The emission bands from site I are around 291 and 310 nm, while those from site II are around 329 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. The measurements on luminescence decay showed that Ce3+ emission from the La3+ site has a relatively shorter decay time of about 12 ns, and that from the Ca2+ site has a relatively longer decay time of about 26 ns. TRES measurements demonstrated the high occupancy probability for the Ce3+ in La3+ site. Acknowledgment. The work is financially supported by the National Basic Research Program of China (973 Program) (Grant No. 2007CB935502), by the National Natural Science Foundation of China (Grant No. 20571088), and by the Science and Technology Project of Guangdong Province (Grant No. 2005A10609001). References and Notes (1) Shimizu, Y.; Sakano, K.; Noguchi, Y.; Moriguchi, T. U.S. Patent No. 59989251998. (2) Holloway, P. H.; Trottier, T. A.; Abrams, B.; Kondoleon, C.; Jones, S. L.; Sebastian, J. S.; Thomes, W. J.; Swart, H. J. Vac. Sci. Technol. B. 1999, 172, 758. (3) Blasse, G.; Grabmaier, B. C. Luminescent Materials; SpringerVerlag: Berlin, Germany, 1994. (4) Dorenbos, P. J. Lumin. 2000, 91, 155. (5) Dorenbos, P. Phys. ReV. B 2000, 62, 15640. (6) Dorenbos, P. Phys. ReV. B 2000, 62, 15650. (7) Dorenbos, P. Phys. ReV. B 2001, 64, 125117. (8) Dorenbos, P. J. Lumin. 2002, 99, 283. (9) Wu, Y.; Liu, J.; Fu, P.; Wang, J.; Zhou, H.; Wang, G.; Chen, C. Chem. Mater. 2001, 13, 753. (10) Jing, F.; Wu, Y.; Fu, P. J. Cryst. Growth 2005, 285, 270. (11) Jing, F.; Wu, Y.; Fu, P. J. Cryst. Growth 2006, 292, 454. (12) Liang, H. B.; Zeng, Q.; Tian, Z. F.; Lin, H. H.; Su, Q.; Zhang, G. B.; Fu, Y. B. J. Electrochem. Soc. 2007, 154, J177. (13) He, H.; Yu, Y.; Liang, H. B.; Wang, S. B.; Su, Q.; Tao, Y. J. Rare Earths 2004, 22, 361. (14) He, H.; Liang, H. B.; Tao, Y.; Wang, S. B.; Su, Q. Chem. J. Chin. UniV. 2003, 24, 1541. (15) Knitel, M. J.; Dorenbos, P.; Eijk, C. W. E.; van; Plasteig, B.; Viana, B.; Kahn-Harari, A.; Vivien, D. Phys. Res. A 2000, 443, 364. (16) Blanzat, B.; Denis, J.-P.; Pannel, C.; Barthou, C. Mater. Res. Bull. 1977, 12, 455. (17) Van der Kolk, E.; Dorenbos, P.; Vink, A. P.; Perego, R. C.; van Eijk, W. E.; Lakshmanan, A. R. Phys. ReV. B. 2001, 64, 195129. (18) Karanjikar, N. P.; Naik, R. C. Solid State Commun. 1988, 65, 1419. (19) Williams, G. M.; Edelstein, N.; Boatner, L. A.; Abraham, M. M. Phys. ReV. B 1989, 40, 4143.
JP804149K
