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Synthesis and luminescent properties of prospective Ce3+ doped silicate garnet phosphors for white LED converters
Journal of Luminescence 192 (2017) 328–336
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Synthesis and luminescent properties of prospective Ce3+ doped silicate garnet phosphors for white LED converters
MARK
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N. Khaidukova, , T. Zorenkob, A. Iskaliyevab, K. Paprockib, M. Batentschukc, A. Osvetc, ⁎ R. Van Deund, Ya. Zhydaczevskiie, A. Suchockie, Yu. Zorenkob, a
Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, 119991 Moscow, Russia Institute of Physics, Kazimierz Wielki University in Bydgoszcz, 85090 Bydgoszcz, Poland c Institute of Materials for Energy and ElectronicTechnology (i-IMEET), Department of Materials Science and Engineering VI, University of Erlangen-Nuremberg, 91058 Erlangen, Germany d L3 – Luminescent Lanthanide Lab, Department of Inorganic and Physical Chemistry, Ghent University, 9000 Gent, Belgium e Institute of Physics, Polish Academy of Sciences, 02-668 Warsaw, Poland b
A R T I C L E I N F O
A B S T R A C T
Keywords: Luminescence LED ceramic converters Ca-Si based garnet Ce3+ dopant
The results on crystallization and investigation of the luminescent properties of prospective ceramic phosphors based on the Ce3+ doped {Ca2R}[ScB](CSi2)O12 (R = Lu, Y, Gd; B = Sc, Ga, C = Ga, Al) silicate garnets are presented for the first time in this work. We have observed the variations of the spectroscopic properties of Ce3+ ions in the mentioned Ca2+-Si4+ garnet hosts depending on the cation content at the dodecahedral {}, octahedral [] and tetrahedral () sites of garnet lattice. These results can be useful for the development of new generation of ceramic phosphor converters for white LEDs based on the garnet compounds under study.
1. Introduction In the last two decades the lamps based on the white light emitting diodes (WLED) have displaced the traditional light sources due to their advantages including high luminous efficiency, energy saving, long lifetime and environmental friendliness. The rate of such a displacement for various applications such as backlighting for displays, automotive and general lighting depends on developing more powerful blue and near UV emitting chips, more efficient phosphors and new schemes for conversion of chip radiation to the white light [1]. At present, a WLED source, manufactured on the basis of a blue LED chip and the yellow emitting YAG:Ce powder phosphor dispersed in epoxy and silicone resin, is a canonical device [2]. While a large number of other different phosphors have been developed to date, YAG:Ce is still the most popular phosphor now for producing the WLED [1,2]. In this context, it should be noted that applying YAG:Ce ceramic or crystal phosphor plates for light conversion in the white LED is now also accessible for manufacturing of high power WLEDs [3]. Therefore, the ceramic phosphors based on different compositions of Ce3+ doped Ln3Al5O12 garnets and techniques for obtaining such phosphor ceramics have been patented repeatedly as well [4]. Due to the flexibility of the garnet structure, which allows replacing ions at the dodecahedral { }, octahedral [ ] and tetrahedral ( ) sites, it is
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possible to conveniently replace the host cations and modify the {Y}3[Al]2(Al)3O12 garnet composition for altering the Ce3+ spectroscopic properties to better meet the requirements for utilization in WLED. To date the spectroscopic properties of Ce3+ in some garnets containing Si4+ at tetrahedral sites, namely Y3Mg2AlSi2O12, Y3MgAl3SiO12, CaY2Al4Si O12, MgY2MgAl2Si2O12, CaLu2Al4SiO12, CaLu2Mg2Si3O12, Ca2YMgScSi3O12 and Ca3Sc2Si3O12 garnets have been published [5–15]. Namely, it has been shown that Ca3Sc2Si3O12:Ce exhibits less thermal quenching than YAG:Ce [5]. At the same time, there is no information concerning the spectroscopic properties of Ce3+ in silicate garnets of the {Ca2Y}[Sc,Al,Ga]2(Ga,Al,Si)3O12 family [5–15]. For this reason, this work is devoted to crystallization and investigation of the luminescent properties of phosphors based on the Ce3+ doped {Ca2R}[Sc,B](Ca,Si2)O12; R = Lu, Y, Gd; B = Sc, Ga, C = Ga, Al silicate garnets, which can be used for producing high power WLEDs having high color rendering index and low correlated color temperature values [5,6]. 2. Samples and method of their preparation In this work, we report on the first results on the crystallization of ceramics based on the Ce3+ doped {Ca2R}[ScB]2(CSi2)O12; R = Lu, Y,
Corresponding authors. E-mail addresses: [email protected] (N. Khaidukov), [email protected] (Y. Zorenko).
http://dx.doi.org/10.1016/j.jlumin.2017.06.068 Received 9 January 2017; Received in revised form 26 June 2017; Accepted 29 June 2017 Available online 01 July 2017 0022-2313/ © 2017 Elsevier B.V. All rights reserved.
Journal of Luminescence 192 (2017) 328–336
N. Khaidukov et al.
It can be seen that there is an increase of the unit cell parameter with the ionic radius of the cations residing in dodecahedral {Lu3+, Y3+, Gd3+}, octahedral [Ga3+, Sc3+] and tetrahedral (Al3+, Ga3+) sites, which determines the peak positions of XRD reflections for these garnets (Fig. 1b). 3. Experimental results The luminescent properties of the silicate garnet phosphors {Ca2R} [Sc,Ga]2(Al,Ga,Si)3O12:Ce, R = Lu, Y, Gd, depending on the cation content, were examined by recording the room temperature (RT) cathodoluminescence (CL) spectra using a SEM JEOL microscope equipped with a Stellar Net grating spectrometer with TE cooled CCD camera working in the 200–1200 nm range, as well as the photoluminescence (PL) emission and PL excitation spectra using a SM 2203 (Solar LTD) spectrofluorometer working in the 200–820 nm range. Decay kinetics of the luminescence of ceramic samples at RT was recorded using a Fluorolog 3 spectrophotometer under excitation in the Ce3+ related absorption bands. The obtained spectroscopic data for {Ca2R} [Sc,Ga]2(Al,Ga,Si)3O12:Ce; R = Lu, Y, Gd were systematized in Table 1 in correlation with the structural features of the garnet hosts resulting in the different parameters of the crystal field strength around Ce3+ ions.
Fig. 1. View of a ceramic phosphor plate based on the Ca2YSc2AlSi2O12:Ce garnet.
Gd; B = Sc, Ga, C = Ga, Al garnets. The nominally undoped Ca2YSc2AlSi2O12 and Ca2YSc2GaSi2O12 ceramic samples were synthesized as well for comparison with luminescent properties of the Ce3+ doped counterparts. The ceramics of these garnets were obtained using the hydrothermal crystallization method (Fig. 1). The precursors of the garnets were synthesized from the NaOH–H2O solutions of CaO, SiO2, R2O3 (R = Y, Gd, Lu) and A2O3 (A = Sc, Ga, Al) oxides with different molar ratios at temperatures of 300–400°C. The purity of initial components was 4N. The nominal concentration of Ce3+ ions was 3 at% with respect to the total content of Ca2+ and Y3+ cations. Then precursors were pressed in tablets and dried in air at 1000–1150°C. Finally, the tablets were annealed: (i) in the reducing (CO) atmosphere at 1300°C for keeping Ce3+ valence state of cerium ions in ceramic samples. For comparison, the tablets of Ca2YSc2AlSi2O12 :Ce and Ca2YSc2GaSi2O12:Ce compounds were also annealed at 1300°C in air for the study of the influence of oxygen-containing atmosphere on formation of Ce4+ and Ce3+ states during the synthesis of the ceramic samples of these garnets. The structural analysis of the ceramic phosphors was performed using scanning electronic microscopy with JEOL JSM-820 (Fig. 2), and XRD measurements (Fig. 3). The images of the surface texture of the {Ca2Y}[Sc2](AlSi)3O12:Ce, {Ca2Y}[Sc2](GaSi2)O12:Ce, {Ca2Y}[ScGa] (GaSi2)O12:Ce and {Ca2Y}[ScGa](AlSi2)O12:Ce ceramic samples at 1000x magnification are presented in Fig. 2a-d, respectively. The chemical composition of ceramics was determined in 5–10 different parts of the samples using EDX analysis with the help of IXRF 500 and LN2 Eumex detectors mounted on the above mentioned microscope. The measurement inaccuracy was in the ± 1% range and the results from different regions are averaged. The determined by EDX compositions of ceramics presented in Fig. 2 were Ca1.95Y1.07Ce0.04Sc1.81Al1.18Si1.95O12, Ca2.05Y0.95Ce0.03Sc2.02Ga0.95Si2O12, Ca2.25Y0.715Ce0.025 Sc0.9 Ga1.94 Si2.17O12 and Ca2.05Y0.95Ce0.03Sc0.955Ga0.95Al1.055Si2.01O12 (Fig. 2e-h, respectively). As can be seen from Fig. 2a-d, the substitution of Al3+ by Ga3+ cations leads to the stabilization of the conditions of garnet phase formation and to uniformity of the grain dimensions and smaller grain size in {Ca2Y}[ScGa](GaSi2)O12:Ce and {Ca2Y}[ScGa](AlSi2)O12:Ce samples (Fig. 2c, d, respectively). At the largest Ga content, the surface of {Ca2Y}[ScGa] (GaSi2)O12:Ce ceramic looks monolithic without visible separation on the grains (Fig. 2c). The structure type and phase purity of the synthesized ceramic samples were characterized with conventional powder X-ray diffraction (XRD) technique. The powder XRD patterns were obtained by using a Bruker D8 Advance X-Ray powder diffractometer with Cu Kα radiation. Identification of synthesized compounds, indexing of X-ray powder diffraction patterns and refinement of unit cell parameters were performed with the Diffrac. Suite. EVA software (Bruker). Unit cell parameters (Table 1) were determined with an accuracy of around 0.001 Å. X-ray phase analysis has confirmed that all the synthesized compounds are single-phase samples containing only the garnet phases and no XRD reflections corresponding to any impurity have been detected. In particular, XRD patterns of garnets {Ca2Y}[Sc2](ASi2)O12:Ce and {Ca2Y} [ScGa](ASi2)O12:Ce (A = Al, Ga) are shown in Fig. 1a. All the XRD patterns of synthesized samples have been indexed on the basis of a cubic unit cell with the lattice constants summarized in Table 1.
3.1. Ca2RSc2AlSi2O12:Ce (R = Lu, Y, Gd) garnets The CL and PL spectra of Ca2RSc2AlSi2O12:Ce (R = Lu, Y, Gd) garnets are presented in Figs. 4a and 5, respectively. These spectra show the dominant doublet emission bands, peaked in the green range at 506–520 nm, related to the allowed 5d1-4f (2F5/2,7/2) transitions of Ce3+ ions in the garnet hosts. The CL spectrum of Ca2GdSc2AlSi2O12:Ce sample contains also the luminescence of Gd3+ ions at 313 nm, which is typical for the diluted Gd3+ containing materials (Fig. 4, curve 3). Apart from the Ce3+ emission band in the visible range, the CL spectra of all the investigated ceramic samples consist of the complex emission bands in the UV range peaking between 382 and 390 nm (Fig. 4a). This UV emission band is dominating also in the CL spectrum of the nominally undoped Ca2GdSc2AlSi2O12 ceramic (Fig. 4a, curve 1). The nature of this complex band is considered in several papers [15,16]. Firstly, the emission band peaked at 375 nm, was observed in the spectra of slow emission component of undoped Ca3Sc2Si3O12 ceramic [16,17]. This luminescence with a decay time of 5.5 μs is excited in the broad band with maximum at 200–206 nm [17]. The authors [16] connected this host emission with “near defect excitons”, namely with the luminescence of excitons localized around defects or direct electron–hole recombination on defects with formation of bound excitons. In our opinion, such host-type emission can be related to the luminescence of excitons localized around or bound to F-centers (oxygen vacancy with two trapped electrons) because their behavior is very close to that of the F center related luminescence in YAG [18,19], YAP [19,20] and Al2O3 crystals [19,21]. Because the mentioned spectral features of the UV emission of Ca2RSc2AlSi2O12:Ce (R = Lu, Y, Gd) hosts are observed mainly under high energy excitation (Fig. 4), they do not interfere with our study of the Ce3+ luminescence in the mentioned garnet ceramics for their possible application in WLED convertors. Under excitation in the Ce3+ absorption band at 410 nm, the PL spectra of Ca2LuSc2Al Si2O12:Ce ceramic samples demonstrate the dominant Ce3+ luminescence band peaked at 534 nm (Fig. 5a, curve 1). Substitution of Lu3+ cations by Y3+ and Gd3+ leads to changing the crystal field strength in the dodecahedral positions of garnet structure, where the Ce3+ ions are localized, which is proportional to the ΔE = E2-E1 values (Table 1). This results in a notable shift of the long-wavelength wings of the Ce3+ emission spectra and increasing their FWHM in Ca2GdSc2 AlSi2O12:Ce and Ca2YSc2AlSi2O12:Ce ceramic 329
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Fig. 2. (a-d) - electronic microscope images of surfaces of 2275 {Ca2Y}[Sc2](AlSi2)O12:Ce (a), 2262 {Ca2Y}[Sc2](GaSi2)O12:Ce (b), 2276 {Ca2Y}[ScGa](GaSi2)O12:Ce (c) and 2277 {Ca2Y} [ScGa] (AlSi2)O12:Ce (d) ceramic plates at magnification 1000x. (e-h) - EDX spectra of ceramic garnet samples with nominal contents of {Ca2Y}[Sc2](AlSi2)O12:Ce (3 at%), {Ca2Y}[Sc2] (GaSi2)O12:Ce (3 at%), {Ca2Y}[ScGa](GaSi2)O12:Ce (3 at%) and {Ca2Y}[ScGa](AlSi2)O12:Ce (3 at%), respectively.
Fig. 3. (a) - XRD pattern of {Ca2Y}[Sc2](AlSi2)O12:Ce (1), {Ca2Y} [Sc2](GaSi2)O12:Ce (2), {Ca2Y}[ScGa](GaSi2)O12:Ce (3) and {Ca2Y}[ScGa](AlSi2)O12:Ce (4) ceramic samples. (b) – detail fragment of XRD pattern around (400) peak.
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Table 1 Lattice parameters and luminescent properties of {Ca2Y}[Sc,Ga]2(Al,Ga,Si)3O12:Ce garnet ceramics under study. λmax(CL) and λmax(PL) – maxima of Ce3+ CL and PL emission bands, E1 and E2 - maxima of Ce3+ PL excitation bands, ΔE = E2-E1. Number of sample
Compounds
a, Å
λmax (CL), nm
λmax (PL), nm
E2, E1, nm
ΔE, eV
Decay time of PL τ1 / τ2, ns
FWHM PL band, eV
Stocks shift, eV
2278 2275 2279 2269 2262 2260 2276 2277
Ca2LuSc2AlSi2O12 Ca2YSc2AlSi2O12 Ca2GdSc2AlSi2O12 Ca2LuSc2GaSi2O12 Ca2YSc2GaSi2O12 Ca2GdSc2GaSi2O12 Ca2YScGa2Si2O12 Ca2YScGaAlSi2O12
12.213 12.226 12.254 12.262 12.306 12.324 12.228 12.165
515 506 520 511 508 516 515 516
534 535 532 535 534 531 536 541
326; 343; 344; 343; 317; 344; 325; 324;
1.008 0.829 0.848 0.821 1.104 0.823 1.056 1.088
15/50 9/46 12/47 14/47 13/49 12/46 13/45 15/53
0.57 0.59 0.61 0.64 0.6 0.59 0.67 0.62
0.470 0.456 0.424 0.474 0.483 0.445 0.441 0.445
444 447 450 444 442 446 450 453
perturbed and the change of crystalline field can be relatively small. Another charge compensation mechanism takes place at substitution by Ce3+ ions of Ca2+ sites. In this case the crystalline field around Ce3+ is strongly perturbed by different distributions of the Al3+ and Si4+ cations, located in the tetrahedral sites in the second coordination sphere and change of the crystal field strength can probably be higher.
samples (Fig. 5a, curves 2 and 3 and Table 1). The excitation spectra of Ca2LuSc2AlSi2O12:Ce ceramic sample show the two E1 and E2 bands related to the 4f-5d1,2 transitions of Ce3+ ions (Fig. 5b, curve 1). We have also noted that the substitution of dodecahedral positions of garnet host by Y3+ and Gd3+ leads to increase of the probability of the 4f-5d1 transitions, and in Ca2GdSc2AlSi2O12:Ce and Ca2YSc2AlSi2O12:Ce samples the UV excitation band E2 of Ce3+ ions is practically negligible (Fig. 5b, curves 2 and 3). Without any doubt, this behavior of the mentioned garnets is more useful for the creation of blue LED excited garnet convertors. The decay kinetics of Ca2RSc2AlSi2O12:Ce (R = Lu, Y and Gd) ceramic samples at RT are presented in Fig. 6. All the decay curves have different form which can be well approximated by the two-exponential approximations I(t) = A1exp(t/τ1) + A2exp(t/τ2) + const with decay times in the τ1 = 8–15 ns and τ2 = 46–50 ns ranges. The second decay time is typical for the lifetime of the Ce3+ luminescence in the garnet compounds [5,15–17]. Usually, the non-exponential form of the decay curves and presence of the fast component of the cerium luminescence in the ns range in the silicate garnet compounds are related to the formation of the Ce3+ multicenters. Generally, the Ce3+ multicenters can be formed in the dodecahedral positions of Ca2RSc2 AlSi2O12:Ce (R = Lu, Y and Gd) garnets due to large local structural disordering related to the substitution of Ce3+ ions for cations (Ca2+ and R3+) at sites with various dimensions and charge compensation mechanisms. The formation of the Ce3+ multicenters in the Si2+-Mg2+-Si4+ based garnets was observed recently in several works, namely in Lu2CaMg2(Si,Ge)3O12 [5], Y3Al5−2x MgxSixO12:Ce [22], Ca2YMgScSi3O12:Ce ceramic [15] as well as in Ca2YMgScSi3O12:Ce single crystalline films [23]. Therefore, we can assume the formation of these centers occurs also in {Ca2R} [Sc,Ga]2(Ga,Al,Si2)O12, R = Y, Gd, Lu ceramic samples. Namely, the charge compensation requirement favours the replacement of Ce3+ ions at R3+ sites. In this case the local surrounding around Ce3+ ions is less
3.2. Ca2RSc2GaSi2O12:Ce (R = Lu, Y, Gd) garnets The CL spectra of Ce3+ doped Ca2RSc2GaSi2O12:Ce (R = Lu, Y, Gd) and undoped Ca2YSc2Ga Si2O12 ceramics are shown in Fig. 7a and b, respectively. Apart from the Ce3+ luminescence in the visible range, the CL spectra of all the investigated ceramic samples consist of the emission bands of defect centers in the UV range peaked around 400 nm (Fig. 4a). This UV emission band prevails in the CL spectrum of nominally undoped Ca2YSc2AlSi2O12 ceramic (Fig. 7b, curve 1). Meanwhile, incomparison with Ca2RSc2AlSi2O12:Ce (R = Lu, Y, Gd) ceramic (Fig. 3a), the substitution of Al3+ cations by Ga3+ ions leads to the notable decrease of the concentration of defect related centers, emitting in the UV range, especially for Ca2LuSc2AlSi2O12:Ce and Ca2GdSc2Al Si2O12:Ce ceramic samples (Fig. 7a, curves 1 and 3). The PL spectra of Ca2RSc2AlSi2O12:Ce (R = Lu, Y, Gd) do not show significant changes at substitution of the dodecahedral positions by Lu, Y and Gd cations (Fig. 8a) as compared to Al containing garnets (Fig. 6a). The excitation spectra also do not show the significant variations at replacing these cations in Ca2RSc2GaSi2O12:Ce (R = Lu, Y, Gd) garnets. We have also observed low intensity of the UV excitation bands of the Ce3+ luminescence in Ga3+ containing garnets (Fig. 3b, curves 2 and 3) indicating the dominant radiative de-excitation from the lowest 5d1 level of these ions. We have also revealed the important behavior of Sc3+ and Ga3+ containing garnets. As opposed to situation in the Ga3+ and Sc3+ substituted YAG garnets [24,25], we do not observe quenching of the
Fig. 4. (a) - CL spectra of Ca2RSc2AlSi2O12:Ce, R = Lu (1), Y (2) and Gd (3) ceramic samples at RT. (b) – comparison of the CL spectra of nominally undoped (1) and Ce3+ doped (2) Ca2YSc2AlSi2O12 ceramic samples.
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Fig. 5. Emission (a) and excitation (b) spectra of Ce3+ luminescence in Ca2LuSc2AlSi2O12:Ce (1), Ca2GdSc2AlSi2O12:Ce (2) and Ca2YSc2AlSi2O12:Ce (3) ceramics at RT under excitation and registration of emission at 410 and 580 nm, respectively.
leading to narrowing of the band gap of garnet host [26,27]. The decay kinetics of the Ce3+ luminescence in Ca2RSc2GaSi2O12:Ce, R = Lu, Y, Gd ceramics at RT under excitation in the vicinity of Ce3+ absorption/excitation bands at 404 nm is shown in Fig. 9. Similarly to the PL spectra, the decay kinetics of Ca2RSc2GaSi2O12:Ce (R = Lu, Y, Gd) ceramic samples also do not show significant changes at replacing the cations in the dodecahedral position of the garnet host. Meanwhile, the non-exponential decay kinetics with two decay times in the τ1 = 12–14 and τ2 = 46–49 ns ranges most probably indicate the Ce3+ multicenter formation in the ceramic samples of these garnets. 3.3. Ca2YScGaAlSi2O12:Ce and Ca2YScGaGaSi2O12:Ce garnets The CL of Ca2YScGa2Si2O12:Ce and Ca2YScGaAlSi2O12:Ce garnet ceramic samples show the dominant Ce3+ luminescence at 516 nm at a very low content of the defect related centers, emitting in the UV range, especially in the first garnet compound. This indicates the stabilized influence of Sc-Ga doping on the solid-state reaction of single phase phosphor preparation (Fig. 2) (Fig. 10). The PL emission and excitation spectra of Ce3+ luminescence in Ca2YScGaGaSi2O12:Ce and Ca2YScGaGaSi2O12:Ce ceramics at RT are shown in Fig. 11. As can be seen from the mentioned spectra, the substitution of Al3+ cations in the tetrahedral sites of garnet lattice by Ga3+ ions leads to the notable blue shift of the Ce3+ emission band from 535 nm to 540 nm (Fig. 11a) and to a decrease of the energy gap between the positions of E1 and E2 bands in the excitation spectra of Ce3+ luminescence. It is worth mentioning that intensity of the UV
Fig. 6. Decay kinetics of Ce3+ luminescence in Ca2LuSc2AlSi2O12:Ce (1), Ca2YSc2AlSi2O12:Ce (2) and Ca2GdSc2AlSi2O12:Ce (3) ceramics at RT under excitation in Ce3+ absorption band at 404 nm (a, curves 1–3; b, c and d, curves 1).
Ce3+ luminescence in the Ca2+-Si4+ garnets even at significantly larger content of Sc3+ and Ga3+ ions which is equal to 2.0 formula units. This indicates that the strong increase of the crystal field strength in the dodecahedral sites of garnet lattice due to doping by large Ca2+ ions (Table 1) enables to keep the Ce3+ radiative levels inside the band gap of the garnet host and results in the bright Ce3+ luminescence in Ca2RSc2AlSi2O12:Ce:Ce and Ca2RSc2GaSi2O12:Ce, R = Lu, Y and Gd ceramic samples even at the large content of Sc3+ and Ga3+ ions
Fig. 7. (a) - normalized CL spectra of Ca2RSc2GaSi2O12:Ce, R = Lu (1), Y (2) and Gd (3) ceramic samples at RT; (b) – comparison of the normalized CL spectra of nominally undoped and Ce3+ doped (3%) Ca2YSc2GaSi2O12:Ce ceramic samples at RT.
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Wavelength (nm)
Wavelength (nm)
Fig. 8. Emission (a) and excitation (b) spectra of Ce3+ luminescence in Ca2LuSc2GaSi2O12:Ce (1), Ca2YSc2GaSi2O12:Ce (2), Ca2GdSc2GaSi2O12:Ce (3) ceramics at RT.
of Al3+ cations in the tetrahedral sites by Ga3+ ions leads to the strong acceleration of the decay kinetics of the Ce3+ luminescence in Ca2YScGa GaSi2O12:Ce ceramics (Fig. 12, curve 2). Namely, the values of the decay times τ1 and τ2 for the last sample notably decrease to 13 and 45 ns in comparison with the ones of the first ceramics. Such an acceleration of the decay kinetics can be caused by the strong influence of the Ga3+ states in the lowering of the bottom of the conductive band of garnets and decreasing the band gap of garnet. Finally this can result in increasing of the probability of thermally activated transitions from the excited levels of Ce3+ ions to the conductive band in the Ca2YScGa GaSi2O12:Ce garnets and leads to a decreased decay time of the Ce3+ luminescence. 4. Coexistence of Ce3+ and Ce4+ centers in Ca2YSc2BSi2O12:Ce (B = Al, Ga) ceramics Fig. 9. Decay kinetics of Ce3+ luminescence in Ca2LuSc2AGaSi2O12:Ce (1), Ca2YSc2Ga Si2O12 :Ce (2) and Ca2GdSc2AlGaSi2O12:Ce (3) ceramics at RT under excitation in the Ce3+ related band at 404 nm and registration of luminescence at 510 nm.
It should be noted here that oxidizing conditions of the Ca2RSc2BSi2O12:Ce (R = Y, Lu, Gd; B = Al, Ga) ceramic sample preparation and the local structural disordering around cerium ions can lead to the formation of the Ce4+ ions and coexistence of Ce3+ and Ce4+ centers. In this case the local charge compensation requires the localization of Ce4+ ions around one or two Ca2+ cations surrounded by Sc3+ and Al3+ or Ga3+ cations at the octahedral and tetrahedral sites of garnet host, respectively. Formation of the Ce4+ centers is also strongly reflected in the optical properties of the Ca2+-Si4+ ceramics, especially in the decay kinetics of Ce3+ luminescence. For confirmation of this conclusion, we investigate the influence of thermal treatment (TT) in air (curves 1) and reducing CO (curves 2) atmospheres at 1300°C on the decay kinetic of Ce3+ luminescence in Ca2YSc2AlSi2O12:Ce (a) and Ca2YSc2GaSi2O12:Ce (b) ceramic samples under excitation at 404 nm (Fig. 13). Generally, the annealing in the reducing CO and air atmospheres can result in the different content of Ce3+ and Ce4+ centers of the ceramic samples and can lead also to the respective changes in their decay kinetics. As can be seen from Fig. 13, the treatment of ceramics in CO atmosphere provides a more flat shape of the decay curves of the Ce3+ luminescence in both garnet samples. That indicates the dominant contribution of the intrinsic transitions of Ce3+ ions in the PL decay kinetics of Ca2YScAlSi2O12:Ce and Ca2YScGaSi2O12:Ce ceramics after TT in CO reducing atmosphere (curves 2). Meanwhile, the annealing in air probably leads to a decreasing content of Ce3+ centers and increasing the content of Ce4+ centers in ceramic samples, especially for the Ca2YScAlSi2O12:Ce garnet (Fig. 13a, curve 1). Generally, the presence of the fast component of the cerium luminescence in the τ1 = 8–15 ns range is typical for the garnet compounds co-doped by two valence ions A2+ = Ca2+, Mg2+ and can 9be caused by co-existence of the Ce3+ and Ce4+ states (see, for example
Fig. 10. CL spectra of Ca2YScGa2Si2O12:Ce (1) and Ca2YScGaAlSi2O12:Ce (2) ceramics at RT.
excitation band E2 is practically negligible in Ca2YScGaGaSi2O12:Ce ceramics. The decay kinetics of the Ce3+ luminescence in the Ca2YScGaGaSi2O12:Ce and Ca2YScGaAl Si2O12:Ce ceramic samples under excitation in Ce3+ related band at 404 nm is shown in Fig. 12. Similarly to other garnet samples, the decay curves of Ca2YScGaAlSi2O12:Ce ceramics can be presented by the two components with decay times of τ1 = 15 ns and τ 2 = 53 ns, most probably related to the Ce3+ multicenter formation. The additional substitution 333
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Fig. 11. PL emission (a) and excitation (b) spectra of a Ca2YScGaGaSi2O12:Ce (1) and Ca2YSc GaGaSi2O12:Ce (2) ceramics at RT excited at 440 nm (a) and detected at 540 nm (b).
[28,29]. For this reason, under 304 and 404 nm excitation (Fig. 14), we can observe the luminescence of Ce3+ ions excited both via intrinsic absorption transitions of these ions and via the charge transformation of Ce4+ ions: Ce4+ + hv(304, 404 nm) → (Ce3+)* + p → Ce3+ (534 nm) + p → Ce4+ (see also reference [15]). In the frame of this assumption, the Ce4+ centers can be responsible for the presence of the fast components of the cerium luminescence with lifetime in the 15–35 ns range in Ca2YSc2AlSi2O12:Ce (a) and Ca2YSc2Ga Si2O12:Ce ceramics after TT in air atmosphere under excitation at 304 and 404 nm (Figs. 13 and 14). Faster decay kinetics is observed for Ca2YSc2AlSi2O12:Ce ceramic, especially under excitation at 304 nm in range closer to the maximum of O2–→Ce4+ CTT band whereas in the case of 404 nm excitation the decay curve is more flat (Fig. 14, curves 1 and 2, respectively). Meanwhile, for the Ca2Y3Sc2GaSi2O12:Ce ceramic the decay curves of the Ce3+ luminescence are less perturbed by the expected co-existence with Ce4+ ions (Fig. 14a). The slower components of the luminescence in these garnets with decay time in the 53–73 ns range are related to the intrinsic radiative transitions of Ce3+ ions.
Fig. 12. Decay kinetics of Ce3+ luminescence in Ca2YScGaGaSi2O12:Ce (1) and Ca2YScGaAl Si2O12:Ce (2) ceramic samples under excitation in Ce3+ related band at 404 nm and registration of luminescence at 510 nm at RT.
[15,28,29] and references therein). The presence of Ce4+ ions as very effective electron trapping centers can strongly accelerate the decay of the Ce3+ luminescence in the case of excitation with the energy above garnet band gap or with the energy close to the onset of the O2- - Ce4+ charge transfer transitions (CTT) [28,29]. The presence of O2–→Ce4+ transitions is also possible under excitation in the vicinity of UV and visible 4 f-5d1 absorption bands of Ce3+ ions, due to the extension of the long-wavelength wing of the mentioned CTT absorption band peaked approximately at 240 nm in the garnets even up to 450 nm
5. Conclusions The crystallization of ceramic phosphors based on the Ce3+ doped {Ca2R}[Sc,Ga](Al,Ga, Si2)O12 (R = Lu, Y, Gd) garnets and investigation of their luminescent properties were firstly performed in this work. We have observed some trends in variations of the spectroscopic behavior of the mentioned Ce3+ doped Ca2+-Si4+ based garnets due to the substitution of the dodecahedral {} sites of garnet host by Lu3+, Y3+ and Gd3+ ions, octahedral [] sites by Sc3+ and Ga3+ ions and
Fig. 13. Decay kinetics of Ce3+ luminescence in Ca2YSc2AlSi2O12:Ce (a) and Ca2YSc2Ga Si2O12:Ce (b) ceramic samples, annealed in air (1) and CO (2) atmosphere at 1300°C under excitation in Ce3+ related band at 404 nm and registration of luminescence at 510 nm at RT.
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Time (ns)
Time (ns)
Fig. 14. Decay kinetics of Ce3+ luminescence in Ca2YSc2AlSi2O12:Ce (a) and Ca2YSc2Ga Si2O12:Ce (b) ceramic samples, annealed in air at 1300 °C under excitation at 304 nm (1) and 404 nm (2) and registration of luminescence at 510 nm at RT.
tetrahedral ( ) sites by Al3+ and Ga3+ ions, which can be suitable for the development of white converters based on the compounds under study. The cathodoluminescence spectra of Ca2YSc2Ga Si2O12:Ce, Ca2GdSc2GaSi2O12:Ce, Ca2YScGa2Si2O12:Ce and Ca2YScGaAlSi2O12:Ce garnet ceramic samples show the dominant Ce3+ luminescence in the green-yellow range and very low content of the defect related centers, emitting in the UV range. This indicates the stabilizing influence of ScGa doping on the solid-state reaction of single phase garnet phosphor preparation. The observed variation of photoluminescence spectra under different excitation wavelengths and non-exponential decay kinetics of the cerium photoluminescence in the {Ca2R}[Sc,Ga](Al,Ga, Si2)O12 (R = Lu, Y, Gd) garnet compounds under study can probably be connected mainly with the Ce3+ multicenters formation due to the large structural disordering in the decahedral positions of garnet hosts related to the substituting of different (Ca2+ and R3+) cations with various dimensions and charge states and different local surrounding in the second coordination sphere, formed by A3+ (A = Sc, Al, Ga) and Si4+ cations at the octahedral and tetrahedral sites. Due to the Ce3+ multicenter formation, the non-exponential decay kinetics of the Ce3+ emitting centers is observed in {Ca2R} [Sc,Ga](Al,Ga,Si2)O12 (R = Lu, Y, Gd) ceramic samples under excitation in the different Ce3+ related absorption bands. The formation of the Ce4+ and Ce3+ valence states is also expected in the Ca2+-Si4+ based garnets due to the non-uniformity of local surrounding of cerium ions in the dodecahedral positions of the garnet host. Such an assumption can be also confirmed by comparative investigation of the decay kinetics of the Ce3+ luminescence in the ceramic samples under study annealed in the reducing CO and air atmospheres. Specifically, we have supposed that the Ce4+ centers can be responsible for the visible acceleration of non-exponential decay kinetics of the Ce3+ luminescence in the Ca2YSc2Al Si2O12:Ce and Ca2YSc2GaSi2O12:Ce ceramics, annealed in air atmosphere, and for the presence of the fast components of the cerium luminescence with the lifetime in the 9–16 ns range under excitation in the range of the onset of O2-→Ce4+ charge transfer transitions. Meanwhile, the decay component of the Ce3+ luminescence in these garnets with the lifetime in the 46–53 ns range is related to the radiative transitions of Ce3+ multicenters in the garnet hosts. The future detailed spectroscopic research of {Ca2R} [Sc,Ga]2(Ga,Al,Si2)O12, R = Y, Gd, Lu silicate garnets, having the dodecahedral sites for simultaneous localization of Ce3+ ions and other rare-earth and transition metal ions in the different valence states, can
open rich perspectives for designing the novel generations of garnet phosphors with the wide possibilities for effective tuning of the blue, green and red components of white LED converters, which can be excited both by the blue or near UV light. Acknowledgements This work was realized within NANOLUX 2014 # 286 project in the ERA NET RusPlus S & T program and partly in frame of Polish NCN 2016/21/B/ST8/03200 project. References [1] Solid-State Lighting R & D. Multi-Year Program Plan. US Energy Depart, Apr, 2014. [2] Ching-Cherng Sun, Yu-Yu Chang, Tsung-Hsun Yang, Te-Yuan Chung, ChengChien Chen, Tsung-Xian Lee, Dun-Ru Li, Chun-Yan Lu, Zi-Yan Ting, Benoit Glorieux, Yi-Chun Chen, Kun-Yu Lai, Cheng-Yi Liu, J. Sol. State Light 1 (2014) 19. [3] M. Raukas, J. Kelso, Y. Zheng, K. Bergenek, D. Eisert, A. Linkov, F. Jermann, ECS J. Solid State Sci. Technol. 2 (2013) R3168. [4] Patents # US7554258; US7879258; US8123981; US8137587; US8298442; US8339025; US8496852; US8664678; US8728835; US0313603. [5] Anant A. Setlur, William J. Heward, Yan Gao, Alok M. Srivastava, R. Gopi Chandran, Madras V. Shankar, Chem. Mater. 18 (2006) 3314. [6] Yasuo Shimomura, Tetsuo Honma, Motoyuki Shigeiwa, Toshio Akai, Kaoru Okamoto, Naoto Kijima, J. Electrochem. Soc. 154 (2007) J35. [7] A. Katelnikovas, H. Bettentrup, D. Uhlich, S. Sakirzanovas, T. Jüstel, A. Kareiva, J. Lumin. 129 (2009) 1356. [8] A. Katelnikovas, T. Bareika, P. Vitta, T. Jüstel, H. Winkler, A. Kareiva, A. Žukauskas, G. Tamulaitis, Opt. Mater. 32 (2010) 1261. [9] A. Katelnikovas, J. Jurkevičius, K. Kazlauskas, P. Vitta, T. Jüstel, A. Kareiva, A. Žukauskas, G. Tamulaitis, J. Alloy. Compd. 509 (2011) 6247. [10] A. Katelnikovas, J.M. Ogiegło, H. Winkler, A. Kareiva, T. Jüstel, J. Sol.-Gel Sci. Technol. 59 (2011) 311. [11] A. Katelnikovas, J. Plewa, D. Dutczak, S. Möller, D. Enseling, H. Winkler, A. Kareiva, T. Jüstel, Opt. Mater. 34 (2012) 1195. [12] A. Katelnikovas, S. Sakirzanovas, D. Dutczak, J. Plewa, D. Enseling, H. Winkler, A. Kareiva, T. Jüstel, J. Lumin. 136 (2013) 17. [13] Jiyou Zhong, Weidong Zhuang, Xianran Xing, Ronghui Liu, Yanfeng Li, Yuanhong Liu, Yunsheng Hu, J. Phys. Chem. C. 119 (2015) 5562. [14] Zaifa Pan, Yu Xu, Qingsong Hu, Weiqiang Li, Huan Zhou, Yifan Zheng, RSC Adv. 5 (2015) 9489. [15] N. Khaidukov, Yu Zorenko, T. Zorenko, A. Iskaliyeva, K. Paprocki, Ya Zhydachevskii, R. Van Deun, M. Batentschuk, Phys. Status Solidi (RRL) (2017), http://dx.doi.org/10.1002/pssr.201700016. [16] K.V. Ivanovskikh, A. Meijerink, F. Piccinelli, A. Speghini, E.I. Zinin, C. Ronda, M. Bettinelli, J. Lumin. 130 (2010) 893. [17] Lei Zhou, Weijie Zhou, Fengjuan Pan, Rui Shi, Lin Huang, Hongbin Liang, Peter A. Tanner, Xueyan Du, Yan Huang, Ye Tao, Lirong Zheng, Chem. Mater. 28 (2016) 2834. [18] Yu. Zorenko, T. Zorenko, T. Voznyak, A. Mandowski, Qi Xia, M. Batentschuk, J. Fridrich, IOP Conference Series: Materials Science and Engineering. 15 2060, 2010. [19] Yu Zorenko, T. Zorenko, T. Voznyak, J. Phys.: Conf. Ser. 289 (2011) 012028.
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Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Synthesis and luminescent properties of prospective Ce3+ doped silicate garnet phosphors for white LED converters
MARK
⁎
N. Khaidukova, , T. Zorenkob, A. Iskaliyevab, K. Paprockib, M. Batentschukc, A. Osvetc, ⁎ R. Van Deund, Ya. Zhydaczevskiie, A. Suchockie, Yu. Zorenkob, a
Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, 119991 Moscow, Russia Institute of Physics, Kazimierz Wielki University in Bydgoszcz, 85090 Bydgoszcz, Poland c Institute of Materials for Energy and ElectronicTechnology (i-IMEET), Department of Materials Science and Engineering VI, University of Erlangen-Nuremberg, 91058 Erlangen, Germany d L3 – Luminescent Lanthanide Lab, Department of Inorganic and Physical Chemistry, Ghent University, 9000 Gent, Belgium e Institute of Physics, Polish Academy of Sciences, 02-668 Warsaw, Poland b
A R T I C L E I N F O
A B S T R A C T
Keywords: Luminescence LED ceramic converters Ca-Si based garnet Ce3+ dopant
The results on crystallization and investigation of the luminescent properties of prospective ceramic phosphors based on the Ce3+ doped {Ca2R}[ScB](CSi2)O12 (R = Lu, Y, Gd; B = Sc, Ga, C = Ga, Al) silicate garnets are presented for the first time in this work. We have observed the variations of the spectroscopic properties of Ce3+ ions in the mentioned Ca2+-Si4+ garnet hosts depending on the cation content at the dodecahedral {}, octahedral [] and tetrahedral () sites of garnet lattice. These results can be useful for the development of new generation of ceramic phosphor converters for white LEDs based on the garnet compounds under study.
1. Introduction In the last two decades the lamps based on the white light emitting diodes (WLED) have displaced the traditional light sources due to their advantages including high luminous efficiency, energy saving, long lifetime and environmental friendliness. The rate of such a displacement for various applications such as backlighting for displays, automotive and general lighting depends on developing more powerful blue and near UV emitting chips, more efficient phosphors and new schemes for conversion of chip radiation to the white light [1]. At present, a WLED source, manufactured on the basis of a blue LED chip and the yellow emitting YAG:Ce powder phosphor dispersed in epoxy and silicone resin, is a canonical device [2]. While a large number of other different phosphors have been developed to date, YAG:Ce is still the most popular phosphor now for producing the WLED [1,2]. In this context, it should be noted that applying YAG:Ce ceramic or crystal phosphor plates for light conversion in the white LED is now also accessible for manufacturing of high power WLEDs [3]. Therefore, the ceramic phosphors based on different compositions of Ce3+ doped Ln3Al5O12 garnets and techniques for obtaining such phosphor ceramics have been patented repeatedly as well [4]. Due to the flexibility of the garnet structure, which allows replacing ions at the dodecahedral { }, octahedral [ ] and tetrahedral ( ) sites, it is
⁎
possible to conveniently replace the host cations and modify the {Y}3[Al]2(Al)3O12 garnet composition for altering the Ce3+ spectroscopic properties to better meet the requirements for utilization in WLED. To date the spectroscopic properties of Ce3+ in some garnets containing Si4+ at tetrahedral sites, namely Y3Mg2AlSi2O12, Y3MgAl3SiO12, CaY2Al4Si O12, MgY2MgAl2Si2O12, CaLu2Al4SiO12, CaLu2Mg2Si3O12, Ca2YMgScSi3O12 and Ca3Sc2Si3O12 garnets have been published [5–15]. Namely, it has been shown that Ca3Sc2Si3O12:Ce exhibits less thermal quenching than YAG:Ce [5]. At the same time, there is no information concerning the spectroscopic properties of Ce3+ in silicate garnets of the {Ca2Y}[Sc,Al,Ga]2(Ga,Al,Si)3O12 family [5–15]. For this reason, this work is devoted to crystallization and investigation of the luminescent properties of phosphors based on the Ce3+ doped {Ca2R}[Sc,B](Ca,Si2)O12; R = Lu, Y, Gd; B = Sc, Ga, C = Ga, Al silicate garnets, which can be used for producing high power WLEDs having high color rendering index and low correlated color temperature values [5,6]. 2. Samples and method of their preparation In this work, we report on the first results on the crystallization of ceramics based on the Ce3+ doped {Ca2R}[ScB]2(CSi2)O12; R = Lu, Y,
Corresponding authors. E-mail addresses: [email protected] (N. Khaidukov), [email protected] (Y. Zorenko).
http://dx.doi.org/10.1016/j.jlumin.2017.06.068 Received 9 January 2017; Received in revised form 26 June 2017; Accepted 29 June 2017 Available online 01 July 2017 0022-2313/ © 2017 Elsevier B.V. All rights reserved.
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It can be seen that there is an increase of the unit cell parameter with the ionic radius of the cations residing in dodecahedral {Lu3+, Y3+, Gd3+}, octahedral [Ga3+, Sc3+] and tetrahedral (Al3+, Ga3+) sites, which determines the peak positions of XRD reflections for these garnets (Fig. 1b). 3. Experimental results The luminescent properties of the silicate garnet phosphors {Ca2R} [Sc,Ga]2(Al,Ga,Si)3O12:Ce, R = Lu, Y, Gd, depending on the cation content, were examined by recording the room temperature (RT) cathodoluminescence (CL) spectra using a SEM JEOL microscope equipped with a Stellar Net grating spectrometer with TE cooled CCD camera working in the 200–1200 nm range, as well as the photoluminescence (PL) emission and PL excitation spectra using a SM 2203 (Solar LTD) spectrofluorometer working in the 200–820 nm range. Decay kinetics of the luminescence of ceramic samples at RT was recorded using a Fluorolog 3 spectrophotometer under excitation in the Ce3+ related absorption bands. The obtained spectroscopic data for {Ca2R} [Sc,Ga]2(Al,Ga,Si)3O12:Ce; R = Lu, Y, Gd were systematized in Table 1 in correlation with the structural features of the garnet hosts resulting in the different parameters of the crystal field strength around Ce3+ ions.
Fig. 1. View of a ceramic phosphor plate based on the Ca2YSc2AlSi2O12:Ce garnet.
Gd; B = Sc, Ga, C = Ga, Al garnets. The nominally undoped Ca2YSc2AlSi2O12 and Ca2YSc2GaSi2O12 ceramic samples were synthesized as well for comparison with luminescent properties of the Ce3+ doped counterparts. The ceramics of these garnets were obtained using the hydrothermal crystallization method (Fig. 1). The precursors of the garnets were synthesized from the NaOH–H2O solutions of CaO, SiO2, R2O3 (R = Y, Gd, Lu) and A2O3 (A = Sc, Ga, Al) oxides with different molar ratios at temperatures of 300–400°C. The purity of initial components was 4N. The nominal concentration of Ce3+ ions was 3 at% with respect to the total content of Ca2+ and Y3+ cations. Then precursors were pressed in tablets and dried in air at 1000–1150°C. Finally, the tablets were annealed: (i) in the reducing (CO) atmosphere at 1300°C for keeping Ce3+ valence state of cerium ions in ceramic samples. For comparison, the tablets of Ca2YSc2AlSi2O12 :Ce and Ca2YSc2GaSi2O12:Ce compounds were also annealed at 1300°C in air for the study of the influence of oxygen-containing atmosphere on formation of Ce4+ and Ce3+ states during the synthesis of the ceramic samples of these garnets. The structural analysis of the ceramic phosphors was performed using scanning electronic microscopy with JEOL JSM-820 (Fig. 2), and XRD measurements (Fig. 3). The images of the surface texture of the {Ca2Y}[Sc2](AlSi)3O12:Ce, {Ca2Y}[Sc2](GaSi2)O12:Ce, {Ca2Y}[ScGa] (GaSi2)O12:Ce and {Ca2Y}[ScGa](AlSi2)O12:Ce ceramic samples at 1000x magnification are presented in Fig. 2a-d, respectively. The chemical composition of ceramics was determined in 5–10 different parts of the samples using EDX analysis with the help of IXRF 500 and LN2 Eumex detectors mounted on the above mentioned microscope. The measurement inaccuracy was in the ± 1% range and the results from different regions are averaged. The determined by EDX compositions of ceramics presented in Fig. 2 were Ca1.95Y1.07Ce0.04Sc1.81Al1.18Si1.95O12, Ca2.05Y0.95Ce0.03Sc2.02Ga0.95Si2O12, Ca2.25Y0.715Ce0.025 Sc0.9 Ga1.94 Si2.17O12 and Ca2.05Y0.95Ce0.03Sc0.955Ga0.95Al1.055Si2.01O12 (Fig. 2e-h, respectively). As can be seen from Fig. 2a-d, the substitution of Al3+ by Ga3+ cations leads to the stabilization of the conditions of garnet phase formation and to uniformity of the grain dimensions and smaller grain size in {Ca2Y}[ScGa](GaSi2)O12:Ce and {Ca2Y}[ScGa](AlSi2)O12:Ce samples (Fig. 2c, d, respectively). At the largest Ga content, the surface of {Ca2Y}[ScGa] (GaSi2)O12:Ce ceramic looks monolithic without visible separation on the grains (Fig. 2c). The structure type and phase purity of the synthesized ceramic samples were characterized with conventional powder X-ray diffraction (XRD) technique. The powder XRD patterns were obtained by using a Bruker D8 Advance X-Ray powder diffractometer with Cu Kα radiation. Identification of synthesized compounds, indexing of X-ray powder diffraction patterns and refinement of unit cell parameters were performed with the Diffrac. Suite. EVA software (Bruker). Unit cell parameters (Table 1) were determined with an accuracy of around 0.001 Å. X-ray phase analysis has confirmed that all the synthesized compounds are single-phase samples containing only the garnet phases and no XRD reflections corresponding to any impurity have been detected. In particular, XRD patterns of garnets {Ca2Y}[Sc2](ASi2)O12:Ce and {Ca2Y} [ScGa](ASi2)O12:Ce (A = Al, Ga) are shown in Fig. 1a. All the XRD patterns of synthesized samples have been indexed on the basis of a cubic unit cell with the lattice constants summarized in Table 1.
3.1. Ca2RSc2AlSi2O12:Ce (R = Lu, Y, Gd) garnets The CL and PL spectra of Ca2RSc2AlSi2O12:Ce (R = Lu, Y, Gd) garnets are presented in Figs. 4a and 5, respectively. These spectra show the dominant doublet emission bands, peaked in the green range at 506–520 nm, related to the allowed 5d1-4f (2F5/2,7/2) transitions of Ce3+ ions in the garnet hosts. The CL spectrum of Ca2GdSc2AlSi2O12:Ce sample contains also the luminescence of Gd3+ ions at 313 nm, which is typical for the diluted Gd3+ containing materials (Fig. 4, curve 3). Apart from the Ce3+ emission band in the visible range, the CL spectra of all the investigated ceramic samples consist of the complex emission bands in the UV range peaking between 382 and 390 nm (Fig. 4a). This UV emission band is dominating also in the CL spectrum of the nominally undoped Ca2GdSc2AlSi2O12 ceramic (Fig. 4a, curve 1). The nature of this complex band is considered in several papers [15,16]. Firstly, the emission band peaked at 375 nm, was observed in the spectra of slow emission component of undoped Ca3Sc2Si3O12 ceramic [16,17]. This luminescence with a decay time of 5.5 μs is excited in the broad band with maximum at 200–206 nm [17]. The authors [16] connected this host emission with “near defect excitons”, namely with the luminescence of excitons localized around defects or direct electron–hole recombination on defects with formation of bound excitons. In our opinion, such host-type emission can be related to the luminescence of excitons localized around or bound to F-centers (oxygen vacancy with two trapped electrons) because their behavior is very close to that of the F center related luminescence in YAG [18,19], YAP [19,20] and Al2O3 crystals [19,21]. Because the mentioned spectral features of the UV emission of Ca2RSc2AlSi2O12:Ce (R = Lu, Y, Gd) hosts are observed mainly under high energy excitation (Fig. 4), they do not interfere with our study of the Ce3+ luminescence in the mentioned garnet ceramics for their possible application in WLED convertors. Under excitation in the Ce3+ absorption band at 410 nm, the PL spectra of Ca2LuSc2Al Si2O12:Ce ceramic samples demonstrate the dominant Ce3+ luminescence band peaked at 534 nm (Fig. 5a, curve 1). Substitution of Lu3+ cations by Y3+ and Gd3+ leads to changing the crystal field strength in the dodecahedral positions of garnet structure, where the Ce3+ ions are localized, which is proportional to the ΔE = E2-E1 values (Table 1). This results in a notable shift of the long-wavelength wings of the Ce3+ emission spectra and increasing their FWHM in Ca2GdSc2 AlSi2O12:Ce and Ca2YSc2AlSi2O12:Ce ceramic 329
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Fig. 2. (a-d) - electronic microscope images of surfaces of 2275 {Ca2Y}[Sc2](AlSi2)O12:Ce (a), 2262 {Ca2Y}[Sc2](GaSi2)O12:Ce (b), 2276 {Ca2Y}[ScGa](GaSi2)O12:Ce (c) and 2277 {Ca2Y} [ScGa] (AlSi2)O12:Ce (d) ceramic plates at magnification 1000x. (e-h) - EDX spectra of ceramic garnet samples with nominal contents of {Ca2Y}[Sc2](AlSi2)O12:Ce (3 at%), {Ca2Y}[Sc2] (GaSi2)O12:Ce (3 at%), {Ca2Y}[ScGa](GaSi2)O12:Ce (3 at%) and {Ca2Y}[ScGa](AlSi2)O12:Ce (3 at%), respectively.
Fig. 3. (a) - XRD pattern of {Ca2Y}[Sc2](AlSi2)O12:Ce (1), {Ca2Y} [Sc2](GaSi2)O12:Ce (2), {Ca2Y}[ScGa](GaSi2)O12:Ce (3) and {Ca2Y}[ScGa](AlSi2)O12:Ce (4) ceramic samples. (b) – detail fragment of XRD pattern around (400) peak.
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Table 1 Lattice parameters and luminescent properties of {Ca2Y}[Sc,Ga]2(Al,Ga,Si)3O12:Ce garnet ceramics under study. λmax(CL) and λmax(PL) – maxima of Ce3+ CL and PL emission bands, E1 and E2 - maxima of Ce3+ PL excitation bands, ΔE = E2-E1. Number of sample
Compounds
a, Å
λmax (CL), nm
λmax (PL), nm
E2, E1, nm
ΔE, eV
Decay time of PL τ1 / τ2, ns
FWHM PL band, eV
Stocks shift, eV
2278 2275 2279 2269 2262 2260 2276 2277
Ca2LuSc2AlSi2O12 Ca2YSc2AlSi2O12 Ca2GdSc2AlSi2O12 Ca2LuSc2GaSi2O12 Ca2YSc2GaSi2O12 Ca2GdSc2GaSi2O12 Ca2YScGa2Si2O12 Ca2YScGaAlSi2O12
12.213 12.226 12.254 12.262 12.306 12.324 12.228 12.165
515 506 520 511 508 516 515 516
534 535 532 535 534 531 536 541
326; 343; 344; 343; 317; 344; 325; 324;
1.008 0.829 0.848 0.821 1.104 0.823 1.056 1.088
15/50 9/46 12/47 14/47 13/49 12/46 13/45 15/53
0.57 0.59 0.61 0.64 0.6 0.59 0.67 0.62
0.470 0.456 0.424 0.474 0.483 0.445 0.441 0.445
444 447 450 444 442 446 450 453
perturbed and the change of crystalline field can be relatively small. Another charge compensation mechanism takes place at substitution by Ce3+ ions of Ca2+ sites. In this case the crystalline field around Ce3+ is strongly perturbed by different distributions of the Al3+ and Si4+ cations, located in the tetrahedral sites in the second coordination sphere and change of the crystal field strength can probably be higher.
samples (Fig. 5a, curves 2 and 3 and Table 1). The excitation spectra of Ca2LuSc2AlSi2O12:Ce ceramic sample show the two E1 and E2 bands related to the 4f-5d1,2 transitions of Ce3+ ions (Fig. 5b, curve 1). We have also noted that the substitution of dodecahedral positions of garnet host by Y3+ and Gd3+ leads to increase of the probability of the 4f-5d1 transitions, and in Ca2GdSc2AlSi2O12:Ce and Ca2YSc2AlSi2O12:Ce samples the UV excitation band E2 of Ce3+ ions is practically negligible (Fig. 5b, curves 2 and 3). Without any doubt, this behavior of the mentioned garnets is more useful for the creation of blue LED excited garnet convertors. The decay kinetics of Ca2RSc2AlSi2O12:Ce (R = Lu, Y and Gd) ceramic samples at RT are presented in Fig. 6. All the decay curves have different form which can be well approximated by the two-exponential approximations I(t) = A1exp(t/τ1) + A2exp(t/τ2) + const with decay times in the τ1 = 8–15 ns and τ2 = 46–50 ns ranges. The second decay time is typical for the lifetime of the Ce3+ luminescence in the garnet compounds [5,15–17]. Usually, the non-exponential form of the decay curves and presence of the fast component of the cerium luminescence in the ns range in the silicate garnet compounds are related to the formation of the Ce3+ multicenters. Generally, the Ce3+ multicenters can be formed in the dodecahedral positions of Ca2RSc2 AlSi2O12:Ce (R = Lu, Y and Gd) garnets due to large local structural disordering related to the substitution of Ce3+ ions for cations (Ca2+ and R3+) at sites with various dimensions and charge compensation mechanisms. The formation of the Ce3+ multicenters in the Si2+-Mg2+-Si4+ based garnets was observed recently in several works, namely in Lu2CaMg2(Si,Ge)3O12 [5], Y3Al5−2x MgxSixO12:Ce [22], Ca2YMgScSi3O12:Ce ceramic [15] as well as in Ca2YMgScSi3O12:Ce single crystalline films [23]. Therefore, we can assume the formation of these centers occurs also in {Ca2R} [Sc,Ga]2(Ga,Al,Si2)O12, R = Y, Gd, Lu ceramic samples. Namely, the charge compensation requirement favours the replacement of Ce3+ ions at R3+ sites. In this case the local surrounding around Ce3+ ions is less
3.2. Ca2RSc2GaSi2O12:Ce (R = Lu, Y, Gd) garnets The CL spectra of Ce3+ doped Ca2RSc2GaSi2O12:Ce (R = Lu, Y, Gd) and undoped Ca2YSc2Ga Si2O12 ceramics are shown in Fig. 7a and b, respectively. Apart from the Ce3+ luminescence in the visible range, the CL spectra of all the investigated ceramic samples consist of the emission bands of defect centers in the UV range peaked around 400 nm (Fig. 4a). This UV emission band prevails in the CL spectrum of nominally undoped Ca2YSc2AlSi2O12 ceramic (Fig. 7b, curve 1). Meanwhile, incomparison with Ca2RSc2AlSi2O12:Ce (R = Lu, Y, Gd) ceramic (Fig. 3a), the substitution of Al3+ cations by Ga3+ ions leads to the notable decrease of the concentration of defect related centers, emitting in the UV range, especially for Ca2LuSc2AlSi2O12:Ce and Ca2GdSc2Al Si2O12:Ce ceramic samples (Fig. 7a, curves 1 and 3). The PL spectra of Ca2RSc2AlSi2O12:Ce (R = Lu, Y, Gd) do not show significant changes at substitution of the dodecahedral positions by Lu, Y and Gd cations (Fig. 8a) as compared to Al containing garnets (Fig. 6a). The excitation spectra also do not show the significant variations at replacing these cations in Ca2RSc2GaSi2O12:Ce (R = Lu, Y, Gd) garnets. We have also observed low intensity of the UV excitation bands of the Ce3+ luminescence in Ga3+ containing garnets (Fig. 3b, curves 2 and 3) indicating the dominant radiative de-excitation from the lowest 5d1 level of these ions. We have also revealed the important behavior of Sc3+ and Ga3+ containing garnets. As opposed to situation in the Ga3+ and Sc3+ substituted YAG garnets [24,25], we do not observe quenching of the
Fig. 4. (a) - CL spectra of Ca2RSc2AlSi2O12:Ce, R = Lu (1), Y (2) and Gd (3) ceramic samples at RT. (b) – comparison of the CL spectra of nominally undoped (1) and Ce3+ doped (2) Ca2YSc2AlSi2O12 ceramic samples.
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Fig. 5. Emission (a) and excitation (b) spectra of Ce3+ luminescence in Ca2LuSc2AlSi2O12:Ce (1), Ca2GdSc2AlSi2O12:Ce (2) and Ca2YSc2AlSi2O12:Ce (3) ceramics at RT under excitation and registration of emission at 410 and 580 nm, respectively.
leading to narrowing of the band gap of garnet host [26,27]. The decay kinetics of the Ce3+ luminescence in Ca2RSc2GaSi2O12:Ce, R = Lu, Y, Gd ceramics at RT under excitation in the vicinity of Ce3+ absorption/excitation bands at 404 nm is shown in Fig. 9. Similarly to the PL spectra, the decay kinetics of Ca2RSc2GaSi2O12:Ce (R = Lu, Y, Gd) ceramic samples also do not show significant changes at replacing the cations in the dodecahedral position of the garnet host. Meanwhile, the non-exponential decay kinetics with two decay times in the τ1 = 12–14 and τ2 = 46–49 ns ranges most probably indicate the Ce3+ multicenter formation in the ceramic samples of these garnets. 3.3. Ca2YScGaAlSi2O12:Ce and Ca2YScGaGaSi2O12:Ce garnets The CL of Ca2YScGa2Si2O12:Ce and Ca2YScGaAlSi2O12:Ce garnet ceramic samples show the dominant Ce3+ luminescence at 516 nm at a very low content of the defect related centers, emitting in the UV range, especially in the first garnet compound. This indicates the stabilized influence of Sc-Ga doping on the solid-state reaction of single phase phosphor preparation (Fig. 2) (Fig. 10). The PL emission and excitation spectra of Ce3+ luminescence in Ca2YScGaGaSi2O12:Ce and Ca2YScGaGaSi2O12:Ce ceramics at RT are shown in Fig. 11. As can be seen from the mentioned spectra, the substitution of Al3+ cations in the tetrahedral sites of garnet lattice by Ga3+ ions leads to the notable blue shift of the Ce3+ emission band from 535 nm to 540 nm (Fig. 11a) and to a decrease of the energy gap between the positions of E1 and E2 bands in the excitation spectra of Ce3+ luminescence. It is worth mentioning that intensity of the UV
Fig. 6. Decay kinetics of Ce3+ luminescence in Ca2LuSc2AlSi2O12:Ce (1), Ca2YSc2AlSi2O12:Ce (2) and Ca2GdSc2AlSi2O12:Ce (3) ceramics at RT under excitation in Ce3+ absorption band at 404 nm (a, curves 1–3; b, c and d, curves 1).
Ce3+ luminescence in the Ca2+-Si4+ garnets even at significantly larger content of Sc3+ and Ga3+ ions which is equal to 2.0 formula units. This indicates that the strong increase of the crystal field strength in the dodecahedral sites of garnet lattice due to doping by large Ca2+ ions (Table 1) enables to keep the Ce3+ radiative levels inside the band gap of the garnet host and results in the bright Ce3+ luminescence in Ca2RSc2AlSi2O12:Ce:Ce and Ca2RSc2GaSi2O12:Ce, R = Lu, Y and Gd ceramic samples even at the large content of Sc3+ and Ga3+ ions
Fig. 7. (a) - normalized CL spectra of Ca2RSc2GaSi2O12:Ce, R = Lu (1), Y (2) and Gd (3) ceramic samples at RT; (b) – comparison of the normalized CL spectra of nominally undoped and Ce3+ doped (3%) Ca2YSc2GaSi2O12:Ce ceramic samples at RT.
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Wavelength (nm)
Wavelength (nm)
Fig. 8. Emission (a) and excitation (b) spectra of Ce3+ luminescence in Ca2LuSc2GaSi2O12:Ce (1), Ca2YSc2GaSi2O12:Ce (2), Ca2GdSc2GaSi2O12:Ce (3) ceramics at RT.
of Al3+ cations in the tetrahedral sites by Ga3+ ions leads to the strong acceleration of the decay kinetics of the Ce3+ luminescence in Ca2YScGa GaSi2O12:Ce ceramics (Fig. 12, curve 2). Namely, the values of the decay times τ1 and τ2 for the last sample notably decrease to 13 and 45 ns in comparison with the ones of the first ceramics. Such an acceleration of the decay kinetics can be caused by the strong influence of the Ga3+ states in the lowering of the bottom of the conductive band of garnets and decreasing the band gap of garnet. Finally this can result in increasing of the probability of thermally activated transitions from the excited levels of Ce3+ ions to the conductive band in the Ca2YScGa GaSi2O12:Ce garnets and leads to a decreased decay time of the Ce3+ luminescence. 4. Coexistence of Ce3+ and Ce4+ centers in Ca2YSc2BSi2O12:Ce (B = Al, Ga) ceramics Fig. 9. Decay kinetics of Ce3+ luminescence in Ca2LuSc2AGaSi2O12:Ce (1), Ca2YSc2Ga Si2O12 :Ce (2) and Ca2GdSc2AlGaSi2O12:Ce (3) ceramics at RT under excitation in the Ce3+ related band at 404 nm and registration of luminescence at 510 nm.
It should be noted here that oxidizing conditions of the Ca2RSc2BSi2O12:Ce (R = Y, Lu, Gd; B = Al, Ga) ceramic sample preparation and the local structural disordering around cerium ions can lead to the formation of the Ce4+ ions and coexistence of Ce3+ and Ce4+ centers. In this case the local charge compensation requires the localization of Ce4+ ions around one or two Ca2+ cations surrounded by Sc3+ and Al3+ or Ga3+ cations at the octahedral and tetrahedral sites of garnet host, respectively. Formation of the Ce4+ centers is also strongly reflected in the optical properties of the Ca2+-Si4+ ceramics, especially in the decay kinetics of Ce3+ luminescence. For confirmation of this conclusion, we investigate the influence of thermal treatment (TT) in air (curves 1) and reducing CO (curves 2) atmospheres at 1300°C on the decay kinetic of Ce3+ luminescence in Ca2YSc2AlSi2O12:Ce (a) and Ca2YSc2GaSi2O12:Ce (b) ceramic samples under excitation at 404 nm (Fig. 13). Generally, the annealing in the reducing CO and air atmospheres can result in the different content of Ce3+ and Ce4+ centers of the ceramic samples and can lead also to the respective changes in their decay kinetics. As can be seen from Fig. 13, the treatment of ceramics in CO atmosphere provides a more flat shape of the decay curves of the Ce3+ luminescence in both garnet samples. That indicates the dominant contribution of the intrinsic transitions of Ce3+ ions in the PL decay kinetics of Ca2YScAlSi2O12:Ce and Ca2YScGaSi2O12:Ce ceramics after TT in CO reducing atmosphere (curves 2). Meanwhile, the annealing in air probably leads to a decreasing content of Ce3+ centers and increasing the content of Ce4+ centers in ceramic samples, especially for the Ca2YScAlSi2O12:Ce garnet (Fig. 13a, curve 1). Generally, the presence of the fast component of the cerium luminescence in the τ1 = 8–15 ns range is typical for the garnet compounds co-doped by two valence ions A2+ = Ca2+, Mg2+ and can 9be caused by co-existence of the Ce3+ and Ce4+ states (see, for example
Fig. 10. CL spectra of Ca2YScGa2Si2O12:Ce (1) and Ca2YScGaAlSi2O12:Ce (2) ceramics at RT.
excitation band E2 is practically negligible in Ca2YScGaGaSi2O12:Ce ceramics. The decay kinetics of the Ce3+ luminescence in the Ca2YScGaGaSi2O12:Ce and Ca2YScGaAl Si2O12:Ce ceramic samples under excitation in Ce3+ related band at 404 nm is shown in Fig. 12. Similarly to other garnet samples, the decay curves of Ca2YScGaAlSi2O12:Ce ceramics can be presented by the two components with decay times of τ1 = 15 ns and τ 2 = 53 ns, most probably related to the Ce3+ multicenter formation. The additional substitution 333
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Fig. 11. PL emission (a) and excitation (b) spectra of a Ca2YScGaGaSi2O12:Ce (1) and Ca2YSc GaGaSi2O12:Ce (2) ceramics at RT excited at 440 nm (a) and detected at 540 nm (b).
[28,29]. For this reason, under 304 and 404 nm excitation (Fig. 14), we can observe the luminescence of Ce3+ ions excited both via intrinsic absorption transitions of these ions and via the charge transformation of Ce4+ ions: Ce4+ + hv(304, 404 nm) → (Ce3+)* + p → Ce3+ (534 nm) + p → Ce4+ (see also reference [15]). In the frame of this assumption, the Ce4+ centers can be responsible for the presence of the fast components of the cerium luminescence with lifetime in the 15–35 ns range in Ca2YSc2AlSi2O12:Ce (a) and Ca2YSc2Ga Si2O12:Ce ceramics after TT in air atmosphere under excitation at 304 and 404 nm (Figs. 13 and 14). Faster decay kinetics is observed for Ca2YSc2AlSi2O12:Ce ceramic, especially under excitation at 304 nm in range closer to the maximum of O2–→Ce4+ CTT band whereas in the case of 404 nm excitation the decay curve is more flat (Fig. 14, curves 1 and 2, respectively). Meanwhile, for the Ca2Y3Sc2GaSi2O12:Ce ceramic the decay curves of the Ce3+ luminescence are less perturbed by the expected co-existence with Ce4+ ions (Fig. 14a). The slower components of the luminescence in these garnets with decay time in the 53–73 ns range are related to the intrinsic radiative transitions of Ce3+ ions.
Fig. 12. Decay kinetics of Ce3+ luminescence in Ca2YScGaGaSi2O12:Ce (1) and Ca2YScGaAl Si2O12:Ce (2) ceramic samples under excitation in Ce3+ related band at 404 nm and registration of luminescence at 510 nm at RT.
[15,28,29] and references therein). The presence of Ce4+ ions as very effective electron trapping centers can strongly accelerate the decay of the Ce3+ luminescence in the case of excitation with the energy above garnet band gap or with the energy close to the onset of the O2- - Ce4+ charge transfer transitions (CTT) [28,29]. The presence of O2–→Ce4+ transitions is also possible under excitation in the vicinity of UV and visible 4 f-5d1 absorption bands of Ce3+ ions, due to the extension of the long-wavelength wing of the mentioned CTT absorption band peaked approximately at 240 nm in the garnets even up to 450 nm
5. Conclusions The crystallization of ceramic phosphors based on the Ce3+ doped {Ca2R}[Sc,Ga](Al,Ga, Si2)O12 (R = Lu, Y, Gd) garnets and investigation of their luminescent properties were firstly performed in this work. We have observed some trends in variations of the spectroscopic behavior of the mentioned Ce3+ doped Ca2+-Si4+ based garnets due to the substitution of the dodecahedral {} sites of garnet host by Lu3+, Y3+ and Gd3+ ions, octahedral [] sites by Sc3+ and Ga3+ ions and
Fig. 13. Decay kinetics of Ce3+ luminescence in Ca2YSc2AlSi2O12:Ce (a) and Ca2YSc2Ga Si2O12:Ce (b) ceramic samples, annealed in air (1) and CO (2) atmosphere at 1300°C under excitation in Ce3+ related band at 404 nm and registration of luminescence at 510 nm at RT.
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Time (ns)
Time (ns)
Fig. 14. Decay kinetics of Ce3+ luminescence in Ca2YSc2AlSi2O12:Ce (a) and Ca2YSc2Ga Si2O12:Ce (b) ceramic samples, annealed in air at 1300 °C under excitation at 304 nm (1) and 404 nm (2) and registration of luminescence at 510 nm at RT.
tetrahedral ( ) sites by Al3+ and Ga3+ ions, which can be suitable for the development of white converters based on the compounds under study. The cathodoluminescence spectra of Ca2YSc2Ga Si2O12:Ce, Ca2GdSc2GaSi2O12:Ce, Ca2YScGa2Si2O12:Ce and Ca2YScGaAlSi2O12:Ce garnet ceramic samples show the dominant Ce3+ luminescence in the green-yellow range and very low content of the defect related centers, emitting in the UV range. This indicates the stabilizing influence of ScGa doping on the solid-state reaction of single phase garnet phosphor preparation. The observed variation of photoluminescence spectra under different excitation wavelengths and non-exponential decay kinetics of the cerium photoluminescence in the {Ca2R}[Sc,Ga](Al,Ga, Si2)O12 (R = Lu, Y, Gd) garnet compounds under study can probably be connected mainly with the Ce3+ multicenters formation due to the large structural disordering in the decahedral positions of garnet hosts related to the substituting of different (Ca2+ and R3+) cations with various dimensions and charge states and different local surrounding in the second coordination sphere, formed by A3+ (A = Sc, Al, Ga) and Si4+ cations at the octahedral and tetrahedral sites. Due to the Ce3+ multicenter formation, the non-exponential decay kinetics of the Ce3+ emitting centers is observed in {Ca2R} [Sc,Ga](Al,Ga,Si2)O12 (R = Lu, Y, Gd) ceramic samples under excitation in the different Ce3+ related absorption bands. The formation of the Ce4+ and Ce3+ valence states is also expected in the Ca2+-Si4+ based garnets due to the non-uniformity of local surrounding of cerium ions in the dodecahedral positions of the garnet host. Such an assumption can be also confirmed by comparative investigation of the decay kinetics of the Ce3+ luminescence in the ceramic samples under study annealed in the reducing CO and air atmospheres. Specifically, we have supposed that the Ce4+ centers can be responsible for the visible acceleration of non-exponential decay kinetics of the Ce3+ luminescence in the Ca2YSc2Al Si2O12:Ce and Ca2YSc2GaSi2O12:Ce ceramics, annealed in air atmosphere, and for the presence of the fast components of the cerium luminescence with the lifetime in the 9–16 ns range under excitation in the range of the onset of O2-→Ce4+ charge transfer transitions. Meanwhile, the decay component of the Ce3+ luminescence in these garnets with the lifetime in the 46–53 ns range is related to the radiative transitions of Ce3+ multicenters in the garnet hosts. The future detailed spectroscopic research of {Ca2R} [Sc,Ga]2(Ga,Al,Si2)O12, R = Y, Gd, Lu silicate garnets, having the dodecahedral sites for simultaneous localization of Ce3+ ions and other rare-earth and transition metal ions in the different valence states, can
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