Synthesis, Crystal Structures, and Photoluminescence Properties of

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Synthesis, Crystal Structures, and Photoluminescence Properties of Ce Doped CaLaZrGaO : New Garnet Green-Emitting Phosphors for White LEDs 2

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Jiyou Zhong, Weidong Zhuang, Xianran Xing, Ronghui Liu, Yanfeng Li, Yuanhong Liu, and Yunsheng Hu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp508409r • Publication Date (Web): 10 Nov 2014 Downloaded from http://pubs.acs.org on February 18, 2015

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The Journal of Physical Chemistry

Synthesis, Crystal Structures, and Photoluminescence Properties of Ce3+-Doped Ca2LaZr2Ga3O12: New Garnet Green-Emitting Phosphors for White LEDs Jiyou Zhonga,b, Weidong Zhuanga*, Xianran Xingb, Ronghui Liua, Yanfeng Lia, Yuanhong Liua, Yunsheng Hua a

National Engineering Research Center for Rare Earth Materials, General Research Institute for

Nonferrous Metals, and Grirem Advanced Materials Co., Ltd., Beijing 100088, PR China b

Department of Physical Chemistry, University of Science & Technology Beijing, Beijing 100083, PR

China

ABSTRACT: A new family of garnet compounds, Ca2LnZr2Ga3O12 (Ln=La, Y, Lu, Gd) have been synthesized by high-temperature solid-state reaction method. The crystal structures were characterized by the X-ray diffraction (XRD) and refined by the Rietveld method. The photoluminescence properties, morphology, CIE value, quantum efficiency and thermal stability of Ca2LaZr2Ga3O12:Ce3+ phosphors were investigated in detail to evaluate the use in w-LEDs. The photoluminescence results revealed that these phosphors have a broad excitation band in the blue region ranging from 400 nm to 470 nm and a broad green emission band centered at about 515 nm. The above results indicated that the phosphors could be effectively excited by blue light and may have potentials to be served as green-emitting phosphors for application in w-LEDs. * Corresponding author: E-mail:[email protected]. (Prof. Weidong Zhuang.)

ACS Paragon Plus Environment


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1. INTRODUCTION White light emitting diodes (w-LEDs) have been attracted much attention due to their considerable impact on issues such as energy consumption, environment and even the health of individuals

1-2

. Therefore, w-LEDs are thought to have brought about a revolution in

energy-efficient lighting and are promising to replace conventional light sources such as incandescent and fluorescent lamps in the near future 3. Currently, w-LEDs are commonly generated by the combination of the blue LEDs chip and the yellow-emitting phosphors 4, 5. But, the white light obtained by this approach would result in a low color-rendering index (CRI) and high correlated color temperature (CCT) due to red and green emission deficiency in the visible spectrum, which could not meet the requirements of human’s increasing pursuit of high quality lighting, and generally red- or green- emitting phosphors were added to modify the spectrum deficiency 6, 7. In recent years, w-LEDs fabricated using blue light LEDs coupled with red- and green- emitting phosphors with advantages of color stability and excellent color rendering is also a significant alternative for achieving high quality white light 8, 9. Thus the green emitting phosphors play an important role in fabrication of high quality w-LEDs. During the past few years’ development, two commercial green emitting phosphors Lu3Al5O12:Ce3+ and Ca3Sc2Si3O12:Ce3+ have been developed

10

, and both of them exhibit better chemical stability and higher efficiency

than Y3Al5O12:Ce3+ yellow emitting phosphor

11

. However, the expensive Lu2O3 and Sc2O3 raw

materials, to some extent, limit the application of these phosphors

12

. The situation may be

depressed; nevertheless, the relatedness of these two phosphors or even of all the garnets phosphor should not be ignored. Garnet structure, which can be represented by the general formula A3B2X3O12, where A, B and X


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are eight-, six- and four-coordinated with the surrounding O and forming dodecahedron, octahedron and tetrahedron, respectively

13, 14

. Here, A is a distorted site for the alternately

arranged two kinds of A-O bonds and always to be filled by alkali metal, alkaline earth metal and rare earth metal, which can be partially replaced by luminescence center ion. For most of the Ce3+ activated garnet phosphors, a broad intense excitation band can be observed in the blue region, and presenting a broad emission band corresponding to 5d-4f transition of Ce3+ with emission peaks ranging from blue to red region, which is attractive to be used as phosphors for w-LED 15-17. Therefore, many studies have been performed in developing garnet structure new cheap phosphors. Typically, by following the rule: incorporating the Mg2+-Si4+ couple on the A and X sites can shift the Ce3+ emission band to low energies in garnet hosts; and a new red emitting garnet phosphor Lu2CaMg2(Si,Ge)3O12:Ce3+ has been designed and successfully developed by Anant A. Setlur et al 13

. The phosphor exhibits comparable quantum efficiency to commercial Y3Al5O12:Ce3+ phosphor.

Thereupon, more and more garnet phosphors have been developed successively, such as Y3Mg2AlSi2O12:Ce3+, Ca2AB2V3O12:Eu3+ (A= Li, Na, K; B= Mg, Zn), Li5La3B2O12:Eu3+ (B= Nb, Ta) etc 14-19. Furthermore, garnet synthesis by conventional solid state reactions or at ambient pressure synthesis is even more limited

13

. Though previous studies give phenomenological crystal

chemistry rules for synthesis of garnets, such as replacing Si4+ with Ge4+ or replacing Al3+ with Ga3+ enhances the substitution of larger ions on the dodecahedral A sites and steadies the garnet structures

13

, garnet phase stability is still a hot issue. By a comparison of nearly all reported

garnets, we have founded that the B site trends to be filled by a single kind of atom, which is helpful to maintain the cubic structure with minimum lattice stress. And we also found that the



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divalent metal (Mg, Zn), trivalent metal (Al, Sc), and even pentavalent metal (Nb, Ta) are commonly individually occupying the B site. It is reasonable to believe that tetravalent metal like Zr may play a role in B site as well. In order to stable the phase, Ga was introduce into X site. And therefore, to balance the valence, the A site’s rare earth consumption is expected to be lowered by alkaline earth metal substitution, which could lower the cost as well. Hence, the assumed formula Ca2LnZr2Ga3O12 (Ln= La, Y, Lu, Gd) was proposed. Herein, we present the synthesis, crystal structure of a new family of garnets with Zr occupying the B site, and discuss the luminescent properties of Ca2LaZr2Ga3O12:Ce3+ garnet phosphors for application in w-LEDs. The morphology, CIE value, quantum efficiency and thermal stability as well as concentration quenching mechanism of Ca2LaZr2Ga3O12:Ce3+ phosphors were investigated in detail.

2. EXPERIMENTAL 2.1. Materials and Synthesis The Ca2LnZr2Ga3O12 (Ln= La, Y, Lu, Gd) garnets and Ca2LaZr2Ga3O12:Ce3+ garnet phosphors were synthesized by high-temperature solid-state reaction method. The starting materials CaCO3 (Aldrich, 99.95%), Ga2O3 (Aldrich, 99.9%), ZrO2 (Aldrich, 99.9%) and CeO2 (Aldrich, 99.995%) were weighed out according to the stoichiometric ratio. The mixed powder was grounded evenly in an agate mortar, and then homogeneous mixtures were put in an alumina crucible and continually fired at 1350 °C in a reducing atmosphere (CO) for 4h. 2.2. Characterization Methods The crystal structure determination of the as-prepared Ca2LnZr2Ga3O12 (Ln= La, Y, Lu, Gd) garnets were performed

by an X-ray diffraction (XRD) analysis using an X-ray powder


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diffractometer (Rigaku, Japan) with Co-Ka radiation (λ = 0.178892 nm). The data were collected over a 2θ range from 10° to 100° at intervals of 0.02° with a count time of 5 s per step. The structural parameters of Ca2LaZr2Ga3O12 garnet was refined by the Rietveld method using the Fullprof software. The photoluminescence spectra and thermal quenching were measured by a spectrofluorometer (Fluoromax-4, Edison, USA), which are composed of a Xe high-pressure arc lamp, a photomultiplier tube and a heating apparatus. Quantum efficiency was measured using the integrating sphere on the QE-2100 quantum yield measurement system (Otsuka Electronics Co., Ltd., Japan), and a Xe lamp was used as an excitation source and white BaSO4 powder as a reference.

3. RESULTS AND DISCUSSION 3.1. Phases and Structures of Ca2LnZr2Ga3O12 (Ln= La, Gd, Y, Lu) Garnets According to the well-known “lanthanide contraction” effect, La3+ has the largest ionic radius and Lu3+ has the smallest ionic radius. If the Ln3+ site can be occupied by La3+ and Lu3+, respectively, it is possible to be replaced by other rare earth ions with ionic radius between them. Hence, the proposed Ca2LnZr2Ga3O12 (Ln= La, Gd, Y, Lu) compounds were selected to be synthesized as host for phosphor and the powder X-ray diffraction profiles were shown in Figure 1a. As presented, all of them show pure phase and have a similar crystal structure; but with the Ln3+ ionic radius becoming larger, the diffraction peaks shift toward lower angles, which reflects the expansion of lattices. In order to confirm the garnets structure, Rietveld structural refinements of powder diffraction pattern for Ca2LaZr2Ga3O12 was performed and shown in Figure 2. The structural parameters of well-known Y3Al5O12 garnet were used as initial model in the Rietveld analysis. The refined unit cell parameters, atoms coordinates and residual factors are summarized



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in Table 1. The as-obtained goodness of fit parameters and cell parameters are χ2 = 3.18, Rp = 8.32% and a=b=c= 12.7539(8) Å for Ca2LaZr2Ga3O12, which can ensure the garnet structure. Figure 3 presents the crystal structure of Ca2LaZr2Ga3O12. In the structures of these compounds, Zr4+ and Ga3+ are octahedral and tetrahedral coordinated, respectively. The [ZrO6] octahedrons and [GaO4] tetrahedrons are connected by O2- points, and they share edges or corners with [Ca/LnO8] dodecahedra to form a three-dimensional network, which can be observed clearly from Figure 3b. Furthermore, from the viewpoint of exploring new green-emitting garnets phosphors, a series of Ca2La1-xZr2Ga3O12:xCe3+ (x=0.02~0.2) garnets phosphors were synthesized, the powder X-ray diffraction profiles were shown in Figure 1b. It was found that all the XRD patterns agree well with the verified Ca2LaZr2Ga3O12 garnet phase, and no impurities were detected, indicating that the doped Ce3+ ions did not generate any impurities or induce significant changes in the host structure. 3.2. Photoluminescence Properties of Ce3+ in Ca2LaZr2Ga3O12 The excitation (PLE) and emission (PL) spectra of a selected Ca2La1-xZr2Ga3O12:xCe3+ (x=0.06) sample is shown in Figure 4. The PLE spectrum (monitored at 512 nm) shows two main broad excitation bands with peaks at about 330 nm and 430 nm, which are attributed to the crystal field splitting of Ce3+ 5d states 13, 20, and the position of the lowest Ce3+ 4f1-5d1 absorption transition in Ca2LaZr2Ga3O12 is at higher energy compared with YAG: Ce3+, which indicated that a weaker crystal field on Ce3+ in this garnet host. Moreover, the PL spectrum consists of a broad asymmetric emission band attributed to the spin-allowed d-f transitions of Ce3+ and centered at about 515nm under 430nm excitation. However, the PL spectrum can be decomposed into two Gaussian profiles with peaks centered at 495 nm (20202 cm-1) and 544 nm (18382 cm-1), respectively, on an energy


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scale with an energy difference ∆k of about 1820 cm-1, which is in agreement with the theoretical difference between the 2F5/2 and 2F7/2 levels (1800~2000 cm-1)

21, 22

. And this indicates that there

is only one type of emission center in the Ca2LaZr2Ga3O12 host lattice, which shows agreement with the structure analysis. Figure 5 shows the PL spectra of Ca2La1-xZr2Ga3O12:xCe3+ (x=0.02~0.2) under 430 nm excitation. The emission intensities increase with Ce3+ content increasing until a maximum intensity is reached when x=0.06, and then decrease with x beyond the critical concentration. The concentration quenching occurs mainly because the nonradiative energy transfers between Ce3+ ions within a certain distance. And three mechanisms may be responsible for the nonradiative energy transfers, which are exchange interaction, radiation reabsorption and multipolar interaction. According to the critical concentration, the critical distance Rc can be calculated by the following formula: 23

Rc ≈ 2[

3V 1 / 3 ] 4 πx c N

(1)

Where V is the volume of the crystallographic unit cell, xc is the critical concentration and N is the number of lattice sites in the unit cell that can be occupied by the activator ion. In the present case, the values V=2075.87 Å3, N=24, and xc=0.06, and Rc is calculated to be 14 Å. Thus, it can be inferred that the mechanism of exchange interaction plays no role in energy transfer between Ce3+ ions in the Ca2LaZr2Ga3O12 since the exchange interaction requires a forbidden transition and a typical critical distance of 5 Å 24. Therefore, according to Dexter’s theory 25, if the energy transfer occurs only by electric multipolar interaction, the probability of energy transfer between two activator ions is inversely proportional to the nth power of R’ (n= 6, 8, or 10), where R’ is the distance between the activator ions . However, in this case, by linear fitting the relationship of



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lg(I/x) vs. lg(x) with a slope of -1.48 (as shown in Figure 6), the calculated n is 3.54, which is inconsistent with the assumed conditions. Thus, the multipolar interaction should not be the only mechanism. As shown in Figure 4, the excitation and emission spectra overlap to some extent, indicating that the radiation reabsorption mechanism may also play an important role in energy transfer of Ce3+-doped Ca2LaZr2Ga3O12 phosphors. It is also found that the PL spectra shifts to longer wavelengths (red shift) with the increase of Ce3+ content. To explain this phenomenon, three possible reasons are proposed as follows. Firstly, as mentioned above, the probability of the energy transfer from Ce3+ ions at higher levels of 5d to those at lower levels of 5d was promoted with an increase of Ce3+ concentration, which makes it possible that the transitions of low 5d excited state to the 4f ground state enhanced, and results in red shifts in the emission spectrum

13

. In addition, with increasing the Ce3+ concentration, the

reabsorption begins to reduce the high-energy wing of Ce3+ emission band, which also causes the red-shift of the emission band. Furthermore, since the Ce3+ ionic radius is smaller than La3+, the lattice will shrink with the increasing of Ce3+ content, which will lead to the distance between ligands and luminescence centers reduced. Thus the crystal field may become stronger, which results in a larger split of 5d levels and exhibits a red shift in the spectra. Interestingly, we also found that the PLE spectra (as shown in Figure 7) exhibits an obvious trend of red shift at the lower-energy excitation band (around 430nm, defined as 4f-5d1 transitions

26

) with Ce3+ content

increasing, while the higher-energy excitation band (around 330nm, defined as 4f-5d2 transitions 26

) just shows a hardly discernibly trend of blue shift, which reflects the lowering of 5d1 levels but

a bit increase of 5d2 levels, as the increase of Ce3+ content. Generally, the red shift of excitation spectra is caused by changing crystal field splitting or centroid shift. Since the centroid shift will


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result in all the excited 5d energy levels of Ce3+ shifting toward the same direction with an approximate step value, thus the centroid shift should be excluded or at least not the main reason for red shift of spectra in these garnets phosphors. While the crystal field splitting is reasonable for this situation, and the schematic diagram of crystal field splitting model is shown in Figure S1 (Supporting Information). Therefore, as a consequence, the lattice’s shrink induced larger crystal field splitting should be the dominant factor for the red shift of spectra. The thermal quenching property is one of the key application criteria for practical phosphors. Generally, the emission intensity decreases gradually, due to the enhanced nonradiative transition probability while the phosphor is heated. The emission intensity of Ca2La0.94Zr2Ga3O12:0.06Ce3+ phosphor excited by 430 nm light was investigated as a function of temperature in the range of 25-200 °C, as shown in Figure 8. It is revealed that the emission intensity of Ca2La0.94Zr2Ga3O12:0.06Ce3+ drops rapidly with temperature increases, and only about 48% emission intensity remains when the temperature was raised up to 100 °C, which is far lower than YAG:Ce3+ (no less than 90%). To explain the large thermal quenching behavior of Ce3+ in Ca2LaZr2Ga3O12, two possible models are proposed as follows. One is the well known nonradiative relaxation model: the thermal relaxation process from the potential curve of the 5d excited state to the potential curve of the 4f ground state through the crossing point in the configuration coordinate diagram. The other is the thermal ionization model: thermal excitation of the 5d electron to conduction band states. If the main quenching process is the nonradiative relaxation process, the activation energy of Ca2LaZr2Ga3O12:Ce3+ would become higher than YAG because the energy difference between the minimum of the potential curve of the excited 5d level and its crossing point with the 4f ground state in the configuration coordinate diagram (shown in



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Figure S2, Supporting Information) would become larger

27

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. However, if the quenching process

only caused by the nonradiative relaxation process, the activation energy can be calculated by the Arrhenius equation: 24, 28

IT =

I0 1 + c exp(

(2)

- ∆E ) kT

Here I0 is the initial emission intensity, IT is the intensity at different temperatures, ∆E is activation energy of thermal quenching, c is a constant for a certain host, and k is the Boltzmann constant (8.629×10-5 eV). By linear fitting the relationship of ln[(I0/IT)-1] vs. 1000/T for the Ca2La0.94Zr2Ga3O12:0.06Ce3+ phosphor with a slope of -2.35 (as shown in Figure 10), the activation energy was calculated to be 0.203 eV, which is really quite lower than that of YAG:Ce3+ (0.81eV)

29

. Thus, the thermal relaxation process should not be responsible for the

thermal quenching of Ca2LaZr2Ga3O12:Ce3+. And the model of thermal excitation of the 5d electron to conduction band states should be considered for Ga-containing garnet phosphors. Though incorporating Ga3+/Ge4+ into garnets could enhance the phase formation, the energy band gap will be lowered as well and this will reduce the energy barrier for thermal ionization 13, 30. The energy gap of Ca2LaZr2Ga3O12 calculated by absorption spectrum analysis is estimated to be 4.54 eV (as shown in Figure 9), which is much lower than YAG (6.5 eV) 31. For Ce3+ activated YAG phosphor, the energy gap between conduction band and excited 5d-states is 1.07 eV 32, that is to say, the conduction ban is only 0.26 eV above the crossing point between potential curve of the 5d excited state and the potential curve of the 4f ground state, whereas this small energy separation is large enough to prevent the thermal ionization process. But for Ca2LaZr2Ga3O12:Ce3+ phosphor or even most of Ga-containing garnets phosphors, the energy band gap between conduction band and


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valence band is smaller and the energy levels of excited 5d-states is higher than YAG: Ce3+, it is conceivable that the thermal ionization is more likely to occur in these Ga-containing garnets phosphors. According to the electronic structure study of RE3(Al,Ga)5O12 by Pieter Dorenbos 32, by incorporating Ga into RE3Al5O12, the valence band raises 0.4~0.7 eV and the conduction band is lowered by 0.2~0.8 eV. It can be inferred that the conduction band of Ca2LaZr2Ga3O12 should be lowered by 0.8 eV at least than YAG, which means that the energy gap between conduction band and excited 5d-states is no more than 0.27 eV. It can also be inferred that the bottom of conduction band is below the crossing point between potential curve of the 5d excited state and the potential curve of the 4f ground state, because the energy gap between excited 5d-states and the crossing point is more than 0.81 eV. That’s the main reason for the poor thermal quenching properties of Ca2LaZr2Ga3O12:Ce3+. Furthermore, the morphology, CIE value and quantum efficiency of Ca2LaZr2Ga3O12: Ce3+ phosphors have also been studied and measured to investigate the possible practical applications in w-LEDs. The morphology of the selected Ca2La0.94Zr2Ga3O12:0.06Ce3+ sample was determined using SEM (shown in Figure 11). It is revealed that the particles have uniform smooth morphology and narrow size distribution with average diameter of about 2µm. It is believed that such fine particles are suitable in practical terms in the fabrication of white LEDs devices, especially in application of remote phosphor technology for white LEDs. The CIE chromaticity coordinates for Ca2La1-xZr2Ga3O12:xCe3+ phosphors excited at 430 nm were determined and the CIE coordinates are calculated to be ranged from (0.271, 0.479) to (0.285, 0.502) (depicted in Figure 12). A digital photo of the Ca2La0.94Zr2Ga3O12:0.06Ce3+ phosphor under 254 nm UV lamp is shown in the inset of Figure 12, revealing an intense green light emission. Additionally, we



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have also measured the internal quantum efficiency (QE) of Ca2La0.94Zr2Ga3O12:0.06Ce3+ phosphor, which is determined to be 35.2% under 430nm excitation. The QE is possible to be improved by synthesis and composition optimization.

4. CONCLUSION In summary, a series of new compounds has been successfully synthesized by the high-temperature solid-state reaction method. The XRD and Rietveld refinement results confirmed the formation of pure phase compounds with garnet structure. In addition, the photoluminescence properties of Ca2LaZr2Ga3O12:Ce3+ were investigated to explore new green-emitting phosphors for w-LEDs. The photoluminescence results revealed that these phosphors have a broad excitation band in the blue region and a broad green emission band centered at about 515 nm, and both of the excitation and emission spectra show red shift with Ce3+ content increasing, mainly due to large crystal field splitting. The thermal stability, morphology, CIE value and quantum efficiency of the obtained phosphors were investigated to evaluate the practical applications. The results indicate that these garnet phosphors are promising to develop into green phosphors for w-LEDs.


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Supporting Information The schematic diagram of crystal field splitting model of Ce3+ in CaLa2Zr2Ga3O12 and the configuration coordinate diagram are shown in Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGEMENTS This present work was financially supported by the National Basic Research Program of China (2014CB643801), and the National Natural Science Foundation of China (51102021, 51302016).



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Chem. Mater. 2006, 18, 3314-3322. (14) Jia, Y.; Huang, Y.; Guo, N.; Qiao, H.; Zheng, Y.; Lv, W.; Zhao, Q.; You, H. Mg1.5Lu1.5Al3.5Si1.5O12:Ce3+,Mn2+: A Novel Garnet Phosphor with Adjustable Emission Color for Blue Light-Emitting Diodes. RSC Advances 2012, 2, 2678-2681. (15) Liu, Y.; Zhuang, W.; Hu, Y.; Gao, W.; Hao, J. Synthesis and Luminescence of Sub-Micron Sized Ca3Sc2Si3O12:Ce Green Phosphors for White Light-Emitting Diode and Field-Emission Display Applications. Journal of Alloys and Compounds 2010, 504, 488-492. (16) Katelnikovas, A.; Bettentrup, H.; Uhlich, D.; Sakirzanovas, S.; Jüstel, T.; Kareiva, A. Synthesis and Optical Properties of Ce3+-Doped Y3Mg2AlSi2O12 Phosphors. Journal of Luminescence 2009, 129, 1356-1361. (17) Wu, J. L.; Gundiah, G.; Cheetham, A. K. Structure-Property Correlations in Ce-Doped Garnet Phosphors for Use in Solid State Lighting. Chemical Physics Letters 2007, 441, 250-254. (18) Chen, X.; Xia, Z.; Yi, M.; Wu, X.; Xin, H. Rare-Earth Free Self-Activated and Rare-Earth Activated Ca2NaZn2V3O12 Vanadate Phosphors and Their Color-Tunable Luminescence Properties. Journal of Physics and Chemistry of Solids 2013, 74, 1439-1443. (19) Zhang, X.; Meng, F.; Wei, H.; Seo, H. J. Eu3+ Luminescence in Novel Garnet-Type Li5La3Nb2O12 Ceramics. Ceramics International 2013, 39, 4063-4067 (20) Ueda, J.; Tanabe, S.; Nakanishi, T. Analysis of Ce3+ Luminescence Quenching in Solid Solutions between Y3Al5O12 and Y3Ga5O12 by Temperature Dependence of Photoconductivity Measurement. J. Appl. Phys. 2011, 110, 053102-053102-6. (21) Blasse, G.; Grabmaier, B. C. Luminescent Materials; Springer: Berlin, 1994. (22) Xia Z.; Liu, R. S. Tunable Blue-Green Color Emission and Energy Transfer of Ca2Al3O6F:Ce3+,Tb3+ Phosphors for Near-UV White LEDs. J. Phys. Chem. C. 2012, 116, 15604-15609 (23) Blasse, G. Energy Transfer Between Inequivalent Eu2+ Ions. J. Solid State Chem. 1986, 62, 207-211. (24) Xia, Z.; Liu, R. S.; Huang, K. W.; Drozd, V. Ca2Al3O6F:Eu2+: A Green-Emitting Oxyfluoride Phosphor for White Light-Emitting Diodes. J. Mater. Chem. 2012, 22, 15183-15189 (25) Dexter, D. L. A Theory of Sensitized Luminescence in Solids. J. Chem. Phys. 1953, 21, 836-850. (26) Luo, Y.; Xia, Z. Effect of Al/Ga Substitution on Photoluminescence and Phosphorescence Properties of Garnet-Type Y3Sc2Ga3−xAlxO12:Ce3+ Phosphor. J. Phys. Chem. C 2014, 118,



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23297-23305. (27) Ueda, J.; Aishima, K.; Tanabe, S. Temperature and Compositional Dependence of Optical and Optoelectronic Properties in Ce3+-Doped Y3Sc2Al3-xGaxO12 (x = 0, 1, 2, 3). Optical Materials 2013, 35, 1952-1957. (28) Huang, C. H.; Chen, T. M. Novel Yellow-Emitting Sr8MgLn(PO4)7:Eu2+ (Ln = Y, La) Phosphors for Applications in White LEDs with Excellent Color Rendering Index. Inorg. Chem. 2011, 50, 5725-5730. (29) Bachmann, V.; Ronda, C.; Oeckler, O.; Schnick,W.; Meijerink, A. Temperature Quenching of Yellow Ce3+ Luminescence in YAG:Ce. Chem. Mater. 2009, 21, 2077-2084. (30) Kalaji, A.; Saines, P. J.; George, N. C.; Cheetham, A. K. Photoluminescence of Cerium-Doped (Ca1-xSrx)3RE2Ge3O12 Garnet Phosphors for Solid State Lighting: Relating Structure to Emission. Chemical Physics Letters 2013, 586, 91-96. (31) Slack, G. A.; Oliver, D. W.; Chrenko, R. M.; Roberts, S. Optical Absorption of Y3Al5O12 from 10to 55 000-cm-1 Wave Numbers. Phys. Rev. 1969, 177, 1308-1314. (32) Dorenbos, P. Electronic Structure and Optical Properties of the Lanthanide Activated RE3(Al1-xGax)5O12 (RE= Gd,Y,Lu) Garnet Compounds. J. Lumin. 2013, 134, 310-318.


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Tables Table 1 The results of structure refinement of Ca2LaZr2Ga3O12. formula symmetry space group a = b = c (Å) α =β= γ (deg) V (Å3) Z Rp (%) Rwp (%) χ2 atom Ca La Zr Ga O

site 24c 24c 16a 24d 96h

Ca2LaZr2Ga3O12 cubic Ia-3d 12.7539(8) 90 2074.61(3) 8 8.32 11.6 3.18 x 0.12500 0.12500 0.00000 0.37500 0.97016

y 0.00000 0.00000 0.00000 0.00000 0.05468

z 0.25000 0.25000 0.00000 0.25000 0.15353

occu. 0.16667 0.08333 0.16667 0.25000 1.00000


Biso 0.16903 0.16903 0.01778 0.13025 0.11939


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Captions for Figures Figure 1. Powder XRD patterns of (a) Ca2LnZr2Ga3O12 (Ln= La, Gd, Y, and Lu) and (b) Ca2La1-xZr2Ga3O12: xCe3+ (x=0~0.2).

Figure 2. Powder XRD pattern (red circles) of Ca2LaZr2Ga3O12 with its corresponding Rietveld refinement (black solid line) and residuals (blue line in the bottom).

Figure 3. (a) Crystal structure of Ca2LaZr2Ga3O12 and (b) the coordination environment of cations.

Figure 4. PLE and PL spectra of Ca2La0.94Zr2Ga3O12:0.06Ce3+ phosphor. Figure 5. PL (λex=430 nm) spectra of Ca2La1-xZr2Ga3O12: xCe3+ with varying Ce3+ concentrations; inset shows the emission intensity as a function of the Ce3+ content.

Figure 6. The linear fitting of the relationship of lg(I/x) vs. lg(x).

Figure 7. PLE (λem=515 nm) spectra of Ca2La1-xZr2Ga3O12: xCe3+ with varying Ce3+ concentrations.

Figure 8. Temperature-dependent PL spectra of Ca2La0.94Zr2Ga3O12:0.06Ce3+ phosphor; inset shows the emission intensity as a function of temperature (λex=430 nm).

Figure 9. The absorption spectrum and calculated energy gap of Ca2LaZr2Ga3O12 by Tauc Equation.

Figure 10. The Arrhenius fitting of the emission intensity of Ca2La0.94Zr2Ga3O12:0.06Ce3+ phosphor and the calculated activation energy for thermal quenching.

Figure 11. SEM image of the selected Ca2La0.94Zr2Ga3O12:0.06Ce3+ phosphor. Figure 12. The CIE coordinates of Ca2La1-xZr2Ga3O12: xCe3+ (x=0.02~0.2) and a digital photo under 254 nm UV lamp of the selected Ca2La0.94Zr2Ga3O12:0.06Ce

3+

phosphor.


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Figure 1



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 2


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Figure 3



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 4


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Figure 5



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Figure 6


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Figure 7



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Figure 8


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Figure 9



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Figure 10


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Figure 11



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Figure 12


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TOC CaLa2Zr2Ga3O12: Ce3+: a new garnet green-emitting phosphor was explored for white-LEDs.


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

I n t e n s it y ( a .u .)


Page 32 of 44

C a 2L a Z r 2G a 3O

1 2

C a 2G d Z r 2G a 3O

1 2

C a 2Y Z r 2G a 3O

C a 2L u Z r 2G a 3O

2 0

3 0

4 0

5 0


2 -T h e ta (d e g r e e )

6 0

1 2

1 2

7 0

Page 33 of 44

x = 0 .2 0 x = 0 .1 6 x = 0 .1 2


x = 0 .0 8 x = 0 .0 6 x = 0 .0 4

2 0

4 0

6 0

2 T h e ta (d e g r e e ) ACS Paragon Plus Environment

(8 4 0 ) (8 4 2 ) (6 6 4 )

(8 0 0 )

(4 4 4 ) (6 4 0 ) (6 4 2 )

(6 1 1 ) (6 2 0 )

(4 3 1 )

(3 3 2 )

(5 2 1 )

x = 0 .0 0

(4 2 2 )

(4 2 0 )

(3 2 1 ) (4 0 0 )

x = 0 .0 2

(2 2 0 )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47


8 0


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47


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λe m = 5 1 5 n m λe x = 4 3 0 n m


5 4 4 n m 4 9 5 n m

P L

P L E

2

5 d →F

7 /2

2

5 d →F

Δ

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

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3 0 0

3 5 0

4 0 0

4 5 0

5 /2

k = 1 8 2 0 c m

5 0 0

W a v e le n g th (n m )


5 5 0

-1

6 0 0

6 5 0

Page 37 of 44

R e d S h ift

1 0 0

5 I n te n s ity (%

)

8 .0 x 1 0


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47


9 0

8 0

5

6 .0 x 1 0

7 0 0 .0 0

0 .0 4

0 .0 8 0 .1 2 0 .1 6 3 + C e c o n c e n tr a tio n

0 .2 0

x 1= 0 .0 2

4 .0 x 1 0

5

x 2= 0 .0 4

λe x = 4 3 0 n m

x 3= 0 .0 6 x 4= 0 .0 8

2 .0 x 1 0

5

x 5= 0 .1 2 x 6= 0 .1 6 x 7= 0 .2 0

0 .0 4 5 0

5 0 0

5 5 0

W a v e le n g th (n m ) ACS Paragon Plus Environment

6 0 0

6 5 0


7 .2

C e

3 +

)

7 .0

lg (I /x

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

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6 .8 S lo p e = - 1 .4 8 6 .6

6 .4 - 1 .3

- 1 .2

- 1 .1

- 1 .0

lg (x

)

- 0 .9

3 + ACS Paragon PlusC Environment e

- 0 .8

- 0 .7

Page 39 of 44

x 1= 0 .0 2

R e d S h ift

x 2= 0 .0 4 x 3= 0 .0 6 x 4= 0 .0 8


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47


x 5= 0 .1 2

λe m = 5 1 5 n m

x 6= 0 .1 6 x 7= 0 .2 0

3 0 0

3 5 0

4 0 0

W a v e le n g th (n m )


4 5 0


℃ 5 0 ℃ 1 0 0 ℃ 1 5 0 ℃ 2 0 0 ℃

1 .0

0 .8

N o r m a liz e d I n t e n s it y ( a .u .)

2 5

N o r m a liz e d I n t e n s it y ( a .u .)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

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1 .0 0 .8 0 .6 0 .4 0 .2

0 .6

5 0

℃

1 0 0 1 5 0 T e m p e r a tu r e ( )

2 0 0

0 .4

0 .2

0 .0 4 5 0

5 0 0

W

5 5 0

a v e le n g th (n m )


6 0 0

6 5 0

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2 .5

1 .0

2 .0 1 /2

0 .8

(α h v )

A b s o r b a n c e ( a .u .)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47


0 .6

1 .5

1 .0

E g

= 4 .5 4 e V

0 .5

0 .4

0 .0 3 .5

0 .2

4 .0

4 .5

5 .0

5 .5

6 .0

h v (e V )

0 .0 2 0 0

3 0 0

W

4 0 0

5 0 0

a v e le n g th (n m )


6 0 0

7 0 0


1

ln [(I 0/I T )-1 ]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

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0

S lo p e = - 2 .3 5 3

-1 2 .0

2 .2

2 .4

2 .6

1 0 0 0 /T (K

-1


)

2 .8

3 .0

3 .2

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Synthesis, Crystal Structures, and Photoluminescence Properties of

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