Ga substitution on the structural and luminescence properties of Y3(Al1-xGax)5O12: Ce3+ phosphors

Optical Materials 75 (2018) 619e625

Contents lists available at ScienceDirect

Optical Materials journal homepage: www.elsevier.com/locate/optmat

Effect of Al/Ga substitution on the structural and luminescence properties of Y3(Al1-xGax)5O12: Ce3þ phosphors Sheng Fu a, Jin Tan a, b, *, Xin Bai a, Shanjie Yang a, Lei You a, Zhengkang Du a a b

Faculty of Materials Science and Chemistry, China University of Geosciences, Wuhan 430074, China Engineering Research Center of Nano-Geomaterials of Ministry of Education, China University of Geosciences, Wuhan 430074, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 4 August 2017 Received in revised form 8 November 2017 Accepted 12 November 2017 Available online 22 November 2017

As candidates for display and lighting materials, a series of gallium-substituted cerium-doped yttrium aluminum garnet (Y3(GaxAl1-x)5O12: Ce3þ) phosphors were synthesized by high temperature solid-state reaction. The phases, morphology, luminescence spectra and thermal stability of the phosphors were investigated. The volatilization of Ga2O3 induces the constituents out of stoichiometric ratio and different impurities in the system. The excitation and emission spectra occur red shift (339 nm - 351 nm) and blue shift (465 nm - 437 nm), and blue shift (541 nm - 517 nm), respectively. The spectra have no further blue shift and the luminescence intensity decrease with x over 0.4. Combining crystal structure with PL spectrum, the distortion of dodecahedron and crystal field splitting of 5d level of Ce3þ are influenced by Ga3þ in octahedral coordination polyhedron rather than tetrahedron. The crystalline perfection and Ga3þ occupying the tetrahedron induce less garnet phase formation, more impurities and the 5d level located in the conductive bands, thus accounting for the x ¼ 0.4 turning points of the PL and PLE intensity. Based on the thermal quenching and CIE, the Y3(GaxAl1-x)5O12: Ce3þ0.06 phosphors have great potential for use on the w-LED. © 2017 Elsevier B.V. All rights reserved.

Keywords: Phosphor YAGG: Ce3þ Photoluminescence Crystal structure Blue shift

1. Introduction White light-emitting diodes (w-LED) have emerged as the new generation lighting source due to their excellent properties such as high energy efficiency, long lifetime, energy conservation and environment friendliness [1e4]. Although the white radiation can be generated from many methods, the most common approach for fabricating the w-LED is to combine a blue InGaN chip with yellowemitting phosphors of cerium-doped yttrium aluminum garnet (YAG: Ce3þ). However this type of w-LED mainly suffers from problems of spectrum defect of emission around 500 nm and around 650 nm [5e7]. The enhancement of the green and red component phosphors dominates the market to modify the drawback [8,9]. Therefore, it is significant to synthesize the high performance green phosphor to supplement the absent emission around 500 nm. Recently, three methods are used: i) synthesize the new compound and structure phosphors [7,10], ii) introduce of other rare earth ions (Eu3þ, La3þ, Dy3þ, Gd3þ) [1,11,12] with Ce3þ into the host crystal, or iii) replace of Y3þ and Al3þ ions with other

* Corresponding author. E-mail address: [email protected] (J. Tan). https://doi.org/10.1016/j.optmat.2017.11.021 0925-3467/© 2017 Elsevier B.V. All rights reserved.

adaptive cations according the stoichiometry to modify the emission band of Ce3þ [3,9,13,14]. The Y3(Al1-xGax)5: Ce3þ phosphor is a preferred candidate. Some researchers have reported the studies on Y3(Al1-xGax)5: Ce3þ phosphor. For example, M. Y. Kim surveyed the new synthesis methods, and M. Ayvacikli researched the influence of defects in phosphors photoluminescence [14,15]. In order to clarify the occupancy sites of Ga3þ and Al3þ in garnet structure, some researchers employed the DFT (Density Functional Theory) to tet calculate the formation energy of Gaoct Al and GaAl and concluded 3þ that the Ga ions preferred to occupy the 16a sites (octahedron) [16]. Inspiring by the calculation, X. W. He and X. F. Liu clarified the preferential occupation (octahedron) of Ga3þ ions in YAGG: Ce3þ structure by nuclear magnetic resonance with the concentration of Ga below 0.3 synthesized via a facile sol-gel method [17]. But there is rare research on the occupancy sites of Ga3þ and Al3þ in garnet structure and their effects on optical properties in large Ga3þ concentration range. As a straightforward and commercially produced method, the traditional high temperature solid-state reaction can achieve the desired phase purity and required particle size [18,19]. Therefore, in this paper, we synthesized a series of Y3(Al1-xGax)5O12: Ce3þ (x ¼ 0,

620

S. Fu et al. / Optical Materials 75 (2018) 619e625

0.2, 0.3, 0.35, 0.4, 0.45, 0.5, 0.6, 0.8, 1.0) by high temperature solidstate reaction with BaF2, H3BO3 and NaF as the composite flux. We have investigated the effects of Al/Ga ratio on the Y3(Al1-xGax)5O12: Ce3þ phase, crystal structure, luminescence and temperaturedependent photoluminescence properties. The relationship between ions occupation in crystal structure and the spectroscopic properties will be discussed in detail. 2. Experimental aspects 2.1. Material and synthesis A series of crystal samples of Y3(Al1-xGax)5O12: Ce3þ (x ¼ 0, 0.2, 0.3, 0.35, 0.4, 0.45, 0.5, 0.6, 0.8, 1.0) phosphors have been synthesized by the conventional high temperature solid-state reaction with controlling reaction atmosphere comprised with 10% H2 and 90% N2. The raw materials of Y2O3(99.99%), Ga2O3(99.99%), Al2O3(99.999%) and CeO2(99.99%) are all purchased from Sinopharm Chemical Reagent Co. Ltd without any afterprocessing. These starting materials are weighted according to the stoichiometric amounts with 1.5% BaF2, 1.5% H3BO3 and 2% NaF as the fluxes, and then mix they with ethyl alcohol in the agate mortar by grinding for 1 h to ensure uniform of each parts. Then, the mixture is fired at 1350  C with duration of 2 h and cools down to the room temperature. Finally, a series of Y3(Al1-xGax)5O12: Ce3þ are prepared for the following measurements. 2.2. Characterization methods The crystal structure and phase determination of Y3(AlxGa13þ garnets were performed by an X-ray powder diffraction (D8-FOCUS, Cu Ka radiation, l ¼ 1.54056 Å, 40 kV, 40 mA) and scanned from 10 to 70 with step length of 0.01. The structural parameters of Y3(Al0.6Ga0.4)5O12:Ce3þ garnet were refined by the Rietveld method using the Fullprof software. The morphologies of the samples were inspected using scanning electron microscope (SEM, Hitachi S-8010) operating at 10 kV. Room temperature photoluminescence excitation (PLE) and emission (PL) spectra were characterized by Fluoromax-4P fluorescence spectrophotometer with a photomultiplier tube operating at 400 V, and a 150 W Xe lamp used as the excitation source. The thermal stability of the luminescence was measured by Fluoromax-4P spectrometer connected a heating equipment (Orient KOJI, TAP-02), and the samples were heated from 25  C to 250  C with a 25  C interval. The CIE x-y color coordinates were given by CIE 1931.

x)5O12:Ce

3. Results and discussion 3.1. Phase and structures of Y3(GaxAl1-x)5O12:Ce3þ phosphors Fig. 1 shows the XRD patterns of Y3(GaxAl1-x)5O12: Ce3þ garnets and the both diffraction patterns exhibit intense reflection corresponding to the ICSD29249. It indicates that the garnet structure is unalterable with the Ga3þ substitution [11]. As x  0.4, the products completely phase transform to the pure garnet structure. The impurities appear when the concentration x is beyond 0.4 (0.4 < x < 1: Y4Al2O9 and x ¼ 1: Y3GaO6). It should note that the peaks tend to migrate toward the low degree with the increasing of Ga concentration, which can be explained by the comparison of ionic radii of Ga3þ (0.76 Å) with Al3þ (0.53 Å) and Vegard law [20]. The existence of impurities in high Ga content samples is due to the volatility of Ga2O3 and the difficulty of Ga incorporation in garnet structure [21]. The more Ga2O3 in the ingredients, the larger volatile in the synthesis processing, which results in the worsening of the deviation from the stoichiometry. And the impurities (Y4Al2O9 phase

Fig. 1. Powder XRD patterns of Y2.94(GaxAl1-x)5O12:Ce3þ0.06 (x ¼ 0, 0.2, 0.4, 0.8, 1.0) and the standard pattern of YAG (ICSD29249).

with 0.4 < x < 1, Y3GaO6 with x ¼ 1) are consistent with the aforementioned. The impurities have definite influence to optical properties [18], and the spectroscopic phenomena are determined by the garnet phase. In order to further prove the reliability of garnets structure, Rietveld structure refinements of powder diffraction pattern for Y2.94(Ga0.4Al0.6)5O12:Ce3þ0.06 is chosen and the results are shown in Fig. 2 and Table 1. The phosphors agree well with the garnet structures. The occupancy ratio of all ions are nearly to theoretical value based on Table 1. And the Rietveld analysis results indicates the weighted profile R-factor (Rwp) and the expected R factor (Rp) are 15.6% and 18.8%. Although the values are large due to the escape of Ga2O3 from the system and inducing deviation from stoichiometric ratio, they can also prove that the results on phase analysis and the occupation of Ga3þ and Ce3þ ions are credible. With substitutions, the space group of crystal is also Ia-3d and the sites of all ions are not changed comparing with the host [10]. As shown in Table 1, the lattice parameter of synthesized phosphors is between the YAG (12.01 Å) and YGG (12.28 Å) [15] which corresponds to the result of XRD. The crystal structure patterns are shown in Fig. 3 and the ratio of octahedral (16a) and tetrahedral (24d) equals to 2: 3 (Fig. 3b). The Ga3þ mainly occupies the 16a lattice sites and Al3þ occupies the 24d lattice sites which consistent with References [16,17]. 3.2. Morphology analyzing of Y3(GaxAl1-x)5O12:Ce3þ phosphors Generally, the emission efficiency of the phosphors mainly depends on the surface crystallinity, particle size and internal structure. The morphology, dispersity, particle diameters and the growth mechanism of these phosphors were investigated by SEM

Fig. 2. Powder XRD patterns for Y2.94(Ga0.4Al0.6)5O12:Ce3þ0.06 phosphor.

Rietveld

structure

of

the

selected

S. Fu et al. / Optical Materials 75 (2018) 619e625 Table 1 Results of Rietveld structure of Y2.94(Ga0.4Al0.6)5O12:Ce3þ0.06. Formula

Y2.94(Ga0.35Al0.65)5O12:Ce3þ0.06

Symmetry Space group a ¼ b ¼ c (Å) a ¼ b ¼ g (degree) V (Å3) Z Rp (%) Rwp (%)

Cubic Ia-3d 12.108 (2) 90 1774.989 (8) 8 15.6 18.8 6.78

c2 Atom

x

Y1 Ce1 Al1 Ga 1 Al2 Ga 2 O1

0.000 0.000 0.000 0.000 0.000 0.000 0.971

y (0) (0) (0) (0) (0) (0) (1)

0.250 0.250 0.000 0.000 0.250 0.250 0.052

z (0) (0) (0) (0) (0) (0) (1)

0.125 0.125 0.000 0.000 0.375 0.375 0.148

Biso (0) (0) (0) (0) (0) (0) (0)

0.455 0.455 0.411 0.411 0.473 0.473 0.500

Occ (3) (3) (4) (3) (3) (3) (2)

0.978 0.022 0.052 0.948 0.954 0.046 1.000

Site (2) (2) (3) (3) (4) (4) (0)

24c 24c 16a 16a 24d 24d 96h

(shown in Fig. 4). As shown, the particle size of Y2.94(Al0.6Ga0.4)5O12:Ce3þ0.06 is larger than that of Y2.94Al5O12: Ce3þ0.06, meanwhile the morphology becomes more regular, and even the regular crystal planes are observed, which indicates the greater luminescence properties for x ¼ 0.4. From References [22], synthesizing powders by adding fluxes includes two steps of nucleation and particle growth. The nucleation process depends on the dissolution rate of different reactants and particle growth decides the morphology of powders. In a local fluid environment induced by fluxes, crystal growth process is divided into two situations: (a) The growth process controlled by interface reaction can synthesis particles with crystal face; (b) The growth process controlled by diffusion rate can develop spherical particles [23]. The emerged crystal planes of particles, especially in Fig. 4b, suggest that the grains development of Y2.94(GaxAl1-x)5O12:Ce3þ0.06 are controlled by interface reactions. Therefore, the phosphors have good dispersity, large and regular grain, which are the bases of excellent luminescence properties [24]. 3.3. Photoluminescence properties of Y3(GaxAl1-x)5O12:Ce3þ phosphors Room temperature photoluminescence excitation (PLE) and emission (PL) spectra of the YAG samples, where increasing amounts of Ga3þ substitute Al3þ up to x ¼ 0.6, doped with 2% Ce3þ are presented in Fig. 5 and Fig. 6. The high substitution

621

concentrations when x ¼ 0.8 and x ¼ 1 respectively emit the low intensity and no luminescence observed, corresponding to the literature [25]. The PLE spectrum (monitored at 517 nm) shows two main broad excitation bands with peaks at about 345 nm and 450 nm. We observe that the PLE spectra (as marked in Fig. 5) exhibits an obvious red shift at the higher-energy excitation band (shifted from 339 nm to 351 nm) with Ga3þ content increasing, while the lower-energy band just shows a hardly discernibly trend of blue shift (shifted from 465 nm to 437 nm). The PL spectra under 430 nm excitation is only found with the blue shift (shifted from 541 nm to 517 nm). It should be pointed out that the sharp lines in the excitation spectra are the background peaks of fluorescence spectrophotometer. Interestingly, as shown in Fig. 7, the prepared samples of the Y3(GaxAl1-x)5O12:Ce3þ phosphors show the increasing emission intensity and the decreasing wavelength with the increase of x and respectively reach the strongest and shortest (517 nm) when x ¼ 0.4. The higher-energy of PLE spectrum expresses the similar tendency with the wavelength shifting to the longest (351 nm). With the further increase of Ga concentration, the PLE and PL intensities decrease accordingly but the peaks are without further shifting. Due to the allowed 5d-4f transition and 5d energy level splitting of activator ions Ce3þ which are influenced by crystal field, the phosphors are endowed with the luminescence [26,27]. The energy-level and light emitting mechanism diagram can be given in Fig. 8a. According to the crystal field theory, the crystal field strength is relevant to covalency, site symmetry, ligand charge and bong length. The crystal field splitting (Dq) can be expressed as below [28,29]:

Dq ¼

ze2 r 4 6R5

(1)

where the Dq is a measure of the crystal field strength, z is the charge or valence of the anion, e is the charge of electron, r is the radius of the d wave function, and R is the bond distance between the Ce3þ and O2. Ga3þ ions with the bigger ion radii and stronger electronegativity comparing with Al3þ have shorter bond and the stronger interactions with the coordinate O2, which will induce the ligands of Al3þ sites shrinking [11,26,30]. So, the Dq decreases and the 5d2 energy level declines while the 5d1 raises for the extending of R with the substitution concentration increasing. The red shift around 350 nm can be well explained for energy to excite 2 F5/2 / 5d2 decreasing with x increasing, and the increase to excite 2 F7/2 / 5d1 for blue shift around 450 nm in PLE spectra. And the blue shift of PL spectra is easily understood for the 5d1 energy raising. The two peaks of 492 nm (20202 cm1) and 544 nm

Fig. 3. The crystal structure pattern of the Y3(Al1-xGax)5:Ce3þ:a) coordinating polyhedron structure of the Y3(Al1-xGax)5:Ce3þ, the Al/Ga atom occupied tetrahedral (24d) & octahedral (16a) and the Y/Ce occupied the dodecahedron relatively. b) the tetrahedral (24d) and octahedral (16a) pattern.

622

S. Fu et al. / Optical Materials 75 (2018) 619e625

Fig. 4. SEM images of Y3(GaxAl1-x)5O12:Ce3þ phosphor powders. a) Y2.94Al5O12:Ce3þ0.06 b) Y2.94(Al0.6Ga0.4)5O12:Ce3þ0.06. c) Y2.94Ga5O12:Ce3þ0.06.

Fig. 5. The photoluminescence excitation (PLE) spectrum of Y3(GaxAl1-x)5O12:Ce3þ (x ¼ 0, 0.1, 0.2, 0.3, 0.35, 0.4, 0.45, 0.5 and 0.6) with the emission wavelength lem ¼ 517 nm.

(18382 cm1) are observed by Gaussian fitting peaks for x ¼ 0.6 and the energy difference △k equaled to 1820 cm1. It is in accordance with the theoretical energy difference (1800-2000 cm1) between the 2F7/2 and 2F5/2 levels [11,31,32]. In order to clarify the interesting phenomenon that x ¼ 0.4 is a turning point of PLE and PL spectrum, the crystal structure is taken into consideration. According to Pauling coordination polyhedron rule, the relative size of the positive and negative ions directly affects the crystal mode and the coordination polyhedron formation of the ions. If Rþ/R equals to 0.225e0.414, the polyhedron prefers to forming tetrahedral coordination while octahedral coordination

Fig. 6. The photoluminescence emission (PL) spectrum of Y3(GaxAl1-x)5O12:Ce3þ (x ¼ 0, 0.1, 0.2, 0.3, 0.35, 0.4, 0.45, 0.5 and 0.6) with the emission wavelength lex ¼ 450 nm.

when Rþ/R equals to 0.414e0.732. And the radius ratio of Ga3þ ions and O2 is:

RGa3þ 0:76 ¼ 0:628 ¼ 1:21 RO2

(2)

The incorporation of Ga3þ is preferred to form Ga-O octahedral coordination polyhedron comparing with the 0.438 of Al3þ and O2. The ratio of octahedral and tetrahedron coordination polyhedron in garnet structure is 2: 3 as shown in Fig. 3b, and the octahedral sites occupies for 0.4 in the whole sites of Al(Ga) lattice,

S. Fu et al. / Optical Materials 75 (2018) 619e625

623

Table 2 Lattice constant, band gap energy (Eg) in Ce3þ-doped YAG, YAGG and YGG garnet crystal [23]. Samples

Referenced lattice constant (Å)

Eg (eV)

YAG: Ce3þ YAGG: Ce3þ YGG: Ce3þ

12.00 12.19 12.27

65 5.9 5.5

Fig. 7. The relationship of the center wavelength and intensity vs substituted concentration of Ga3þ based on the emission spectra.

which exactly equals to the substitution concentration x ¼ 0.4. The result coincides with the Rietveld structure refinements that Ga3þ ions mainly occupy octahedral (l6a sites) in crystal structure. As combining ions occupancy with the luminescence phenomenon, it concludes that the crystal field of Ce3þ is mainly affected by the octahedral central ions (16a sites) and the splitting energy reached to minimum until all sites are occupied by Ga3þ. The difficulty of Ga3þ to occupy the tetrahedron coordination (24d sites) leads to less garnet phase formation and more impurities. The Ga3þ ions in 24d sites have little influence on the 5d level splitting of Ce3þ and the PL spectra almost do not have blue shift with the further increase of Ga concentration. The intensity of the PLE and PL spectra decrease with Ga3þ concentration over 0.4 is a common phenomenon [14,18,25,26]. The garnet crystal structure becomes more integral due to the compensation of internal strain and less defects when the 16a sites are occupied by Ga3þ gradually, thus benefitting for the luminescence from Ce3þ ions. So, the intensity of PL and PLE increase with the substitution concentration increasing at Ga < 0.4. Combing these references with the synthesized phosphors, the energy level diagram can be provided as shown in Fig. 8 [25,33]. With the increase of Ga concentration, the band gap of garnet host decrease (Table 2). The 5d2 move to the conduction band and the 5d1 levels gradually close to the conduction band while the crystal field gradually decreases. When 16a sites are completely occupied by the Ga3þ (x ¼ 0.4), the 5d2 levels locates in the conduction band and reach the largest blue

Fig. 9. The excitation spectra of Y2.94(Ga0.4Al0.6)5O12:Ce3þ0.06 with lem ¼ 517 nm and the emission spectrum of Y3-x (Ga0.4Al0.6)5O12:Ce3þx (x ¼ 0.02, 0.04, 0.06, 0.08, 0.10, 0.12) with lex ¼ 450 nm. The inset shown the relation of center wavelength and intensity vs activator concentrations of the emission spectrum. The circle marked intersection area of PLE excitation and PL emission.

shift (Fig. 8b), while the both 5d2 and 5d1 locate in conduction band with x ¼ 1 (Fig. 8c). The thermally stimulated ionization process easily occurs for the low activation energy and resulting in the intensity of PLE and PL decreasing and no luminescence observed when x ¼ 1. The Ga3þ prefers to occupy the octahedral site, only when the octahedral site is fully occupied and then the Ga3þ is forced to enter the tetrahendral site, so x ¼ 0.4 acts as a turning point in the crystal structure and luminescence properties. As shown in Fig. 9, it is found that the PL spectra shifts to longer wavelength (red shift) with the increase of Ce3þ content and exhibits the maxima intensity in 0.06 (x ¼ 0.06 is used in above experiments). To explain this phenomenon, three possible reasons are proposed as follows. Firstly, the energy transfer from Ce3þ ions at higher level of 5d to the lower level of 5d makes it possible that for the transitions of low 5d excited state to the 4f enhances [10,34]

Fig. 8. Energy-level diagram of Ce3þ in YAG, YGAG, YGG and light emitting mechanism.

624

S. Fu et al. / Optical Materials 75 (2018) 619e625

and leads to the red shift of the emission spectra with the increase of Ce3þ concentration. In addition, as marked in Fig. 9, the reabsorption occurs and reduce the high-energy wing of Ce3þ emission band. Furthermore, since the Ce3þ with larger ions replaces the smaller Y3þ, the R decreases and the Dq increase, so a red shift occurs in the emission spectra. To deeply understand the concentration quenching, the nonradiative energy transfers between Ce3þ are given [10,25,35]. There are three mechanisms for nonradiative energy transfers to explain concentration quenching: exchange interaction, radiation reabsorption and electric multipolar interaction. To clarify the mechanisms of this system, the critical distance Rc is taken into consideration and calculated by function below [10,36]:

1
3V 3 Rc z2 4pxc N

(3)

Here V is the volume of unit cell, xc was the critical concentration, and N is the number of lattice sites in the unit cell that can be occupied by the activator ion. In Y3-x(Ga0.4Al0.6)5O12:Ce3þx, the value V ¼ 1774.989 Å3, N ¼ 24, xc ¼ 0.06 and Rc is calculated: Rc ¼ 6.65 Å. It is affirmed that the mechanism of exchange interaction can be excluded because the critical distance of this mechanism was not more than 5 Å [10,25]. Thus, electric multipolar interaction plays the leading role in the energy transfer between Ce3þ ions. In order to judge the electric multipolar interaction, the following formula is quoted to calculate and observe [36,37]:

h i Q 1 I ¼ K 1 þ bx 3 x

(4)

where x is the concentration of activator ions, I/x is the emission intensity (I) per activator concentration (x), k and b are constants for the same excitation condition for the certain host crystal and Q is the function of the multipole-multipole interaction. Q ¼ 6, 8 and 10 corresponded to dipole-dipole (d-d), dipole-quadrupole (d-q) and quadrupole-quadrupole (q-q) interactions, respectively. As shown in Fig. 10, the value of slope ¼ 0.96842 obtains Q ¼ 2.90536 which imply that there is a mixed mechanism of energy transfer between Ce3þ ions. Combining with Fig. 9, there is intersection area between excitation and emission spectra which indicates the radiation reabsorption existed in system [12]. Therefore, the electric multipolar interaction and radiation reabsorption coexist in the energy transfer mechanisms.

Fig. 10. Linear fitting of relationship between log x and log I/x.

Fig. 11. Temperature-dependent luminescence intensity of Y3(GaxAl1-x)5O12:Ce3þ (x ¼ 0, 0.2, 0.4).

As is known, the thermal quenching is one of the important parameters for phosphors, which must be low for practical w-LED applications to maintain the stability of chromaticity and the brightness of white light output at high temperature (>150  C). The emission intensities of Y3(GaxAl1-x)5O12:Ce3þ (x ¼ 0, 0.2, 0.4) phosphors excited by 460 nm are investigated as a function of temperature in the range of 25e250  C, as shown in Fig. 11. It is revealed that the all intensities drop gradually with temperature increase, and 82.7%, 73.4%, 71.9% emission intensity remain when the temperature is raised up to 150  C for x ¼ 0, 0.2, 0.4, respectively, corresponding to the literature [10,38]. The thermal quenching may be resulted from the two main reasons: i) the

Fig. 12. The CIE color coordinates of Y2.94(GaxAl1-x)5O12: Ce3þ0.06 (x ¼ 0, 0.1, 0.2, 0.3, 0.4) under the room temperature (RM) and 150  C (HT). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

S. Fu et al. / Optical Materials 75 (2018) 619e625

garnet structure of Y3(GaxAl1-x)5O12:Ce3þ (x ¼ 0, 0.2, 0.4) phosphors become more and more relaxed and rigidity gradually decreases with x increasing, ii) the thermal ionization is more likely to arise as the Ga concentration increases, consisting with Fig. 8 based on References [10,38]. The CIE 1931 x-y color coordinates are taken to elevate the blue shift and thermal stability of the phosphors, as shown in Fig. 12. It exhibits a large blue-shift with x value increasing, from (0.4063, 0.5693) to (0.3225, 0.5861). And the x ¼ 0.4 shows more thermal stability than x ¼ 0 under 150  C with color coordinates (0.4039, 0.5618) and (0.3366, 0.5907), respectively. The results indicate that Y2,94(Ga0.4Al0.6)5O12:0.06Ce3þ phosphors can be employed in the w-LED [39,40]. 4. Conclusion In conclusion, a series of garnet-type Y3(GaxAl1-x)5O12:Ce3þ have been synthesized by the conventional high temperature solid-state reaction at 1350  C with duration of 2 h. The photoluminescence properties closely related to the Ga concentration. Ga3þ ions reduce the 5d level splitting which results in the PLE excitation spectrum red shift from 339 nm to 351 nm and blue shift from 465 nm to 437 nm, and PL emission spectrum blue shift from 541 nm to 517 nm. With all the octahedral centers (16a lattice) in garnet structure replaced by Ga (x ¼ 0.4), which is the turning point for structure and luminescence, the intensity and blue shift of emission reached to the largest. The difficulty of Ga3þ to occupy tetrahedral site leads to the appearance of impurities. The Ga3þ ions in tetrahedral site have less influence on the crystal field splitting of 5d level of Ce3þ. The occupation behavior of Ga3þ in garnet structure can account for the PL intensity decreasing with x > 0.4 and without further blue shift. Combining the thermal quenching with CIE color coordinates, the blue-shift Y3(GaxAl1-x)5O12: Ce3þ0.06 phosphors can ameliorate the absent part of w-LED. Acknowledgment This work was supported by National Natural Science Foundation of China (Grant No. 40643018). The authors thank Prof. Fei Zhuge for related help and discussion. References [1] C.C. Lin, R.S. Liu, Advances in phosphors for light-emitting diodes, J. Phys. Chem. Lett. 2 (2011) 1268e1277. [2] R. Praveena, L. Shi, K.H. Jang, V. Venkatramu, C.K. Jayasankar, H.J. Seo, Solegel synthesis and thermal stability of luminescence of Lu3Al5O12: Ce3þ nanogarnet, J. Alloys Compd. 503 (2011) 859e863. [3] S.J. Yoon, S.J. Dhoble, K. Park, Synthesis and photoluminescence properties of La1-xAlO3: xTb3þ green phosphors for white LEDs, Ceram. Int. 40 (2014) 4345e4350. [4] Y. Peng, R.X. Li, H. Cheng, Z. Chen, H. Li, M.X. Chen, Facile preparation of patterned phosphor-in-glass with excellent luminous properties through screen-printing for high-power white light-emitting diodes, J. Alloys Compd. 693 (2017) 279e284. [5] T. Cheng, X. Luo, S. Huang, S. Liu, Thermal analysis and optimization of multiple LED packaging based on a general analytical solution, Int. J. Therm. Sci. 49 (2010) 196e201. [6] S. Ye, F. Xiao, Y.X. Pan, Y.Y. Ma, Q.Y. Zhang, Phosphors in phosphor-converted white light-emitting diodes: recent advances in materials, techniques and properties, Mater. Sci. Eng. R. 71 (2010) 1e34. [7] G. Zhu, Z. Ci, S. Xin, Y. Wen, Y. Wang, Warm white light generation from Dy3þ doped NaSr2Nb5O15 for white LEDs, Mater. Lett. 91 (2013) 304e306. [8] J.S. Lee, P. Arunkumar, S. Kim, I.J. Lee, H. Lee, W.B. Im, Smart design to resolve spectral overlapping of phosphor-in-glass for high-powered remotetype white light-emitting devices, Opt. Lett. 39 (2014) 762e765. [9] A. Katelnikovas, T. Bareika, P. Vitta, T. Jüstel, H. Winkler, A. Kareiva, A.Z. ukauskas, G. Tamulaitis, Y3-xMg2AlSi2O12: Ce3þx phosphors e prospective for warm-white light emitting diodes, Opt. Mater. 32 (2010) 1261e1265. [10] J.Y. Zhong, W.D. Zhuang, X.R. Xing, R.H. Liu, Y.F. Li, Y.H. Liu, Y.S. Hu, Synthesis, crystal structures, and photoluminescence properties of Ce3þ-doped

[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]

625

Ca2LaZr2Ga3O12: new garnet green-emitting phosphors for white LEDs, J. Phys. Chem. C 119 (2015) 5562e5569. J.Y. Zhong, W.D. Zhuang, X.R. Xing, R.H. Liu, Y.F. Li, Y.H. Liu, Y.S. Hu, Blue-shift of spectrum and enhanced luminescent properties of YAG: Ce3þ phosphor induced by small amount of La3þ incorporation, J. Alloys Compd. 674 (2016) 93e97. M.Y. Ding, Y.T. Li, D.Q. Chen, H.W. Lu, J.H. Xi, Z.G. Ji, Hexagonal crown-capped NaYF4:Ce3þ/Gd3þ/Dy3þ microrods: formation mechanism, energy transfer and luminescence properties, J. Alloys. Compd 658 (2016) 952e960. C. Zhao, D.C. Zhu, M.X. Ma, T. Han, M.J. Tu, Brownish red emitting YAG: Ce3þ,Cuþ phosphors for enhancing the color rendering index of white LEDs, J. Alloys. Compd 523 (2012) 151e154. M.Y. Kim, D.S. Bae, Photoluminescence of Y3(Al, Ga)5O12: Ce3þ nanoparticles by a reverse micelle process, J. Mater. Res. 23 (2003) 31e35. M. Ayvacikli, A. Canimoglu, L.E. Muresan, L. Barbu Tudoran, Structural and luminescence effects of Ga Co-doping on Ce-doped yttrium aluminate based phosphors, J. Alloys Compd. 666 (2016) 447e453. A.B.M. Garcia, L.S. Jo, Structure, electronic, and spectroscopic effects of Ga codoping on Ce-doped yttrium aluminum garnet: first-principles study, Phys. Rev. B 82 (2010) 184118e184123. X.W. He, X.F. Liu, C.Y. You, Y.Z. Zhang, R.F. Li, R.H. Yu, Clarifying the preferential occupation of Ga3þ ions in YAG: Ce, Ga nanocrystals with various Ga3þdoping concentrations by nuclear magnetic resonance spectroscopy, J. Mater. Chem. C 4 (2016) 10691e10700. K.J. Rajan, S.V. Manorama, Formation of hierarchical macro porous YAlO: Ce multifunctional nanophosphors, J. Appl. Phys. 119 (2016) 114902. H.I. Won, H.H. Nersisyan, C.W. Won, K.H. Lee, Effect of metal halide fluxes on microstructure and luminescence of Y3Al5O12: Ce3þ phosphors, Mater. Chem. Phys. 129 (2011) 955e960. B.T. Liou, S.H. Yen, Y.K. Kuo, Proc. SPIE Int. Soc. Opt. Eng. 5628 (2005) 296e305. Y. Zhang, X.Q. Chen, H.M. Qin, Z.H. Luo, J. Jiang, H.C. Jiang, Fabrication of Cedoped Gd3(Al,Ga)5O12 powders using Co-precipitation method, J. Inorg. Mater. 31 (2016) 1151e1156. C. Manjunatha, B.M. Nagabhushana, D.V. Sunitha, H. Nagabhushana, S.C. Sharma, G.B. Venkatesh, R.P.S. Chakradhar, Effect of NaF flux on microstructure and thermoluminescence properties of Sm3þ doped CdSiO3 nanophosphor, J. Lumin 134 (2013) 432e440. Saito Yasuyoshi, EP: No 1032057, 2000, pp. 2e23. M. Kottaisamyk, R.J. Agannathan, R.P. Rao, et al., On the Formation of fluxgrown Y2O2S: Eu3þ red phosphors, J. Electrochem. Soc. 143 (1996) 189e197. J. Ueda, S. Tanade, T. Nakanishi, Analysis of luminescence quenching in solid solutions between Y3Al5O12 and Y3Ga5O12 by temperature dependence of photoconductivity measure, J. Appl. Phys. 110 (2011) 053102. J.M. Ogiegło, A. Katelnikovas, A. Zych, T. Justel, A. Meijerink, C.R. Ronda, Luminescence and luminescence quenching in Gd3(Ga,Al)5O12 scintillators doped with Ce3þ, J. Phys. Chem. A 117 (2013) 2479e2484. G. Blasse, A. Bril, A new phosphor for lying-spot cathode-ray tubes for color televisions, J. Appl. Phys. Lett. 11 (1967) 53e54. H.S. Jang, W.B. Im, D.C. Lee, et al., Enhancement of red spectral emission intensity of Y3Al5O12 phosphors via Pr3þ co-doping and Tb3þ substitution for application, J. Lumin 126 (2007) 371e377. Y.R. Shi, G. Zhu, M.S. Mikami, Y. Shimomura, Y.H. Wang, A novel Ce3þactivated Lu3MgAl3SiO12 garnet phosphor for blue chip light-emitting diodes with excellent performance, J. Dalton Trans. 44 (2014) 1775e1781. A.B. Munoz-García, L. Seijo, Crystallization of Y2O3-Al2O3 rich glasses: synthesis of YAG glass-ceramics, J. Phys. Chem. C 115 (2011) 815. Z. Xia, R.S. Liu, Tunableblue-green color emission and energy transfer of Ca2Al3O6F: Ce3þ, Tb3þ phosphors for near-UV white LEDs, J. Phys. Chem. C 116 (2012) 15604e15609. Paolo Ghigna, et al., Local structure of the Ce3þ ion in the yellow emitting phosphor YAG: Ce, Opt. Mater 34.1 (2011) 19e22. H.S. Jang, D.Y. Jeon, Yellow-emitting Sr3SiO5:Ce3þ,Liþ phosphor for whitelight-emitting diodes and yellow-light-emitting diodes, Appl. Phys. Lett. 90 (2007) 041906. A.A. Setlur, W.J. Heward, Y. Gao, A.M. Srivastava, R.G. Chandran, M.V. Shankar, Crystal chemistry and luminescence of Ce3þ-doped Lu2CaMg2(Si,Ge)3O12 and its use in LED based lighting, Chem. Mater. 18 (2006) 3314e3322. M. Nikl, K. Nitsch, E. Mihokova, N. Solovieva, J.A. Mares, P. Fabeni, G.P. Pazzi, M. Martini, A. Vedda, S. Baccaro, Efficient radioluminescence of the Ce3þdoped NaeGd phosphate glasses, Appl. Phys. Lett. 77 (2000) 2159. P.A.M. Berdowski, G. Blasse, Luminescence and energy transfer in a highly symmetrical system: Eu2Ti2O7, J. Solid State Chem. 62 (1986) 317e327. G. Blasse, Energy transfer in oxidic phosphors, Phys. Lett. A 28 (1968) 444e445. Y.C. Qiang, Y.X. Yu, G.L. Chenb, J.Y. Fang, Synthesis and luminescence properties of Y2.94Al5-mGamO12:0.06Ce3þ (1 m 2.5) green phosphors for white LEDs, Ceram. Int. 42 (2016) 767e773. T. Smith, J. Guild, The C.I.E. colorimetric standards and their use, Trans. Opt. Soc. 33 (1931) 75e134. C.S. McCamy, Correlated color temperature as an explicit function of chromaticity coordinates, J. Color Res. Appl. 17 (1992) 142e144.