New Y 2 BaAl 4 SiO 12 : Ce 3 + yellow microcrystal-glass powder phosphor with high thermal emission stability †

To decrease the rare earth element usage and synthesis cost of Y3Al5O12:Ce phosphor, the Y2BaAl4SiO12 compound is developed as a new host for Ce employing the solid solution design strategy. The design uses polyhedron substitution where YO8/AlO4 are partially replaced by BaO8/SiO4, respectively. Structure analysis of Y2BaAl4SiO12 proves that it successfully preserves the garnet structure, crystallizing in the cubic Ia % 3d space group with a = b = c = 12.00680(5) Å. Barium (Ba) atoms occupy the Y site and silicon (Si) atoms occupy the Al site in the AlO4 tetrahedrons. An expanded study on Y2MAl4SiO12 (M = Ba, Ca, Mg, Sr) series shows a cation size (of M)-dependent phase formation behavior. The lattice stability can be related with the M type in the M–Si pair and substitution level of M–Si for Y–Al. Doping Ce into Y2BaAl4SiO12 yields bright yellow photoluminescence peaking at around 537 nm upon excitation by 460 nm light. The emission intensity is quite stable against thermal quenching whereas the peak wavelength shows a slight red-shift as the ambient temperature increases. The crystallization behavior of Y2BaAl4SiO12 is suggested as melt-assisted precipitation/growth based on cathodoluminescence analysis. The highly crystalline nature of the microcrystals explains the stable emission against thermal quenching. This study may provide an inspiring insight into preparing phosphor with new morphology-structure of ‘‘microcrystal-glass powder phosphor’’, which distinguishes it from conventional ‘‘ceramic powder phosphor’’ or ‘‘single-crystal phosphor’’.


Introduction
White light emitting diodes (WLEDs) are playing a key role in lighting and illumination. 1The main way to produce white light from a monochromatic light emitting diode (LED) combines a blue indium gallium nitride (InGaN) with a down-converting phosphor such as (Y,Gd) 3 (Al,Ga) 5 O 12 :Ce (YAG:Ce), or (Tb,Re) 3 (Al,Ga) 5 O 12 :Ce (TAG:Ce). 2,3Applying YAG and TAG in WLED were patented by the Nichia Corporation and Osram, respectively.Thus, it is of practical interest to find alternative candidates free from intellectual property conflict with YAG/TAG.Several (oxy)nitride candidates such as Ca-a-Sialon:Eu 2+ , 2 (Sr,Ba)-Si 2 O 2 N 2 :Eu 2+ , 4 (La,Ca) 3 Si 6 N 11 :Ce 3+ , 5 CaAlSiN 3 :Ce 3+ , 6 and SrAl-Si 4 N 7 :Ce 3+ , 7 have been developed and patented.Also, several oxide candidates such as (Lu/Y) 3 MgAl 3 SiO 12 :Ce 3+ , 8,9 Lu 3 (Al,Mg) 2 -(Al,Si) 3 O 12 :Ce 3+ , 10 Y 3 Mg 2 AlSi 2 O 12 :Ce 3+ , 11,12 or LaSr 2 AlO 5 :Ce 3+ , 13 are free from patent restriction and may be considered.Compared to the (oxy)nitride candidates, the oxide candidates have a lower synthesis cost and are easier to use in the form of ceramic plate (for application in emerging packaging techniques such as the remote phosphor arrangement 14 which can reduce the thermal effect of the LED p-n junction).
Intending to develop a new yellow candidate with less rare earth element usage than YAG:Ce, in this paper, we report the Y 2 BaAl 4 SiO 12 :Ce composition artificially created from YAG:Ce by using the solid state design, [15][16][17] where a YO 8 polyhedron is replaced by a MO 8 [M = barium (Ba), calcium (Ca), magnesium (Mg) or strontium (Sr)] and for charge neutrality, an AlO 4 tetrahedron is simultaneously substituted by a SiO 4 tetrahedron.This process is also called ''chemical unit substitution''. 18,19The phase formation temperature for Y 2 MAl 4 SiO 12 is also expected to decrease because of the M-Si pair introduction.An essential concern regarding such a design is whether the new phosphor can retain the garnet structure and whether the luminescence will be readily comparable to YAG:Ce; this paper presents the results of these two questions.In addition, the Y 2 MAl 4 SiO 12 :Ce powder sample contains highly crystalline micro semi-single crystals, which permits stable emission intensity against thermal quenching.This microstructure, which differs from the conventional ceramic-powder-phosphor or the single-crystal-phosphor, provides an inspiring insight for an important new type of phosphor, i.e., the microcrystal-glass powder phosphor.

Experimental
(Y,Ce) 2 MAl 4 SiO 12 (M = Ba, Ca, Mg, Sr) phosphors were prepared by firing the mixtures of high purity oxides (499.9%),yttrium(III) oxide (Y 2 O 3 ), cerium(IV) oxide (CeO 2 ), aluminium oxide (Al 2 O 3 ), silicon dioxide (SiO 2 ), and MCO 3 (M = Ba, Ca, Sr) or magnesium oxide (MgO) in respective stoichiometric ratios.The powder mixtures were homogeneously ground and then placed in boron nitride crucibles, which were loaded into an alumina tubular furnace and heated at 1350-1450 1C with a holding time of 1-5 h in a reducing hydrogen (5%)-nitrogen (95%) atmosphere.Phosphor compositions referred to later in the paper are their nominal ones.The powder X-ray diffraction (XRD) data were collected on an X-ray diffractometer (SmartLab, Rigaku, Tokyo, Japan) with CuK a radiation (1.54056 Å), operating at 45 kV and 200 mA with a scan speed of 21 min À1 .Crystal structure refinements employing the Rietveld method were implemented using TOPAS 4.2 software. 20Photoluminescence spectra were measured using a spectrofluorometer (FP-6500 Jasco, Tokyo, Japan) at room temperature.The emission stability against temperature increase (30-200 1C) was investigated using a combined setup including a xenon lamp, a multichannel photodetector (MPCD-7000, Hamamatsu) and a computer controlled electric heater.The internal quantum efficiency (IQE) and external quantum efficiency (EQE) of the phosphor were determined on a phosphor quantum efficiency spectrophotometer (QE-1100, Otsuka Electronics, Japan), following the relationship of EQE = a Â IQE (a: absorption efficiency).The reflection spectrum of barium sulfate (BaSO 4 ) white standard was used for calibration.The crystallization behavior study was carried out on a scanning electron microscope (SEM; S-4300, Hitachi) equipped with a cathodoluminescence system (CL; MP32S/M, Horiba).The phosphor particles were embedded in epoxy resin and then cut using an argon ion cross section polisher (SM-09010, Jeol Ltd, Tokyo, Japan) for 18 h.The beam current of the CL measurement was fixed at 100 pA and the electron beam energy was set at 5 kV, which corresponds to a penetration depth of about 350 nm.

Results and discussion
3.1 Cation-size dependent phase formation behaviors Different phase formation behaviors depending on cation (M) size were observed in the (Y,Ce) 2 MAl 4 SiO 12 series.As seen from Fig. 1, after heating at 1450 1C for 5 h, the M = Ba/Sr analogues formed the crystalline garnet phase, whereas the M = Ca/Mg ones contained the garnet main phase and a secondary phase (such as CaAl 2 SiO 7 or MgAl 2 O 4 ).A broad diffraction band in the 25-351 (2y) range suggesting the existence of a secondary amorphous glass phase appears in the M = Ba pattern and the intensity of this band decreases in that of M = Sr and then disappears in those of the Ca/Mg ones.As for the secondary crystalline phase, the corresponding amount in the M = Mg analogue is larger than that in the Ca analogue.Since some of the main phase atoms are in the glass phase, real chemical compositions of the Ba/Sr-members should differ slightly from the designed ones, which explains the fact that the evolution of cell volume (V) against ionic radius (IR) of M does not show a wholly linear trend (Fig. S1, ESI †).Such observations indicate an M cation size dependent phase formation behavior in the (Y,Ce) 2 MAl 4 SiO 12 series: an M cation with a bigger size favors the single crystalline phase formation.If only considering the formation of crystalline phases, one may recognize it from the viewpoint of the garnet structure stability: when YO 8 /AlO 4 are substituted by MO 8 /SiO 4 , a bigger M cation favors the stabilization of the artificial crystal lattice; whereas for smaller M cations, the lattice becomes less stable, which would be more likely to induce the formation of residue secondary phases.Al 3+ and Si 4+ with a coordination number (CN) of 4 have an effective IR of 0.39 and 0.26 Å, respectively.Y 3+ , Ba 2+ , Sr 2+ , Ca 2+ and Mg 2+ with a CN of 8 have IR of 1.019, 1.42, 1.26, 1.12 and 0.89 Å, respectively. 21Thus, substitution of Al by Si will contract the unit cell and a reasonable hypothesis is that the replacement of Y by a foreign cation with a bigger size will be more likely to stabilize the lattice.In contrast, the Ca or Mg that replaces Y is not favorably big, leading to a high tendency of impurity formation.A similar case with garnet where the cation size influences the structure stability has been reported for Gd 3 Al 5 O 12 :Ce which is derived from Lu 3 Al 5 O 12 :Ce by replacing lutetium (Lu; 0.977 Å) with gadolinium (Gd; 1.503 Å).The difference in IR between Gd and Lu is attributed as the intrinsic reason which makes GAG thermodynamically metastable above 1500 1C as well as the formation of the gadolinium aluminum perovskite (GdAlO 3 ) by-product. 22,23In 2013, the preparation of Y 2 CaAl 4 SiO 12 using a sol-gel combustion route was reported (firstly annealing at 1000 1C for 2 h and subsequently sintering at 1450 1C for 4 h). 24he sample contained a small amount of Ca 2 Al 2 SiO 7 impurity.In the case of Lu 2 CaAl 4 SiO 12 , however, it was found that Lu 2 CaAl 4 SiO 12 prepared at 1400 1C for 4 h formed a pure garnet phase. 25This suggests that the crystal lattice of Lu 2 CaAl 4 SiO 12 is more likely to be stable than that of Y 2 CaAl 4 SiO 12 , which confirms that the phase formation behavior depends on the cation size.A recent study 26 demonstrated the synthesis of Y 2 MgAl 4 SiO 12 :Ce at 1300-1400 1C with a prolonged holding period of 12 h together with an intermediate preheating (at 1000 1C for 8 h) gave an almost pure phase.Thus, the claim of ''M cation size dependent phase formation behavior in the (Y,Ce) 2 MAl 4 SiO 12 series'' stresses that a bigger M cation favors a pure garnet phase formation, but it does not necessarily mean that the M = Ca/Mg analogues are not able to form a pure phase.
In addition, the amorphous phase formation in the M = Ba pattern may be related to the level of Ba-Si substituting for Y-Al in YAG.To verify this effect, a set of Y 2.96Àx Ba x Ce 0.04 Al 5Àx Si x O 12 (x = 0.8, 0.6) samples were prepared by heating at 1400 1C for 2 h.The broad diffraction band in the XRD patterns can still be observed in these samples; however, when comparing their normalized ones (Fig. S2, ESI †), it is clear that with decreasing Ba-Si substitution level from x = 1.0 to x = 0.6, the intensity of this broad band gradually decreases, suggesting a decreased glass-phase/crystalline-phase ratio in these products.Thus, in addition to the M cation type, the amount of glass phase in the Y 2 MAl 4 SiO 12 phosphors also depends on the M-Si/Y-Al substitution level.

Photoluminescence of Y 2 MAl 4 SiO 12 :Ce
The emission spectra of the powder samples under l ex = 460 nm are shown in Fig. 2. The broad asymmetric emission of Y 1.96 Ce 0.04 BaAl 4 SiO 12 with a doublet structure feature originates from the energy transition of the lowest Ce 3+ 5d excited state to the two 4f ground states ( 2 F 5/2 and 2 F 7/2 ).From M = Ba to Mg, the emission maximums gradually red-shift from 534 to 552 nm (Table 1), suggesting an increasing crystal field splitting of the Ce 5d levels.This is because of the fact that with the decreasing M cation size in the lattice, the unit cell volume decreases and the bond lengths of Ce with coordinating O atoms become shortened.The evolution from the M = Ba to M = Sr bands does not show the obvious red-shift effect, probably because of amorphous phase formation in the M = Ba samples which leads the intrinsic formula away from the designed Y 2 BaAl 4 SiO 12 :Ce.The full-width at half-maximum (fwhm) values of the spectra show an increasing trend from 0.4242 eV to 0.4522 eV by 6.7% (Table 1) and such a broadening of the spectra suggest a more diverse coordination environments for Ce ions.The spectra were further analyzed using Gaussian fitting (Fig. 2) and the component fit-band 2 shows a more extreme red-shift and broadening than the fit-band 1 (Table 1).Generally, there is no difference in red-shift between these two bands because the energy difference between the two 4f states should remain the same.This feature observed here together with the broadening may be caused by some inhomogeneity in the emission as a consequence of Ce ions in slightly different local environments having different emission energy.In addition, the relative intensities of the two Gaussian components vary among these spectra and therefore, the well resolved fine structure feature becomes gradually less distinguishable and the spectra become somewhat symmetric for the M = Ca/Mg analogues.The M = Ba and Ca samples give a relatively high emission intensity, whereas the emissions of the Sr/Mg analogue are low (probably because of the co-existence of impurity phases).
The peaking emission of Y 2 CaAl 4 SiO 12 :Ce has been reported by Katelnikovas et al., 24 which shows a shift from 542 to 560 nm upon variation of the Ce concentrations.The emission maximum of the M = Ca phosphor in this study locates at 544 nm, which is consistent with previous results.In addition, the emission maximum of Lu 2 CaAl 4 SiO 12 :Ce is reported to be in the range of 520-542 nm, 25 which shows a blue-shift compared to Y 2 CaAl 4 SiO 12 :Ce because of a lower crystal field splitting.
3.3 Crystal structure of Y 2 BaAl 4 SiO 12 Fig. 3 shows the XRD refinement plot of Y 1.96 Ce 0.04 BaAl 4 SiO 12 sample and the scale of the y-axis in the difference Rietveld plot was square-rooted twice to clearly reveal the broad diffraction band and the high angle diffractions.All crystalline peaks are able to be indexed by cubic cell (Ia% 3d) with parameters close to Y 3 Al 5 O 12 whose crystal structure was then taken as a starting model for the refinement of this pattern.In the process, the site of the Al1 ion (in the tetrahedral site) was occupied by Al/Si ions with a fixed occupancy of 2/3 and 1/3, respectively.The Y site was occupied by the Ba/Y/Ce ions with fixed occupancy according to the suggested formula.The process was stable and ended with low R-factors (Table 2 and Fig. 3), indicating the validity of the refinement.Coordinates of each atom and the main bond lengths are listed in Tables 3 and 4

Emission optimization of Y 2 BaAl 4 SiO 12 :Ce
The synthesis conditions were further varied on aspects of temperature and holding period.First, a set of samples were prepared at 1400 1C and held for 1, 2 or 4 h.In this group, the one sintered for 2 h gives a higher emission intensity.Then, with a holding time of 2 h, two other samples were heated at 1300 1C or 1350 1C.However, these samples contained the garnet main phase and different amounts of barium aluminate (BaAl 2 O 4 ) secondary phase (Fig. S3, ESI †).Thus, the sintering parameters of 1400 1C for 2 h were chosen.Under this condition, a series of Y 2Ày Ce y BaAl 4 SiO 12 (y = 0.02, 0.04, 0.06, 0.08) were prepared to optimize the Ce doping concentration; among these samples, the y = 0.08 sample gave the highest emission intensity.The concentration quenching effect does not occur with such a high Ce doping level, probably because of the fact that some Ce may exist in the amorphous phase.The emission peak slightly     red-shifts to a longer wavelength as the Ce content increases.The y = 0.08 phosphor shows a broad emission band (Fig. 5a) with an intense yellow body-color (Fig. 5b).The excitation spectrum shows the maximum intensity at 455 nm, which matches well with the emission of the current efficient blue LEDs (420-460 nm). 3 The fwhm of the emission band is 104 nm, which is similar to that of Y 3 Al 5 O 12 :Ce (B104 nm), but smaller than that of other garnet phosphors such as (Lu,Y) 3 MgAl 3 SiO 12 :Ce (137-147 nm). 8The higher energy band (320-400 nm) of the excitation is much weaker than that of its low energy counterpart (400-520 nm).In comparison, the excitation of a typical YAG:Ce powder shows high and low energy bands of nearly equal intensity. 24A similar feature has been reported for Y 2 CaAl 4 SiO 12 :Ce, 24 which experienced more severe photoionization at lower excitation energies in comparison to YAG:Ce; such a mechanism is also expected to explain the features observed for Y 2 BaAl 4 SiO 12 :Ce.
Under l ex = 460 nm, the y = 0.08 sample exhibits an absorption efficiency of 80.2%, IQE of 95.2% and EQE of 76.4%, which shows a relatively high efficiency performance for the Y 2 BaAl 4 SiO 12 :Ce 3+ phosphor.The existence of a glass phase may affect the absorption of incident photons by phosphor particles; thus, either a post-treatment (such as removal of the glass with etching) or designing the substitution of Ba/Si for Y/Al at a lower level may probably help improve the absorption efficiency.

Temperature dependent emission of Y 2 BaAl 4 SiO 12 :Ce
Although new packaging configurations such as the phosphorin-glass, the remote-phosphor-arrangement and the singlecrystal-phosphor were proposed as promising routes to reduce the thermal effect of the LED p-n junction on the phosphor emission, direct coating of the phosphor-polymer mixture onto the LED chip is still the mainstream in the current market.Thus, it is of practical interest to evaluate the thermal emission stability of a new phosphor.
Emission spectra (l ex = 460 nm) of Y 1.92 Ce 0.08 BaAl 4 SiO 12 over the temperature range 30-200 1C are shown in Fig. 6.The emission intensity decreased with increasing temperature because of thermal quenching.The intensity of the emission peak, when tested at 200 1C, becomes 91.5% of that measured at 30 1C, demonstrating a high stability against the thermal effect, which is quite close to that reported for a single-crystal YAG:Ce phosphor. 27As the temperature increases, the emission maximum shifts towards longer wavelengths and a similar red-shift effect was also observed for a YAG:Ce single-crystal phosphor, 28 which was explained by the temperature dependent absorption and the temperature dependent emission decay of the two Gaussian components.Overall, the emission stability against thermal effect of the Y 2 BaAl 4 SiO 12 :Ce 3+ phosphor is relatively high which may enable it to be used in high-powder WLEDs.

Crystallization behavior of Y 2 BaAl 4 SiO 12 :Ce
The combined technique of scanning electron microscope and cathodoluminescence (SEM-CL) was employed to characterize the binding situation and respective morphology/emission of the amorphous and crystalline phases.The characterization was performed on the cross-sectional area of Y 1.92 Ce 0.08 BaAl 4 SiO 12 .The SEM image shows isolated crystals embedding in the particle matrix (Fig. 7a).The CL emission spectrum measured for the phosphor particle consists of a dominating band peaking at 540 nm and a weak shoulder peaking at 360 nm.Monochromatic CL mapping images taken for these two emissions suggest that both of the two emission bands originate from the crystalline crystals because the bright sections are almost identical (Fig. 7b and c).The emitting sections at 540 nm in Fig. 7b indicate a relatively uniform distribution, and the local bright crystals (diameter of B1 mm) tend to be of spherical shape, which may be induced by the intrinsic cubic structure governed crystallization.The point-dependent CL studies show that the dark points (points 5 and 10) of the particles give no emission and different bright points give the 540 nm and 360 nm emissions with different relative intensities.Previously, this ultraviolet band (360 nm) emission has been observed for undoped YAG powder and attributed to an excitonic emission localized near the anti-site defect Y 3+ Al . 29,30he CL mapping demonstrates that the bright luminescent crystals are uniformly embedded in the dark non-emitting amorphous phase within the cross-sectional area.Such a binding situation may suggest crystallization behavior of Y 2 BaAl 4 SiO 12 crystals from the melt during high temperature sintering or, the nucleation starts at low temperature with a conventional solid  state reaction with the following crystal growth significantly favoured by the melt generation at a higher temperature.More elaborate studies are awaited to verify these claims.The micro Y 2 BaAl 4 SiO 12 crystals are highly crystalline and they prefer to be close to the single-crystal rather than to the conventional ceramic powder.This mechanism explains the high thermal emission stability (Fig. 6).
Although the preparation uses the conventional solid state reaction route, the Y 2 BaAl 4 SiO 12 :Ce powder obtained contains micro semi-single-crystals.This feature shares some similarity with the glass-ceramic phosphor 31,32 which also contains both crystals and glass, but is different from either the single-crystalphosphor or the ceramic-powder-phosphor.The differences are compared in Fig. 8.The melt assisted precipitation-growth mechanism is also different from the diffusion-nucleationgrowth in the solid state reaction.The melt formation is different from the situation that occurred in the glass-ceramic phosphor, which is in situ generated because of the formation of some low temperature eutectic components with barium oxide-silicon dioxide (BaO-SiO 2 ) addition.This kind of structure may be referred to as a ''microcrystal-glass powder phosphor'' to demonstrate the combination of ''powder'' and ''microcrystal'' features.The exact substitution contents of Ce into the Y 2 BaAl 4- SiO 12 microcrystals may be lower than the designed nominal ratios, but Ce incorporation should not be that difficult as in the case of the single-crystal-phosphor (in which problems of Ce-doping solubility and gradient segregation need to be addressed 28 ).In comparison, the microcrystal-glass powder phosphor can avoid these disadvantages but shares the merit of high emission stability.A recent tentative study suggests that Y 2 BaAl 4 SiO 12 is highly likely to form a dense ceramic phosphor under vacuum sintering, which opens more application opportunities for this phosphor.the formation of such morphology relates to the melt generation during high temperature sintering.The development of Y 2 BaAl 4 SiO 12 :Ce guarantees the formation of a Y 3Àx Ba x Al 5Àx Si x O 12 : Ce (0 o x o 1) series which provides the possibility to reduce the use of rare earth elements in commercial YAG and potentially eliminate intellectual property conflict.This study also provides an inspiring insight to preparing the microcrystal-glass powder phosphor, which is distinguished from conventional ceramicpowder-phosphor (because of improved emission stability against thermal quenching) or single-crystal-phosphor (because of the lower preparation cost).
Fig.3shows the XRD refinement plot of Y 1.96 Ce 0.04 BaAl 4 SiO 12 sample and the scale of the y-axis in the difference Rietveld plot was square-rooted twice to clearly reveal the broad diffraction band and the high angle diffractions.All crystalline peaks are able to be indexed by cubic cell (Ia% 3d) with parameters close to Y 3 Al 5 O 12 whose crystal structure was then taken as a starting model for the refinement of this pattern.In the process, the site of the Al1 ion (in the tetrahedral site) was occupied by Al/Si ions with a fixed occupancy of 2/3 and 1/3, respectively.The Y site was occupied by the Ba/Y/Ce ions with fixed occupancy according to the suggested formula.The process was stable and ended with low R-factors (Table2and Fig.3), indicating the validity of the refinement.Coordinates of each atom and the main bond lengths are listed in Tables3 and 4, respectively.The crystallographic information file (CIF) is provided in the ESI.† The unit cell of Y 2 BaAl 4 SiO 12 is depicted in Fig. 4, where the coordination situation of polyhedrons (Y/Ba)O 8 , AlO 6 and (Al,Si)O 4 is shown.The Y 2 BaAl 4 SiO 12 is able to preserve the garnet structure and the introduced Ba and Si atoms occupy the Y and Al (in the AlO 4 tetrahedron) sites, respectively.

Fig. 3
Fig. 3 Observed (black), calculated (red), and difference (gray) XRD profiles for the refinement of the Y 1.96 Ce 0.04 BaAl 4 SiO 12 sample using the Rietveld method.Bragg reflections are indicated with vertical marks.

Fig. 4
Fig. 4 Unit cell structure of Y 2 BaAl 4 SiO 12 viewed along the c axis, showing a typical cubic garnet structure.Ba ions occupy the Y site, and Si ions occupy the Al site in the AlO 4 tetrahedron.

Fig. 5
Fig. 5 (a) Emission and excitation spectra of Y 1.92 Ce 0.08 BaAl 4 SiO 12 and the blue curves give the Gaussian fitting components.(b) Digital image of the phosphor under sunlight.

Fig. 6
Fig. 6 (a) Emission spectra (l ex = 460 nm) of Y 1.92 Ce 0.08 BaAl 4 SiO 12 as recorded over the temperature range 30-200 1C.(b) Evolution of the peaking wavelength and the relative peak intensity against temperature increase.

4
Conclusions A new (Y,Ce) 2 BaAl 4 SiO 12 phosphor was developed using a solid solution method starting from Y 3 Al 5 O 12 :Ce using polyhedron substitution (BaO 8 /SiO 4 for YO 8 /AlO 4 ).The Y 2 MAl 4 SiO 12 series (M = Ba, Ca, Mg, Sr) shows a cation (M) size dependent phase formation behavior and a bigger M cation in the formula favors the formation of a single crystalline garnet phase.For the Ba-analogue, the amount of amorphous phase in the product also depends on the Ba-Si for Y-Al substitution level.The newly developed Y 2 BaAl 4 SiO 12 successfully saves the original garnet structure of YAG.When doped with Ce ions, it absorbs blue light and can efficiently emit yellow light in the spectral range of 470-700 nm.The broad asymmetric emission can be decomposed into two Gaussian emission components originating from the energy transition from the lowest Ce 5d state to two 4f states.The highly crystalline nature of the submicron Y 2 BaAl 4 SiO 12 crystals explains the high emission stability against the thermal effect and

Fig. 7
Fig. 7 Cross-sectional SEM images of the phosphor particles (a) (scale bar: 10 mm); CL mapping taken for the 540 nm emission (b) and 360 nm emission (c).Point CL study was performed on the particles (d) and the corresponding emission spectra are given in (e).

Fig. 8
Fig.8Comparison of the preparation/crystallization between the ceramicpowder phosphor, the single-crystal phosphor, the glass-ceramic phosphor and the microcrystal-glass powder phosphor.

Table 1
Peaking wavelength and full-width at half-maximum (fwhm) of the Y 1.96 Ce 0.04 MAl 4 SiO 12 series emissions