# Fileset [Manuscript (garnet).pdf](https://mdr.nims.go.jp/filesets/2d2bd29f-15b3-4cfc-865d-7f11d56d1913/download) ## Creator Zhigang Sun, Ji-Guang Li, Huiyu Qian, [Yoshio Sakka](https://orcid.org/0000-0001-8357-5843), [Tohru S. Suzuki](https://orcid.org/0000-0001-9458-6863), Bin Lu ## Rights [Creative Commons BY Attribution 4.0 International](https://creativecommons.org/licenses/by/4.0/) ## Other metadata [Optical grade (Gd0.95-xLuxEu0.05)3Al5O12 ceramics with near-zero optical loss: Effects of Lu3+ doping on structural feature, microstructure evolution, and far-red luminescence](https://mdr.nims.go.jp/datasets/48ab174a-69e4-49d2-8e13-97fa0256451e) ## Fulltext Astrodynamics Word template Journal of Advanced Ceramics https://mc03.manuscriptcentral.com/jacer 1 1 Optical grade (Gd0.95-xLuxEu0.05)3Al5O12 ceramics with near-zero optical loss: 2 Effects of Lu3+ doping on structural feature, microstructure evolution, and 3 far-red luminescence 4 5 Zhigang SUNa, Ji-Guang. LIb, Huiyu QIANa, Yoshio. SAKKAb, Tohru S. SUZUKIb, Bin LUa,c,* 6 7 aSchool of Materials Science and Chemical Engineering, Ningbo University, Ningbo, Zhejiang 8 315211, China 9 bResearch Center for Electronic and Optical Materials, National Institute for Materials Science, 10 Tsukuba, Ibaraki 305-0044, Japan 11 cKey Laboratory of Photoelectric Detection Materials and Devices of Zhejiang Province, Ningbo, 12 Zhejiang 315211, China 13 14 * Corresponding author. 15 E-mail: lvbin@nbu.edu.cn 16 17 Abstract: A chemical co-precipitation strategy was employed to synthesize a series of (Gd0.95-18 xLuxEu0.05)3Al5O12 (x = 0.1−0.95) powder phosphors, followed by vacuum sintering to achieve 19 transparent garnet ceramic phosphors. The density functional theory indicated Lu3Al5O12 was formed 20 in priority compared with Gd3Al5O12 during solid-phase reaction. Upon high-temperature sintering, 21 the Lu3+ substitution for Gd3+ suppressed point mass diffusion leading to a smaller grain size. The in-22 line transmittances of the bulk specimens with x = 0.1, 0.3, 0.5, 0.7, and 0.95 nm were ~83.5, 80.1, 23 68.8, 73.7, and 82.2% at 710 nm (Eu3+ emission center), respectively, among which the x = 0.1 24 sample exhibited optical grade with near-zero optical loss in agreement with the defect-free single 25 Journal of Advanced Ceramics https://mc03.manuscriptcentral.com/jacer 2 crystal (~100% of the theoretical transmittance). The resulting particle and ceramic materials both 26 presented characteristic Eu3+ emission arising from 5D0→7FJ (J = 1−4) transition, where the dominant 27 far-red emission at ~710 nm arising from 5D0→7F4 transition overlapped with the absorption of 28 phytochrome PFR. The photoluminescence excitation and photoluminescence intensities of (Gd0.95-29 xLuxEu0.05)3Al5O12 powders and ceramics generally increased at a higher Gd3+/Lu3+ ratio. Lu3+ 30 dopants delayed the fluorescence lifetimes while the bulk samples had shorter lifetimes than the 31 particle counterparts. The transparent (Gd0.85Lu0.1Eu0.05)3Al5O12 ceramic phosphor exhibited good 32 thermal stability with a high thermal quenching temperature above 533 K. The designed ceramic 33 phosphor converted light-emitting diode had a saturation injection current of 435 mA and current-34 dependent color rendering index. More importantly, our report marked the developmental stage of 35 transparent ceramic materials towards zero optical loss. 36 37 Keywords: Transparent ceramics, Garnet, Phosphors, LED, Optical properties, Gibbs free energy 38 39 1 Introduction 40 It is well known that the sunlight plays a crucial role in ecological cycle. In this system, the 41 plants absorb the light energy and then convert the carbon dioxide and water into organic matter with 42 the release of oxygen via photosynthesis. Their light absorption ability derives from the complex and 43 well-defined photoreceptors on plant leaves [1,2]. These photoreceptors have different responses to 44 sunlight, that is, they selectively absorb blue (400−500 nm), red (600−690 nm), and far red (700−740 45 nm) light regions for phototropic processes, photosynthesis, and photomorphogenesis, respectively 46 [3-5]. Phytochromes are red (PR)/far-red (PFR) light photoreceptors, which play significant roles in 47 plant growth and development. The phytochrome PR is biologically inactive upon absorption of red 48 photons into the biological-active PFR state [6]. This is a reversible process, viz., PFR can be also 49 converted back to PR by absorbing far-red photons. Under far-red light irradiation, the plants would 50 Journal of Advanced Ceramics https://mc03.manuscriptcentral.com/jacer 3 yield shade-avoidance response as if they are blocked from sunlight [7]. Therefore, it is feasible to 51 increase the productivity or adjust the traits of various crops via varying the spectral components 52 (e.g., PR/PFR ratio) in phytotron chambers or greenhouses [8]. The Mn4+ and Cr3+ activated 53 luminescent materials can emit far-red light overlapped with the absorption of phytochrome PFR, and 54 thus are frequently used as potential artificial light sources for indoor plant cultivation. A phosphor, if 55 emits multiple beneficial fluorescence signals, may realize the functional diversification for plant 56 cultivation. 57 Apart from the spectral component, the phosphor material designed as artificial light source for 58 indoor plant cultivation should also have high luminescence efficiency, good luminescent thermal 59 stability, excellent chemical and physical stability, and low cost. The phosphor-converted light-60 emitting diodes (pc-LEDs) have the advantages over conventional light sources in terms of high 61 luminous efficiency, good durability, low power consumption, compact size, and friendliness to the 62 environment, which make this illuminating system ideal for plant cultivation [9-13]. A ceramic 63 phosphor is superior to the powder counterpart, since the bulk material avoids using the organic 64 polymer as the packaging material which has poor thermal dispersion (thermal conductivity: 0.1−0.4 65 W·m-1·K-1) to readily cause degradation of luminous intensity, acceleration of aging, and changes in 66 emission color upon long-term operation [14]. Besides, the difference in refractive indices between 67 powder phosphor and organic resin would induce light scattering. Hence, the development of 68 transparent ceramic phosphors against using organic resin not only effectively solves these problems 69 but also possesses the merits of more excellent thermal conductivity, optical transmission, chemical 70 stability, luminescent thermal stability, and mechanical property [15]. 71 Body-centered cubic Ln3Al5O12 (LnAG) aluminates (space group: Ia3d, No. 230) comprise 72 160 atoms per unit cell, where the Ln atoms reside in the dodecahedral interstices formed by the 73 corner-sharing arrangement of the AlO4 and AlO6 polyhedra [16,17]. In this chemical formula, Lu3+, 74 Y3+, and Gd3+ cations are three representative elements for Ln3+. They are attractive host materials 75 Journal of Advanced Ceramics https://mc03.manuscriptcentral.com/jacer 4 owing to their low phonon energy, adjustable bandgap energy, good biocompatibility, and excellent 76 chemical and physical properties. After being doped with rare-earth and/or transition-metal 77 activator(s), these luminescent aluminate materials are extensively applied in the systems of lighting, 78 imaging, laser, optical thermometry, biological sensor, photodynamic therapy, and optical amplifier 79 [18-23]. The Eu3+ doped fluorescent material is one of efficient red emission sources arising from 4f–80 4f intra-configurational 5D0→7F1,2 transitions [24-28]. However, a Eu3+ activated phosphor with 81 strong far-red emission from 5D0→7F4 transition has relatively rare reports. GdAG:Eu is considered 82 better than YAG:Eu and LuAG:Eu, since a 4f energy-level overlap between the 6PJ state of Gd3+ and 83 the 5HJ state of Eu3+ makes energy transfer from Gd3+ to Eu3+ possible. However, the pure GdAG 84 would suffer from the issue of low thermal stability, namely, GdAG easily decomposes into GdAlO3 85 perovskite and Al2O3 above 1300 °C. Previous reports indicate either substituting Gd3+ with smaller 86 Lu3+/Y3+ to decrease the average Ln3+ radii or replacing Al3+ with larger Ga3+ to extend the 87 dodecahedral interstices can stabilize the cubic garnet lattice structure [29-31]. 88 The optical loss caused by microdefects in ceramic bodies always exists, leading to an opaque 89 status or a low transparency relative to the corresponding perfect single crystal material. How to 90 achieve a ceramic material with zero optical loss is still a challenge. In the present work, optical 91 grade (Gd0.95-xLuxEu0.05)3Al5O12 ceramics were successfully fabricated by cost-effective vacuum 92 sintering. The compositional effects on structure feature, sintering behavior, and optical performance 93 were investigated in detail. Especially, our developed ceramic phosphors exhibit strong far-red and 94 yellow dual emissions for potential application to plant cultivation. 95 2 Experimental 96 2.1 Preparation 97 In a typical synthetic procedure, Gd2O3 (~99.999% purity, Haoke Technology Co., Ltd., Beijing, 98 China) and Lu2O3 (~99.99% purity, Haoke Technology Co., Ltd., Beijing, China) raw materials are 99 dissolved in excessive hot nitric acid upon heating till dryness to separately prepare the two nitrates. 100 Journal of Advanced Ceramics https://mc03.manuscriptcentral.com/jacer 5 A mother liquor was obtained by mixing the as-prepared two nitrates, commercial Eu(NO3)3·6H2O 101 (~99.99% purity, Diyang Chemical Co., Ltd., Shanghai, China), and NH4Al(SO4)2·12H2O (~99.999% 102 purity, Aladdin Biochemical Technology Co., Ltd., Shanghai, China) together according to the 103 chemical formula of (Gd0.95-xLuxEu0.05)3Al5O12 (x = 0‒0.95). The molar ratio of Eu3+ to total Ln3+ 104 cations was fixed at 5 at.%, since above which luminescence quenching would occur [32]. The 105 mother liquor was dripped into a 1.5 M ammonium hydrogen carbonate (AHC, 99.995% purity, 106 Macklin Biochemical Co., Ltd., Shanghai, China) solution at a rate of ~4 mL/min under magnetic 107 stirring at room temperature and the molar ratio of AHC to total cations was kept at 2.5:1. After 108 being aged for 48 h, the suspension was repeatedly washed with distilled water and absolute ethanol 109 via centrifugal separation, followed by drying at 90 ºC for more than 12 h in an oven. The dried 110 precursor was lightly crushed using an agate mortar, and then calcined in a tube furnace under 111 flowing oxygen (~100 mL/min) at 1200 ºC for 4 h to yield the garnet particle. Any additive was not 112 added during the whole particle synthesis process. The powder was pre-compressed in a stainless-113 steel mold, followed by cold isostatically pressed at 240 MPa. The green body was sintered in a 114 tungsten-heater furnace at 1680‒1750 ºC for 4 h under 10-4‒10-5 Pa vacuum. The sintered body was 115 annealed at 1550 ºC for 7 h in air and was finally polished on both sides to improve surface finish. It 116 should be noted that we selected different temperatures for sintering ceramic samples according to 117 the varying chemical compositions and the specified parameters along with some information on the 118 final sintered products can refer to Table S1 in the the supplementary material. 119 2.2 Characterization 120 The as-prepared powders were characterized by X-ray diffraction (XRD; Model D8 Advance 121 Davinci, Bruker, Karlsruhe, Germany) using nickel-filtered CuKα as incident radiation, Fourier 122 transform infrared spectroscopy (FTIR; Model Nicolet 6700, Thermo, Wisconsin, USA), field 123 emission scanning electron microscopy (FE-SEM; Model S-4800, Hitachi, Tokyo, Japan), and laser 124 diffraction particle size analyzer (Model Nano-ZS90, Malvern Instruments, Malvern, UK). The in-125 Journal of Advanced Ceramics https://mc03.manuscriptcentral.com/jacer 6 line transmittances of polished ceramics were measured on a UV/VIS/NIR spectrophotometer 126 (Model Lambda 950, Perkin-Elmer, Massachusetts, USA) from 200 to 2100 nm. The microstructures 127 of the bulks were observed on a desktop scanning electron microscope (D-SEM; Model EM-30plus, 128 COXEM, Daejeon, Korea). The photoluminescence (PL), photoluminescence excitation (PLE), and 129 fluorescence decay kinetics of phosphors and ceramics were recorded on two different steady-state 130 fluorescence spectrometers, that is, the Model FS5 equipment (Edinburgh Instruments, Edinburgh, 131 UK) for spectral test and the Model FLS 980 one (Edinburgh Instruments) for decay behavior 132 measurement. The quantum efficiency was determined by a professional quantum efficiency 133 measurement system (Model QE-2100, Otsuka Electronics, Shiga, Japan). The electroluminescence 134 (EL) spectra of the assembled LED device at diverse forward bias currents were measured using a 135 multi-channel spectroradiometer (Model SPEC 3000A, Measurefine Instrument, Hangzhou, China). 136 2.3 Computational methodology 137 First-principles calculations are performed in the framework of the density functional theory 138 (DFT) with the projector augmented plane-wave method, as implemented in the Vienna ab initio 139 simulation package. The generalized gradient approximation proposed by Perdew, Burke, and 140 Ernzerhof is selected for the exchange-correlation potential [32]. The calculations are done on the 141 two chemical compositions of Lu3Al5O12 and Gd3Al5O12. The cut-off energy for plane wave is set to 142 450 eV. The energy criterion is set to 10−5 eV in iterative solution of the Kohn-Sham equation. The 143 Brillouin zone integration is performed using a 6×6×6 k-mesh. All the structures are relaxed until the 144 residual forces on the atoms have declined to less than 0.01 eV/Å. 145 3 Results and discussion 146 3. 1 Density functional theory 147 2 3 2 3 3 5 123 ( ) 5 ( ) 2 ( )+ =Gd O s Al O s Gd Al O s (1) 148 2 3 2 3 3 5 123 ( ) 5 ( ) 2 ( )+ =Lu O s Al O s Lu Al O s (2) 149 Journal of Advanced Ceramics https://mc03.manuscriptcentral.com/jacer 7 Equations (1) and (2) are two crucial reactions during the whole particle synthesis process. 150 Current database lacks related thermodynamic functions for the two products of Gd3Al5O12 and 151 Lu3Al5O12, and thus DFT is used for calculation. DFT-optimized configuration for Gd3Al5O12 or 152 Lu3Al5O12 contains 65 atoms. In the standard state (101325 Pa), the changes in the Gibbs free 153 energies (ΔG) for Eqs. (1) and (2) at 1200 ºC are respectively determined to be -4.28 and -4.46 154 eV/atom based on the algorithm as denoted in Eq. (3) [33]. 155 3 5 12 2 3 2 3( , ) ( , ) [ ( , ) ( , )] ( , ) = − + − Ln Al O Ln O Al O fG T P G T P G T P G T P H T P (3) 156 3. 2 Structural features of synthetic products 157 Our co-precipitation method leads to rounded precipitation precursors with frequently empty 158 interiors upon pyrolysis into relatively discrete rounded garnet powders without observed hard 159 agglomeration (Fig. S1 in the supplementary material). Those precursors are primary in amorphous 160 states (Fig. S2 in the supplementary material). The chemical compositions of precipitation precursors 161 are qualitatively analyzed by FTIR spectra and the results reveal that the precursors possess the 162 structure of hydrated basic carbonate sulfate (Fig. S3 in the supplementary material). 163 164 Journal of Advanced Ceramics https://mc03.manuscriptcentral.com/jacer 8 Fig. 1 XRD patterns of the target particle products for (Gd0.95-xLuxEu0.05)3Al5O12 (x = 0.1−0.95) 165 obtained by calcining the precursors at 1200 ºC for 4 h (a) and the corresponding bulk materials 166 fabricated by vacuum sintering (b). The two insets in the right-hand panels show the enlarged view 167 of the main (420) diffraction peak. 168 169 Figure 1(a) exhibits the XRD patterns of target particle products for (Gd0.95-xLuxEu0.05)3Al5O12 170 (x = 0.1−0.95) obtained by calcining the precursors at 1200 ºC for 4 h. Their XRD peaks can be well 171 indexed into the cubic Lu3Al5O12 standard card (JCPDS No. 73-1368) without observed other impure 172 phase, indicating that the precipitation precursors have completely transformed into pure garnet 173 phase via thermal decomposition. TG analysis further reveals that the pyrolytic processes include 174 dehydroxylation, dehydroxylization, decarbonation, desulfuration, and solid-phase reaction (Fig. S4 175 in the supplementary material), while a higher Gd3+/Lu3+ ratio leads to a higher decomposition 176 temperature on each stage due to the higher alkalinity of Gd3+ than that of Lu3+. The crystallite size 177 (DXRD) can be calculated from the full width at half maximum (FWHM) of the (420) diffraction band 178 using Scherrer’s equation: DXRD = Kλ/(βCosθ), where K is the shape factor (K = 0.89), λ is the 179 wavelength of X-ray (λ = 0.15406 nm), β is the FWHM, and θ is the Bragg angle [34-37]. The 180 determined crystallite sizes are ~135, 123, 122, 116, and 97 nm for the specimens with x = 0.1, 0.3, 181 0.5, 0.7, and 0.95, respectively. The reduced crystallite size with more Lu3+ doping is associated with 182 thermodynamics. The whole calcination processes include thermal decomposition and solid-phase 183 reaction. A high-temperature solid-phase reaction primarily comprises chemical reaction and 184 diffusion process, where the chemical reaction is generally faster than the diffusion process for most 185 of solid-phase reaction. On the other hand, the crystallite formation mainly includes nucleation and 186 growth. Therefore, a chemical reaction that is more prone to occur will provide more nuclei, and vice 187 versa. The ΔG value of Eq. (2) is smaller than that of Eq. (1), suggesting that Lu3Al5O12 is formed in 188 priority during high-temperature solid-phase reaction, and thus a higher Lu3+/Gd3+ ratio would create 189 Journal of Advanced Ceramics https://mc03.manuscriptcentral.com/jacer 9 more crystal nuclei to reduce the average crystallite size. 190 After vacuum sintering, all ceramic samples still maintain the cubic garnet structure [Fig.1(b)]. 191 For the purposes of better comparison, we also prepare a ceramic composition according to 192 Gd3Al5O12. After sintering at 1650 ºC, this specimen exhibits main orthorhombic perovskite structure 193 with a small number of Al2O3 phase due to the thermal decomposition behavior: Gd3Al5O12 → 194 3GdAlO3 + α-Al2O3 at a high temperature (Fig. S5 in the supplementary material). Therefore, more 195 than 10 at.% Lu3+ substitution for Gd3+ has been proved to be effective enough for stabilizing the 196 cubic GdAG phase structure via reducing the average Ln3+ radius. The XRD patterns of ceramics 197 become much sharper relative to those of powders. Additionally, the (420) diffraction bands of bulk 198 specimens are slightly splitting in accordance with that of Lu3Al5O12 single crystal [38,39], but the 199 powder samples exhibit symmetrical peak profiles. These phenomena both indicate that the 200 crystallinity of the ceramic materials has been greatly improved via grain growth at high 201 temperatures. It can be obviously noted that the (420) diffraction peaks of the garnet powders and 202 ceramics both shifts towards the high angle side with increasing Lu3+ incorporation. This is because 203 the decreasing average ionic radius of Ln3+ induces the continuous shrinkage of the unit cell with the 204 increase in the amount of Lu3+ substitution for Gd3+, which also confirms that the Lu3+ and Eu3+ 205 dopants have been substantially dissolved into the GdAG lattice. 206 207 Fig. 2 Lattice constants and theoretical densities of (Gd0.95-xLuxEu0.05)3Al5O12 powders (a) and 208 ceramics (b) as a function of x value. 209 Journal of Advanced Ceramics https://mc03.manuscriptcentral.com/jacer 10 210 Figure 2 shows the lattice constants and the theoretical densities of (Gd0.95-xLuxEu0.05)3Al5O12 211 powders and ceramics as a function of Lu3+ content. The lattice constants are calculated according to 212 XRD data using Bragg’s equation. Their lattice constants almost linearly decrease at a higher Lu3+ 213 concentration. Such a phenomenon complies with Vegard’s law [40], suggesting that homogeneous 214 garnet solid solutions have formed. It should be noted that the powder material has a smaller cell 215 parameter than the bulk counterpart at each Lu3+ concentration. From bulks to nanocrystallites, the 216 lattice expansion or contraction phenomena are both possibly observed according to previous reports 217 [41,42], where the former is caused by internal stress and latter is due to surface tension. Herein, our 218 case belongs to surface tension. The theoretical densities (dth) of these solid solutions can be 219 determined from Eq. (4): 220 38{[ (1 ) ] 3 5 12 }= + + − −  + +th Lu Eu Gd Al OAd mM nM m n M M Ma N (4) 221 where a represents for the lattice constant, Mi stands for the atomic weight of element i (i = Lu, Gd, 222 Eu, and O), NA denotes the Avogadro constant, and m and n refer to the atomic percentages of Lu3+ 223 and Eu3+, respectively [43]. Their theoretical densities linearly rise with the increase in Lu3+ content 224 due to the heavier Lu3+ than Gd3+, namely, gradually approaching the theoretical density of 225 Lu3Al5O12 (6.67−6.73 g/cm3) [44,45]. 226 3.3 Transmittances and microstructures of (Gd,Lu)3Al5O12:Eu ceramics 227 228 Journal of Advanced Ceramics https://mc03.manuscriptcentral.com/jacer 11 Fig. 3 Appearances (a) and in-line transmittances (b) of transparent (Gd0.95-xLuxEu0.05)3Al5O12 (x = 229 0.1−0.95) ceramics. The lower part in panel (a) shows the red emission of (Gd,Lu)3Al5O12:Eu 230 ceramics irradiated by a 365 nm ultraviolet lamp. The embedded image in panel (b) displays the 231 thermal etching polished surface of the best specimen with x = 0.1. The samples are all ~1.0 mm in 232 thickness. 233 234 The appearances and in-line transmittances of vacuum sintered (Gd0.95-xLuxEu0.05)3Al5O12 (x = 235 0.1−0.95) ceramics are shown in Fig. 3. The letters under the five ceramics are able to be well read-236 through upon 365 nm ultraviolet lamp irradiation into characteristic orange emission of Eu3+ [Fig. 237 3(a)]. Nevertheless, the orange emission seems not strong to the naked eyes. This is because the most 238 intense excitation band of Eu3+ locates at 394 nm rather than 365 nm [Fig. 5(c)]. Another reason is 239 that the dominant 5D0→7F4 emission of Eu3+ locates at ~710 nm [Fig. 5(d)], whilst the human eye 240 sensitivity greatly decreases at 650 nm and approaches almost nil at 700 nm. In natural light, the two 241 specimens with x = 0.3 and 0.7 exhibit yellowish hue, whereas the other three samples are normally 242 colorless. The color is closely associated with the transmission behaviors. As indicated in Fig. 3(b), 243 the absorption peaks in the transmittance curves are caused by intra-4f6 transitions of Eu3+, but the x 244 = 0.3 and 0.7 samples have stronger absorption extent around 455 nm blue region and thus appear 245 the optical compensatory yellow color. Such a phenomenon may be related to more oxygen defects. 246 Under vacuum sintering at elevated temperature, the oxygen vacancy is easy to form while an extra 247 defect energy level may be introduced. This new energy level may be around 5D2 state of Eu3+ to 248 enhance 7F0,1→5D2 absorption. It should be noted that these oxygen vacancies induced under oxygen 249 deficient condition are unstable, and also difficult to be precisely controlled. Even if the ceramics are 250 treated by post-annealing, the oxygen in air is not always able to fully fill up in the vacancy site via 251 diffusion in the dense sintered body. 252 The in-line transmittances of ceramic specimens with x = 0.1, 0.3, 0.5, 0.7, and 0.95 are ~83.5, 253 Journal of Advanced Ceramics https://mc03.manuscriptcentral.com/jacer 12 80.1, 68.8, 73.7, and 82.2% at 710 nm (Eu3+ emission center), respectively, among which the 10 at.% 254 Lu3+-stabilized GdAG:Eu bulk sample has the best optical quality. The theoretical transmittance (T) 255 for a defect-free Lu3Al5O12 single crystal can be deduced from the relationship between refractive 256 index (n) and wavelength (λ) as follows [Eqs. (5)−(7)]: 257 ( )2)1 (T R exp t= − − (5) 258 ( )( )2211nRn−=+ (6) 259 22210.4335 0.0055n= +− (7) 260 where α is the loss factor and t is the sample thickness [46-48]. Assuming α = 0, the calculated 261 refractive index and theoretical transmittance at 710 nm are ~1.83 and ~83.5%, respectively, and 262 hence the corresponding ceramic specimens with x = 0.1, 0.3, 0.5, 0.7, and 0.95 are equivalent to 263 ~100, 96, 82, 88, and 98% of the theoretical value. Even more importantly, the x = 0.1 sample 264 exhibits optical grade with near-zero optical loss in accordance with the defect-free single crystal. 265 Such an almost perfect optical quality has a scarce report for any transparent ceramic material, 266 further revealing the advancement of our ceramic technology. 267 The thermal etching microstructure of the best x = 0.1 sample is shown in the inset of Fig. 3(b). 268 Its statistic average grain size via WinRoof image analysis software is only around 7.5 μm. The quite 269 fine grain size contributes to improved mechanical property. Additionally, the grain size is relatively 270 uniform, and no abnormal grain growth or residual pores can be observed. The relative density of the 271 ceramic is determined to be ~100% by Archimedes method, in agreement with the pore-free dense 272 structure and the excellent optical quality. 273 Journal of Advanced Ceramics https://mc03.manuscriptcentral.com/jacer 13 274 Fig. 4 D-SEM micrographs showing the surface microstructures of vacuum sintered (Gd0.95-275 xLuxEu0.05)3Al5O12 specimens with x = 0.1 (a) and 0.95 (b). 276 277 Figure 4 compares the surface microstructures of (Gd0.95-xLuxEu0.05)3Al5O12 ceramic specimens 278 with x = 0.1 and 0.95, which are sintered at an identical temperature of 1680 ºC. Their average grain 279 sizes are counted to be ~9.8 μm for x = 0.1 and ~2.5 μm for x = 0.95. Apparently, the former has a 280 larger grain size than the latter. Point mass diffusion, such as grain-boundary diffusion and volume 281 diffusion, mainly determines the grain size. The more Gd3+/Lu3+ ratio in the garnet system leads to 282 faster point mass diffusion, which is somewhat similar to our previously reported (Y,Gd)2O3-based 283 sesquioxide [49]. Considering that the particle crystallite size originally decreases with increasing 284 Lu3+ addition (Fig. 1), these phenomena imply that the crystallites in powders may undergo nearly 285 synchronous growth upon subsequent sintering densification. It should be noted that the same bulk 286 sample (e.g. x = 0.1) has discrepant statistic average grain sizes from the thermal etching polished 287 surface [~7.5 μm in the inset of Fig. 3(b)] and untreated surface [~9.8 μm in Fig. 4(a)]. Apparently, 288 the latter is larger than the former because of the preferential sintering on surface. In practice, the 289 statistic value on the thermal etching polished surface has better representativeness. 290 3.4 Spectral behaviors of the phosphor particles and ceramics 291 Journal of Advanced Ceramics https://mc03.manuscriptcentral.com/jacer 14 292 Fig. 5 PLE (a, c) / PL spectra (b, d) and CIE chromaticity diagrams (e, f) of (Gd0.95-293 xLuxEu0.05)3Al5O12 powders (a, c, e) and ceramics (b, d, f) as a function of x value. 294 295 Figure 5(a) presents the PLE spectra of (Gd0.95-xLuxEu0.05)3Al5O12 (x = 0.1−0.95) powders as a 296 function of Lu3+ concentration. The broad band at ~230–267 nm is caused by charge transfer (CT) 297 from the 2p orbital of O2- to the 4f orbital of Eu3+ and the neighboring peak at ~275 nm is attributed 298 to the 8S7/2→6IJ transition of Gd3+ [49,50]. The other weak excitation peaks beyond 300 nm are 299 assignable to intra-4f6 transition of Eu3+ as indicated in Fig. 5(a), among which the strongest one 300 locates at ~394 nm arising from 7F0→5L6 transition of Eu3+. Under 394 nm excitation, the 301 corresponding PL spectra exhibit characteristic Eu3+ emissions deriving form 5D0→7FJ (J = 1−4) 302 Journal of Advanced Ceramics https://mc03.manuscriptcentral.com/jacer 15 transitions, where the 5D0→7F1 transition at ~592−596 nm belongs to magnetic-dipole transfer, the 303 5D0→7F2,4 transitions (~610−630 nm for 5D0→7F2 transition and ~696−710 nm for 5D0→7F4 304 transition) ascribe to electric-dipole transfer, and the 5D0→7F3 transition at ~650−656 nm is 305 forbidden [25]. Thereinto, the 5D0→7F4 emission dominates in each case. The 5D0→7F0 transition of 306 Eu3+ (generally at ~580 nm) is absent because it only exists in the point symmetric Cs, Cn, and Cnv (n 307 = 1, 2, 3, 4, 6) sites [51]. However, a cubic garnet lattice provides a crystallographic D2 position for 308 Eu3+ substitution. The D2 lattice site has a high degree of symmetry, but Eu3+ ions sometimes reside 309 at the deviation site of dodecahedral interstices. As a result, either the magnetic-dipole 5D0→7F1 310 transition or the electric-dipole 5D0→7F4 transfer likely dominates in the emission spectra of Eu3+ 311 doped garnet systems, while their ratio depends on not only the surrounding environment but also the 312 cationic average electronegativity [52]. In this work, the electric-dipole 5D0→7F4 transition presents 313 the strongest intensity due to the geometric distortion. 314 The PLE and PL spectra of (Gd0.95-xLuxEu0.05)3Al5O12 ceramics are shown in Figs. 5(c) and (d). 315 The band positions of ceramics basically have no change relative to those of powders, but the PLE 316 and PL intensities significantly increase. By taking x = 0.1 sample as an example, the ceramic 317 phosphor exhibits ~20.5-fold higher PL intensity than the particle counterpart. Additionally, the 318 PLE/PL intensities of (Gd0.95-xLuxEu0.05)3Al5O12 powders and ceramics gradually rise at a higher 319 Gd3+/Lu3+ ratio, where the specimen with x = 0.1 has the strongest intensity (~3.8 times that of the 320 counterpart with x = 0.95). The enhanced luminescence cannot ascribe to the energy transfer from 321 Gd3+ to Eu3+, because the 394 nm excitation wavelength (~3.1 eV) is not enough to excite the 322 electrons of the ground state onto the 6PJ level of Gd3+ (~4.0−4.1 eV) as illustrated in Fig. S6 of the 323 supplementary material. The reason can consider the average electronegativity for Ln3+. That is, Gd3+ 324 is less electronegative than Lu3+ (1.20 for Gd3+ and 1.27 for Lu3+), and thus a higher Gd3+/Lu3+ ratio 325 facilitates electron transfer. By comprehensively considering the spectral behaviors as well as the 326 optical transmittances, the optimum ceramic composition is (Gd0.85Lu0.1Eu0.05)3Al5O12. 327 Journal of Advanced Ceramics https://mc03.manuscriptcentral.com/jacer 16 The Commission International de I’Eclairage (CIE) chromaticity diagrams of (Gd0.95-328 xLuxEu0.05)3Al5O12 phosphors and ceramics under 394 nm excitation are drawn in Figs. 5(e) and (f). 329 The CIE chromaticity coordinates of (Gd0.95-xLuxEu0.05)3Al5O12 powders are (0.56, 0.34), (0.58, 0.36), 330 (0.58, 0.36), (0.55, 0.34), and (0.56, 0.34) for the specimens with x = 0.1, 0.3, 0.5, 0.7, and 0.95, 331 respectively, which all lie in the yellowish pink region. The corresponding CIE chromaticity 332 coordinates of ceramic samples are generally consistent at (0.61, 0.38), which all fall into the 333 reddish-orange scope. 334 335 Fig. 6 Emission spectrum of the best (Gd0.85Lu0.1Eu0.05)3Al5O12 ceramic composition together with 336 the absorption spectra of phytochrome PR and PFR. 337 338 For the purpose of exploring the applicability for plant culture, we compare the PL spectrum of 339 the best (Gd0.85Lu0.1Eu0.05)3Al5O12 ceramic composition with the absorption spectra of phytochrome 340 PR and PFR as shown in Fig. 6. The dominate far-red emission peak at ~710 nm from 5D0→7F4 341 transition of Eu3+ has a significant overlap with the absorption of phytochrome PFR. This 5D0→7F4 342 emission peak position is quite analogous to those of previously reported Mn4+ activated CaGdAlO4, 343 La(MgTi)1/2O3, and Ca3La2W2O12 phosphor particles developed for plant cultivation [12,53,54]. On 344 the other hand, the contributions of 5D0→7F1,2,3 emissions are relatively small to phytochrome PR, 345 because either their band intensities are weak or their peak positions are far away from the maximal 346 absorption wavelength of phytochrome PR. However, the second strongest luminescence band from 347 Journal of Advanced Ceramics https://mc03.manuscriptcentral.com/jacer 17 5D0→7F1 transition of Eu3+ appears yellow emission. Yang et al. found that the yellow light was 348 optimal for the growth and accumulation of bioactive flavonoids in some plants compared with white, 349 red, or blue light [55]. In consequence, our developed ceramic phosphor could sufficiently provide 350 both far-red and yellow emissions, implying the application potential to modulate plant growth as 351 artificial light source. 352 353 Fig. 7 Fluorescence decay kinetics of (Gd0.95-xLuxEu0.05)3Al5O12 (x = 0.1−0.95) phosphor particles (a) 354 and ceramics (b) for 710 nm emission under 394 nm excitation of Eu3+. Panels (c) and (d) show the 355 relationships between lifetimes and x values for the powders and ceramics, respectively. 356 357 Figures 7(a) and (b) display the fluorescence decay kinetics of (Gd0.95-xLuxEu0.05)3Al5O12 (x = 358 0.1−0.95) particle and ceramic phosphors for 710 nm emission under 394 nm excitation of Eu3+. The 359 fluorescence lifetime can be deduced by fitting the decay curve with a single exponential model 360 equation: I=Aexp(-t/τ)+B, where I is the instantaneous fluorescence intensity, τ is the lifetime, t is the 361 Journal of Advanced Ceramics https://mc03.manuscriptcentral.com/jacer 18 decay time, and A and B are constants [56-58]. The determined τ, A, and B parameters are listed in 362 Table S2 of the supplementary material. The lifetime values vary from ~5.75 to 7.34 ms for particle 363 phosphors while from ~3.10 to 3.36 ms for ceramic phosphors. The bulk samples have much shorter 364 lifetimes than the particle counterparts due to the elimination of electron-attracting defects via 365 improved crystallinity and grown grain [49,59]. It also can be seen that the lifetimes increase with 366 the rising Lu3+ contents for both the two materials [Figs. 7(c) and (d)]. This is because Lu3+ addition 367 significantly suppresses mass diffusion and crystallite/grain growth during particle calcination and 368 ceramic sintering. 369 370 Fig. 8 Quantum efficiency measurement for the optimum composition of (Gd0.85Lu0.1Eu0.05)3Al5O12 371 under 394 nm excitation. The inset is the enlargement for the wavelength range of Eu3+ emission. 372 373 Figure 8 depicts the response of the optimum ceramic composition of (Gd0.85Lu0.1Eu0.05)3Al5O12 374 to the 394 nm excitation wavelength using solid barium sulfate white standard as a reference material. 375 The quantum efficiency (ηQE) data are collected by an integrating sphere, which could be 376 automatically derived according to Eq. (8) by a built-in software in the spectrophotometer [53,60]. 377 100%SQER SLE E= −  (8) 378 where LS stands for the emission spectrum of the specimen, and ER and ES represent the excitation 379 Journal of Advanced Ceramics https://mc03.manuscriptcentral.com/jacer 19 spectra for the BaSO4 reference material and the ceramic sample, respectively. The resulting 380 quantum efficiency value for our transparent (Gd0.85Lu0.1Eu0.05)3Al5O12 ceramic phosphor is ~42%, 381 which is higher than the other Eu3+ doped aluminate garnet phosphors such as Lu3Al5O12:Eu (~40%) 382 and Y3Al5O12:Eu (~40.5%) [61,62]. 383 3.5 Thermal stability of luminescence for the transparent ceramic phosphor 384 385 Fig. 9 The temperature-dependent PL spectra (a), the two-dimension plot of emission dependence on 386 temperature (b), the normalized emission intensity (c), and the plot of 1/kT versus Ln(I0/IT-1) (d) of 387 our best ceramic composition under 394 nm excitation. 388 389 The luminescent thermal stability is a significant parameter for the realized functionalization of 390 fluorescent materials in solid-state lighting. The temperature-dependent luminescence measurement 391 for the best fluorescent ceramic composition (Gd0.85Lu0.1Eu0.05)3Al5O12 is performed in the 392 temperature range of 118−833 K, and the results are shown in Figs. 9(a) and (b). With the increasing 393 heating temperature, the PL intensity follows a trend of decline due to the thermal quenching effect 394 Journal of Advanced Ceramics https://mc03.manuscriptcentral.com/jacer 20 derived from electron-phonon interaction. Moreover, the change in emission wavelength, if any, is 395 only ~1 nm within the allowable system error (Fig. S7 in the supplementary material). If we assume 396 the fluorescence intensity at room temperature to be 100%, the maximum value achieves at 118 K 397 [Fig. 9(c)], which is ~1.33 times that of room temperature. The thermal quenching temperature, 398 defined as the temperature at which PL intensity is 50% of the value at room temperature, is a little 399 higher than 533 K for our sample. Such a high thermal quenching temperature indicates that our 400 developed material has a high thermal stability of luminescence. The thermal stability of the sample 401 is further evaluated by the activation energy of thermal quenching process based on Arrhenius 402 equation (Eq. 9): 403 01 exp( / )TIIA E kT=+ − (6) 404 where I0 is the luminescence intensity at room temperature, IT is the emission intensity at a certain 405 temperature, k is Boltzmann constant (8.6×10-5 eV/K), A is a constant, and ΔE is the activation 406 energy of thermal quenching [24,63,64]. The ΔE value can be estimated by taking the logarithm on 407 both sides of this equation, viz., the plot of Ln(I0/IT−1) against (kT)-1 yields a straight line whose 408 slope is equal to ‒ΔE. As shown in Fig. 9(d), the resulting ΔE value is ~0.17 eV with a high degree of 409 fitting (R2 = 99.4%), which is equivalent to the commercially available Y2O3:Eu3+ red phosphor (0.17 410 eV) [65]. The probability of a nonradiative transition per unit time (α) is closely associated with the 411 activation energy according to Eq. (10): 412 exp( )EskT−= (10) 413 where s is the frequency factor [66]. Apparently, a high activation energy would effectively suppress 414 the probability of nonradiative transition. 415 3.6 EL properties of the designed LED lamp 416 Journal of Advanced Ceramics https://mc03.manuscriptcentral.com/jacer 21 417 Fig. 10 Appearance photographs (a), thermal radiation images (b), EL emission spectra (c), and EL 418 integral intensities normalized to 1 (d) of the prepared LED lamp at working currents of 50−500 mA. 419 420 The best fluorescent ceramic specimen is directly covered on a near UV (390−395 nm) chip to 421 package a LED device without any organic polymers and its appearance can be seen from Fig. S8 of 422 the supplementary material. Under an injection current of 50 mA, the assembled LED lamp emits 423 weak red emission as shown in Fig. 10(a). Applying a higher working current of more than 150 mA 424 yields the strong glaring red emission. As the current increases, the junction temperature of the 425 packaged red-emitting LED shows an increasing tendency up to maximum of ~120.8 ºC at 435 mA, 426 and then decreases to ~106.9 ºC at 500 mA. Compared with particle phosphor converted LED [67], 427 our ceramic phosphor converted LED exhibits a lower junction temperature and a more uniform 428 Journal of Advanced Ceramics https://mc03.manuscriptcentral.com/jacer 22 thermal radiation at each current due to its higher conductivity via avoiding using the organic 429 polymer and its own feature of pure substance. As presented in Fig. 10(c), the EL emission spectra 430 also display the characteristic 5D0→7FJ (J = 1−4) transition of Eu3+ and their intensities rise with 431 enhanced injection current up to 435 mA, and then decrease at a higher current. That is, the 435 mA 432 current seems to be the saturation point. Such a phenomenon may be attributed to the induced 433 thermal effect referred to Fig. 10(b). From the normalized curve [Fig. 10(d)], the EL integral 434 intensity at 435 mA is ~11-fold higher than that at 50 mA. At this saturation current, the 435 corresponding color coordinates, color rendering index (CRI), and correlated color temperature 436 (CCT) are (0.61, 0.39), 81.6, and 1745 K, respectively. The elevated working current increases the 437 CRI parameter, but has little effect on CCT and color coordinates (Table S3 in the supplementary 438 material). The phenomenon of saturated injection current for electroluminescence has seldom studied 439 for comparison. However, we believe that the pore-free ceramics may have a higher saturated value 440 relative to the powder forms and the translucent ceramics due to the higher thermal conductivity. 441 4 Conclusions 442 A series of (Gd0.95-xLuxEu0.05)3Al5O12 (x = 0.1−0.95) particle and ceramic phosphors were 443 prepared to study the compositional effects on structure features, sintering behaviors, and 444 luminescence properties. All the phosphors exhibit main far-red emission at ~710 nm arising from 445 5D0→7F4 transition, which overlaps with the absorption of phytochrome PFR. A Lu3+ dopant increases 446 the theoretical density, suppresses the crystallite/grain growth, reduces the luminescence intensity, 447 and delays the fluorescence lifetime. The optimum x = 0.1 ceramic composition exhibits optical 448 grade with near-zero optical loss, a high quantum efficiency of ~42%, and excellent thermal stability 449 with an activation energy of ~0.17 eV. The designed ceramic phosphor converted LED possesses a 450 saturation injection current of 435 mA, high color rendering index of 81.6, and low correlated color 451 temperature of 1745 K. Our findings suggest that the developed red-emitting LED has potential 452 application for plant cultivation. 453 Journal of Advanced Ceramics https://mc03.manuscriptcentral.com/jacer 23 454 Acknowledgment 455 The research was supported by Zhejiang Provincial Natural Science Foundation of China under 456 Grant No. LY23F050007. 457 458 Declaration of competing interest 459 The authors have no competing interests to declare that are relevant to the content of this article. 460 461 Appendix A. Supplementary data 462 Supplementary data to this article can be found in the “Supplementary Material” section. 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 Journal of Advanced Ceramics https://mc03.manuscriptcentral.com/jacer 24 References 479 [1] Nagano S, Guan KL, Shenkutie SM, et al. Structural insights into photoactivation and signalling 480 in plant phytochromes. Nat Plants 2020, 6: 581–588. 481 [2] Li L, Cao QW, Xie J, et al. 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Designing multicolor luminescence in Tb3+/Eu3+-codoped 633 KGd2F7 nanoparticles through energy transfer engineering for warm white light-emitting diode. J 634 Lumin 2023, 261: 119916. 635 636 637 638 639 640 641 642 643 644 645 646 647 648 649 650 651 652 Journal of Advanced Ceramics https://mc03.manuscriptcentral.com/jacer 31 Figure captions 653 Fig. 1 XRD patterns of the target particle products for (Gd0.95-xLuxEu0.05)3Al5O12 (x = 0.1−0.95) 654 obtained by calcining the precursors at 1200 ºC for 4 h (a) and the corresponding bulk materials 655 fabricated by vacuum sintering (b). The two insets in the right-hand panels show the enlarged view 656 of the main (420) diffraction peak. 657 Fig. 2 Lattice constants and theoretical densities of (Gd0.95-xLuxEu0.05)3Al5O12 powders (a) and 658 ceramics (b) as a function of x value. 659 Fig. 3 Appearances (a) and in-line transmittances (b) of transparent (Gd0.95-xLuxEu0.05)3Al5O12 (x = 660 0.1−0.95) ceramics. The lower part in panel (a) shows the red emission of (Gd,Lu)3Al5O12:Eu 661 ceramics irradiated by a 365 nm ultraviolet lamp. The embedded image in panel (b) displays the 662 thermal etching polished surface of the best specimen with x = 0.1. The samples are all ~1.0 mm in 663 thickness. 664 Fig. 4 D-SEM micrographs showing the surface microstructures of vacuum sintered (Gd0.95-665 xLuxEu0.05)3Al5O12 specimens with x = 0.1 (a) and 0.95 (b). 666 Fig. 5 PLE (a, c) / PL spectra (b, d) and CIE chromaticity diagrams (e, f) of (Gd0.95-667 xLuxEu0.05)3Al5O12 powders (a, c, e) and ceramics (b, d, f) as a function of x value. 668 Fig. 6 Emission spectrum of the best (Gd0.85Lu0.1Eu0.05)3Al5O12 ceramic composition together with 669 the absorption spectra of phytochrome PR and PFR. 670 Fig. 7 Fluorescence decay kinetics of (Gd0.95-xLuxEu0.05)3Al5O12 (x = 0.1−0.95) phosphor particles (a) 671 and ceramics (b) for 710 nm emission under 394 nm excitation of Eu3+. Panels (c) and (d) show the 672 relationships between lifetimes and x values for the powders and ceramics, respectively. 673 Fig. 8 Quantum efficiency measurement for the optimum composition of (Gd0.85Lu0.1Eu0.05)3Al5O12 674 under 394 nm excitation. The inset is the enlargement for the wavelength range of Eu3+ emission. 675 Fig. 9 The temperature-dependent PL spectra (a), the two-dimension plot of emission dependence on 676 temperature (b), the normalized emission intensity (c), and the plot of 1/kT versus Ln(I0/IT-1) (d) of 677 Journal of Advanced Ceramics https://mc03.manuscriptcentral.com/jacer 32 our best ceramic composition under 394 nm excitation. 678 Fig. 10 Appearance photographs (a), thermal radiation images (b), EL emission spectra (c), and EL 679 integral intensities normalized to 1 (d) of the prepared LED lamp at working currents of 50−500 mA. 680 681 682 683 684 685 686 687 688