Highly Efﬁcient Orange-Red Emission in Sm 3+ -Doped Yttrium Gallium Garnet Single Crystal

: High-quality single crystals with empirical composition Y 2 . 96 Sm 0 . 04 Ga 5 O 12 (YGG: Sm 3+ ) were successfully prepared by the optical ﬂoating zone method for the ﬁrst time and compared with related single crystals of Y 2 . 96 Sm 0 . 04 Al 5 O 12 (YAG: Sm 3+ ). With both crystals, XRD showed that Sm 3+ entered the cubic-phase structure. Optical absorption spectra produced a series of peaks from Sm 3+ in the 250 nm to 550 nm range, and photoluminescence excitation (PLE) spectra detected at 613 nm showed strong excitation peaks at 407 nm and 468 nm. A strong emission peak at 611 nm (orange-red light) was observed in the photoluminescence (PL) spectra under excitations at both 407 and 468 nm, respectively, but it was much brighter under excitation at 407 nm. Furthermore, with both emission spectra, the peaks from the YGG: Sm 3+ crystal were signiﬁcantly more intense than those from the YAG: Sm 3+ crystal, and both experienced a blue shift. In addition, under excitation at 407 nm, the color purity of the emitted orange-red light of YGG: Sm 3+ was higher than that of the YAG: Sm 3+ crystal, and the ﬂuorescence lifetime for the 4 G 5/2 → 6 H 7/2 transition of YGG: Sm 3+ was longer than that of the YAG: Sm 3+ crystal. The optical properties of the YGG: Sm 3+ crystal are better than those of the YAG: Sm 3+ crystal.


Introduction
Rare-earth-based luminescent materials have been widely studied for their excellent spectral properties, which include a high and adjustable luminescence, long fluorescence lifetime, and large Stokes shift [1,2]. Consequently, they have extensive uses in lightemitting diodes (LEDs), lasers, optical temperature sensors, optical communications, display panels, luminescence dosimeters, and biomedical diagnostics [3][4][5][6]. Nevertheless, considerable efforts are still being employed to improve the luminescence properties of such rare-earth-doped materials [7][8][9], and the color, intensity, and luminescence efficiency have been shown to strongly depend on the structure and composition of the luminescent center [8,10]. Furthermore, the crystal structure, ionic radius, luminescence efficiency, refractive index, and phonon energy are key factors in determining the suitability of hosts and dopants, and the usefulness of certain fluorescent materials is dependent on their unique compositions [11].
Oxides with a garnet structure are commonly used as hosts for rare-earth-doped luminescent materials, and commercial w-LED lamps are currently manufactured using a combination of YAG: (Ce 3+ , Sm 3+ ) yellow and red phosphors [12,13]. However, there are manufacturing problems associated with the use of these materials, including an uneven

Physical Measurements
Powder samples ground from crystal were measured by XRD (DX-2700, Dandong Hao Yuan Company, Dandong City, Liaoning Province, China) at room temperature using Cu-Kα radiation (λ = 1.54060 nm). Measurements were performed over the range 10-90° 2θ in steps of 0.02° with sampling times of 3 s, and the resulting XRD patterns were analyzed using Jade software (MDI Jade 6.0). Crystal densities were measured by a high-precision density tester (DE-120M, Daho Meter Company, Dongguan, Guangdong Province, China). By measuring the weight of YGG: Sm 3+ and YAG: Sm 3+ single crystal rods in air and pure water, respectively, the volume of the crystal V can be obtained based on the Archimedes principle: where M is the weight of the crystal in air (in grams), F is the weight of the crystal in pure water (in grams), and w is the density of pure water at room temperature, which is 1.0 g/cm 3 . The density of the crystal can be calculated by the following formula: Absorption spectra were obtained in the 250-550 nm range with a UV-Vis Spectrophotometer (UV-2700, Shimadzu, Kyoto, Japan).
A photoluminescence spectrometer (ZLF-325, Zolix Instruments Co., Ltd., Beijing, China) was used to measure the photoluminescence emission (PL) and excitation spectra (PLE), with a 150 W xenon lamp as the excitation light source.
Fluorescence lifetimes were obtained with an Edinburgh steady/transient fluorescence spectrometer (FLS1000, Edinburgh, UK) under excitation with 407 nm light, and then the average lifetime of the samples was determined by tri-exponential fitting.

X-ray Diffraction
The XRD patterns of powders ground from YGG: Sm 3+ and YAG: Sm 3+ single crystals ( Figure 2) are consistent with the diffraction peaks of the YGG standard card (PDF c-01-071-2151) and YAG standard card (PDF c-01-088-2048), respectively. The diffraction peaks Crystal densities were measured by a high-precision density tester (DE-120M, Daho Meter Company, Dongguan, Guangdong Province, China). By measuring the weight of YGG: Sm 3+ and YAG: Sm 3+ single crystal rods in air and pure water, respectively, the volume of the crystal V can be obtained based on the Archimedes principle: where M is the weight of the crystal in air (in grams), F is the weight of the crystal in pure water (in grams), and ρ w is the density of pure water at room temperature, which is 1.0 g/cm 3 . The density of the crystal ρ can be calculated by the following formula: Absorption spectra were obtained in the 250-550 nm range with a UV-Vis Spectrophotometer (UV-2700, Shimadzu, Kyoto, Japan).
A photoluminescence spectrometer (ZLF-325, Zolix Instruments Co., Ltd., Beijing, China) was used to measure the photoluminescence emission (PL) and excitation spectra (PLE), with a 150 W xenon lamp as the excitation light source.
Fluorescence lifetimes were obtained with an Edinburgh steady/transient fluorescence spectrometer (FLS1000, Edinburgh, UK) under excitation with 407 nm light, and then the average lifetime of the samples was determined by tri-exponential fitting.

X-ray Diffraction
The XRD patterns of powders ground from YGG: Sm 3+ and YAG: Sm 3+ single crystals ( Figure 2) are consistent with the diffraction peaks of the YGG standard card (PDF c-01-071-2151) and YAG standard card (PDF c-01-088-2048), respectively. The diffraction peaks for both samples are narrow and demonstrate that the crystals have a good crystallinity. One can also see in Figure 2 that the YGG: Sm 3+ crystal peaks are shifted to lower angles compared to the YAG: Sm 3+ samples, thus confirming that replacing Al 3+ by Ga 3+ increases the lattice parameter, as expected from its larger size.
for both samples are narrow and demonstrate that the crystals have a good crystallinity. One can also see in Figure 2 that the YGG: Sm 3+ crystal peaks are shifted to lower angles compared to the YAG: Sm 3+ samples, thus confirming that replacing Al 3+ by Ga 3+ increases the lattice parameter, as expected from its larger size. The cell dimensions and cell volumes of the YGG: Sm 3+ and YAG: Sm 3+ crystals calculated using Jade software are shown in Table 1 and demonstrate that the substitution of Ga 3+ (ionic radius 0.61 Å) for Al 3+ (ionic radius 0.53 Å) [37] results in an increase in the lattice constant from 1.201 nm to 1.230 nm and in the cell volume from 1.732 nm 3 to 1.860 nm 3 .

Density Measurement
The measured densities of the YGG: Sm 3+ and YAG: Sm 3+ single crystals are shown in Table 2, where M is the weight of the crystal in air and V is the volume of the crystal. The densities of Y2.96Sm0.04Ga5O12 and Y2.96Sm0.04Al5O12 crystals obtained from the present work (ρ) are 5.756 and 4.533 g/cm 3 , respectively, which are very close to the densities of crystals calculated by Jade (ρcal in Table 2), indicating that our experimental data are effective and reliable. The density of YGG: Sm 3+ crystal is higher than that of YAG: Sm 3+ crystal because the atomic mass of Ga (69.72) is greater than that of Al (26.98).  The cell dimensions and cell volumes of the YGG: Sm 3+ and YAG: Sm 3+ crystals calculated using Jade software are shown in Table 1 and demonstrate that the substitution of Ga 3+ (ionic radius 0.61 Å) for Al 3+ (ionic radius 0.53 Å) [37] results in an increase in the lattice constant from 1.201 nm to 1.230 nm and in the cell volume from 1.732 nm 3 to 1.860 nm 3 .

Density Measurement
The measured densities of the YGG: Sm 3+ and YAG: Sm 3+ single crystals are shown in Table 2, where M is the weight of the crystal in air and V is the volume of the crystal. The densities of Y 2 . 96 Sm 0 . 04 Ga 5 O 12 and Y 2 . 96 Sm 0 . 04 Al 5 O 12 crystals obtained from the present work (ρ) are 5.756 and 4.533 g/cm 3 , respectively, which are very close to the densities of crystals calculated by Jade (ρ cal in Table 2), indicating that our experimental data are effective and reliable. The density of YGG: Sm 3+ crystal is higher than that of YAG: Sm 3+ crystal because the atomic mass of Ga (69.72) is greater than that of Al (26.98).  Figure 3a shows the absorption spectra of YGG: Sm 3+ and YAG: Sm 3+ crystals, and Figure 3b shows the optical band gaps of YGG: Sm 3+ and YAG: Sm 3+ crystals. The absorption spectra of YGG: Sm 3+ and YAG: Sm 3+ single crystals in the range of 250 nm to 550 nm ( Figure 3a) are similar because they are derived from transitions between Sm 3+ 4f electronic energy levels, which are largely shielded from external structural effects. Ten absorption peaks were observed at 345, 362, 377, 390, 407, 419, 439, 468, 482 and 495 nm, corresponding to 6 H 5/2 → 4 D 7/2 , 4 D 3/2 , 6 P 7/2 , 4 L 15/2 , 4 F 7/2 , 6 P 5/2 , 4 G 9/2 , 4 I 13/2 , 4 I 9/2 and 4 G 7/2 transitions, respectively [38]. The transition 6 H 5/2 → 4 F 7/2 (407 nm) appears to be stronger than other transitions and is more intense in the YGG: Sm 3+ crystal than in the YAG: Sm 3+ crystal, thus demonstrating that crystal density and differences in the ionic radii of Al 3+ and Ga 3+ in YAG and YGG can affect the intensity of the Sm 3+ electronic absorption transitions. Moreover, the absorptions at 407 and 468 nm indicate that YGG: Sm 3+ crystals can be excited by InGaN-based light-emitting diodes and are thus fluorescent materials for manufacturing white LEDs [39]. The absorption peak in the range of 250 nm to 330 nm ( Figure 3a) corresponds to the absorption of the matrix, and the absorption edge of YGG: Sm 3+ single crystal shifts to a longer wavelength than YAG: Sm 3+ single crystal, which is because YGG and YAG crystals are different substrates and have different band gaps [40]. energy levels, which are largely shielded from external structural effects. Ten absorpti peaks were observed at 345, 362, 377, 390, 407, 419, 439, 468, 482 and 495 nm, correspon ing to 6 H5/2 → 4 D7/2, 4 D3/2, 6 P7/2, 4 L15/2, 4 F7/2, 6 P5/2, 4 G9/2, 4 I13/2, 4 I9/2 and 4 G7/2 transitions, resp tively [38]. The transition 6 H5/2 → 4 F7/2 (407 nm) appears to be stronger than other tran tions and is more intense in the YGG: Sm 3+ crystal than in the YAG: Sm 3+ crystal, th demonstrating that crystal density and differences in the ionic radii of Al 3+ and Ga 3+ YAG and YGG can affect the intensity of the Sm 3+ electronic absorption transitions. Mo over, the absorptions at 407 and 468 nm indicate that YGG: Sm 3+ crystals can be excited InGaN-based light-emitting diodes and are thus fluorescent materials for manufacturi white LEDs [39]. The absorption peak in the range of 250 nm to 330 nm (Figure 3a) cor sponds to the absorption of the matrix, and the absorption edge of YGG: Sm 3+ single cr tal shifts to a longer wavelength than YAG: Sm 3+ single crystal, which is because YGG a YAG crystals are different substrates and have different band gaps [40].

Absorption Spectra
where α and hv are the absorption coefficients and phonon energy of the sample, resp tively, and A is a constant. The relationship (αhv) 2 versus hv is indicated in Figure 3b, a Eg is the position of the intersection of the linear part of the curve with the hv axis. T optical band gaps in YGG: Sm 3+ and YAG: Sm 3+ single crystals are 4.25 and 4.47 eV, resp tively, showing that YGG: Sm 3+ single crystal has a narrower optical band gap than YA Sm 3+ single crystal.
Under excitation at 468 nm ( Figure 6), the YGG: Sm 3+ crystal also emits orange-red light but is two orders of magnitude weaker than under excitation at 407 nm. Furthermore, the luminescence intensity of YGG: Sm 3+ crystal is higher than that of YAG: Sm 3+ , and the emission spectrum of YGG: Sm 3+ is blue-shifted relative to the YAG: Sm 3+ crystal, with maxima at 611 and 613 nm and FWHM of 4.2 nm.
These results may be a consequence of changing Al 3+ in the YAG lattice to Ga 3+ , and Ga 3+ has a greater atomic mass than Al 3+ and results in the YGG: Sm 3+ crystal having a higher density (Table 2) and smaller phonon energy than YAG: Sm 3+ [42]. The phonon energy directly affects the luminescence efficiency [23]; as a consequence, the emission peak of the YGG: Sm 3+ crystal is much stronger than that of the YAG: Sm 3+ . In addition, the crystal fields of YGG and YAG crystals are different, which affects the 4f energy level position and linewidth of Sm 3+ [43], resulting in a shorter emission wavelength and a blue shift of YGG: Sm 3+ emission spectra compared with YAG: Sm 3+ crystals. Thus, YGG represents an improvement over YAG as a crystal matrix for observing the luminescence of rare-earth ions, and at the same time, the YGG: Sm 3+ crystal has a highly efficient orangered emission (as shown in Figure 5) and has a potential use in w-LEDs and as orange-red solid-state lasers.

Chromaticity Coordinates
The luminescence color and color purity are important parameters for evaluating the quality and potential uses of luminescent materials [22,44]. CIE-1931 [45] was used to calculate the chromaticity coordinates for the emission spectra of YGG: Sm 3+ and YAG: Sm 3+ single crystals under excitation at 407 nm. As shown in Figure 7 and Table 3, the color coordinates for the spectra from both crystals are located in the orange-red light region, and their color purity was calculated by the following formula [46,47]: Under excitation at 407 nm, the YGG: Sm 3+ crystal emits bright orange-red light, as shown in Figure 5, and the intensity of YGG: Sm 3+ crystal is obviously higher than that of YAG: Sm 3+ crystal. Additionally, the emission spectrum of YGG: Sm 3+ is blue-shifted relative to the YAG: Sm 3+ crystal, with maxima at 611 and 613 nm, and the FWHM are 4.1 and 3.8 nm, respectively.
Under excitation at 468 nm ( Figure 6), the YGG: Sm 3+ crystal also emits orange-red light but is two orders of magnitude weaker than under excitation at 407 nm. Furthermore, the luminescence intensity of YGG: Sm 3+ crystal is higher than that of YAG: Sm 3+ , and the emission spectrum of YGG: Sm 3+ is blue-shifted relative to the YAG: Sm 3+ crystal, with maxima at 611 and 613 nm and FWHM of 4.2 nm.
These results may be a consequence of changing Al 3+ in the YAG lattice to Ga 3+ , and Ga 3+ has a greater atomic mass than Al 3+ and results in the YGG: Sm 3+ crystal having a higher density (Table 2) and smaller phonon energy than YAG: Sm 3+ [42]. The phonon energy directly affects the luminescence efficiency [23]; as a consequence, the emission peak of the YGG: Sm 3+ crystal is much stronger than that of the YAG: Sm 3+ . In addition, the crystal fields of YGG and YAG crystals are different, which affects the 4f energy level position and linewidth of Sm 3+ [43], resulting in a shorter emission wavelength and a blue shift of YGG: Sm 3+ emission spectra compared with YAG: Sm 3+ crystals. Thus, YGG represents an improvement over YAG as a crystal matrix for observing the luminescence of rare-earth ions, and at the same time, the YGG: Sm 3+ crystal has a highly efficient orangered emission (as shown in Figure 5) and has a potential use in w-LEDs and as orange-red solid-state lasers.

Chromaticity Coordinates
The luminescence color and color purity are important parameters for evaluating the quality and potential uses of luminescent materials [22,44]. CIE-1931 [45] was used to calculate the chromaticity coordinates for the emission spectra of YGG: Sm 3+ and YAG: Sm 3+ single crystals under excitation at 407 nm. As shown in Figure 7 and Table 3, the color coordinates for the spectra from both crystals are located in the orange-red light region, and their color purity was calculated by the following formula [46,47]: where (x s , y s ), (x i , y i ), and (x d , y d ) are the color coordinates of the crystals, the color coordinates of isoenergetic white light (0.333, 0.333), and the color coordinates of the main peak of the emission spectrum, respectively. As shown in Table 3, under excitation at 407 nm, the color purity is around 85% for the YGG: Sm 3+ crystal and around 83% for YAG: Sm 3+ . Thus, YGG: Sm 3+ single crystals are high-quality materials that emit orange-red light, with notable improvements in efficiency over YAG: Sm 3+ .
where (xs, ys), (xi, yi), and (xd, yd) are the color coordinates of the crystals, the color coordinates of isoenergetic white light (0.333, 0.333), and the color coordinates of the main peak of the emission spectrum, respectively. As shown in Table 3, under excitation at 407 nm, the color purity is around 85% for the YGG: Sm 3+ crystal and around 83% for YAG: Sm 3+ . Thus, YGG: Sm 3+ single crystals are high-quality materials that emit orange-red light, with notable improvements in efficiency over YAG: Sm 3+ .

Fluorescence Lifetime Measurements
The fluorescence lifetime ( ̅ ) is defined as the time required after ceasing excitation for the fluorescence intensity to drop to 1/e of its maximum [48]. The fluorescence decay curves (Figure 8) for the Sm 3+ 4 G5/2 → 6 H7/2 transition in YGG: Sm 3+ and YAG: Sm 3+ crystals under excitation at 407 nm are similar and were fitted with a three-exponential function [49]: where I(t) is the luminescence intensity as a function of time t, and A1, A2, and A3 are the pre-exponential factors of the three lifetimes τ1, τ2 and τ3 (Table 4) [49]. This indicates that

Fluorescence Lifetime Measurements
The fluorescence lifetime (τ) is defined as the time required after ceasing excitation for the fluorescence intensity to drop to 1/e of its maximum [48]. The fluorescence decay curves (Figure 8) for the Sm 3+ 4 G 5/2 → 6 H 7/2 transition in YGG: Sm 3+ and YAG: Sm 3+ crystals under excitation at 407 nm are similar and were fitted with a three-exponential function [49]: where I(t) is the luminescence intensity as a function of time t, and A 1 , A 2 , and A 3 are the pre-exponential factors of the three lifetimes τ 1 , τ 2 and τ 3 ( attributed to the Sm 3+ ions [50]. The second (τ 2 ) and third (τ 3 ) components are longer than the first component (τ 1 ); therefore, τ 2 and τ 3 could be related to the flicker light emitted by excitons associated with antisite defects in the matrix [51]. The average decay time of the sample is then defined by (6) [49]: Crystals 2023, 13, x FOR PEER REVIEW 9 of 12 there are three decay behaviors. The first component (τ1) is dominant and could be attributed to the Sm 3+ ions [50]. The second (τ2) and third (τ3) components are longer than the first component (τ1); therefore, τ2 and τ3 could be related to the flicker light emitted by excitons associated with antisite defects in the matrix [51]. The average decay time of the sample is then defined by (6)   The fluorescence lifetimes for the 4 G5/2 → 6 H7/2 emission peak from YGG: Sm 3+ and YAG: Sm 3+ single crystals were then calculated to be 0.705 ms and 0.466 ms, respectively (Table 4). Thus, the fluorescence lifetime of YGG: Sm 3+ is not only longer than that of the YAG: Sm 3+ crystal, but it is also longer than those of CaGdAlO4: Sm 3+ (0.69 ms) and NaGd(MnO4): Sm 3+ crystals (0.5574 ms) [18,52,53]; this is probably the consequence of the greater intensity of its 4 G5/2 → 6 H7/2 emission peak, which allows for a greater participation of Sm 3+ ions in this transition and results in a longer fluorescence lifetime [54,55].

Conclusions
High-quality Y2.96Sm0.04Ga5O12 (YGG: Sm 3+ ) single crystals were successfully prepared for the first time by the optical floating zone method and were compared with Y2.96Sm0.04Al5O12 (YAG: Sm 3+ ) single crystals. The density of Y2.96Sm0.04Ga5O12 single crystal (5.756 g/cm 3 ) is larger than that of Y2.96Sm0.04Al5O12 single crystal (4.481 g/cm 3 ), because of the larger atomic mass of Ga compared with Al. XRD analysis showed that Sm 3+ successfully entered into the cubic-phase structure of the garnet crystals. Ten absorption peaks  The fluorescence lifetimes for the 4 G 5/2 → 6 H 7/2 emission peak from YGG: Sm 3+ and YAG: Sm 3+ single crystals were then calculated to be 0.705 ms and 0.466 ms, respectively (Table 4). Thus, the fluorescence lifetime of YGG: Sm 3+ is not only longer than that of the YAG: Sm 3+ crystal, but it is also longer than those of CaGdAlO 4 : Sm 3+ (0.69 ms) and NaGd(MnO 4 ): Sm 3+ crystals (0.5574 ms) [18,52,53]; this is probably the consequence of the greater intensity of its 4 G 5/2 → 6 H 7/2 emission peak, which allows for a greater participation of Sm 3+ ions in this transition and results in a longer fluorescence lifetime [54,55].

Conclusions
High-quality Y 2 . 96 Sm 0 . 04 Ga 5 O 12 (YGG: Sm 3+ ) single crystals were successfully prepared for the first time by the optical floating zone method and were compared with Y 2 . 96 Sm 0 . 04 Al 5 O 12 (YAG: Sm 3+ ) single crystals. The density of Y 2 . 96 Sm 0 . 04 Ga 5 O 12 single crystal (5.756 g/cm 3 ) is larger than that of Y 2 . 96 Sm 0 . 04 Al 5 O 12 single crystal (4.481 g/cm 3 ), because of the larger atomic mass of Ga compared with Al. XRD analysis showed that Sm 3+ successfully entered into the cubic-phase structure of the garnet crystals. Ten absorption peaks were observed at 345, 362, 377, 390, 407, 419, 439, 468, 482 and 495 nm, corresponding to 6 H 5/2 → 4 D 7/2 , 4 D 3/2 , 6 P 7/2 , 4 L 15/2 , 4 F 7/2 , 6 P 5/2 , 4 G 9/2 , 4 I 13/2 , 4 I 9/2 and 4 G 7/2 transitions of Sm 3+ , respectively. Excitation peaks at similar wavelengths were observed in the PLE spectra detected at 613 nm, including strong peaks at 407 nm and 468 nm. Both YGG: Sm 3+ and YAG: Sm 3+ crystals emit orange-red light with a wavelength of about 611 nm under excitation at 407 and 468 nm, respectively, and the luminescence intensity is much stronger with 407 nm excitation. Furthermore, with both PL spectra, the emission peaks from YGG: Sm 3+ crystal are both significantly more intense than those from YAG: Sm 3+ , and both experience a blue shift. The YGG: Sm 3+ crystal has a highly efficient orange-red emission. In addition, under the excitation of 407 nm, the color purity of the orange-red light emitted by the YGG: Sm 3+ crystal (85%) is higher than that emitted by the YAG: Sm 3+ crystal (83%). Additionally, the fluorescence lifetime at the 4 G 5/2 → 6 H 7/2 transition of the YGG: Sm 3+ crystal (0.705 ms) is longer than that of the YAG: Sm 3+ crystal (0.466 ms). This shows that the optical properties of YGG: Sm 3+ crystal are better than those of YAG: Sm 3+ crystal and that they have a potential use in w-LEDs and as orange-red solid-state lasers. In other words, YGG: Sm 3+ crystals are promising new materials for use in w-LEDs and orange-red solid-state lasers.

Data Availability Statement:
The data presented in this study are available on reasonable request from the corresponding author.