Deep Red Photoluminescence from Cr3+ in Fluorine-Doped Lithium Aluminate Host Material

Deep red phosphors have attracted much attention for their applications in lighting, medical diagnosis, health monitoring, agriculture, etc. A new phosphor host material based on fluorine-doped lithium aluminate (ALFO) was proposed and deep red emission from Cr3+ in this host material was demonstrated. Cr3+ in ALFO was excited by blue (~410 nm) and green (~570 nm) rays and covered the deep red to near-infrared region from 650 nm to 900 nm with peaks around 700 nm. ALFO was a fluorine-doped form of the spinel-type compound LiAl5O8 with slightly Li-richer compositions. The composition depended on the preparation conditions, and the contents of Li and F tended to decrease with preparation temperature, such as Al4.69Li1.31F0.28O7.55 at 1100 °C, Al4.73Li1.27F0.17O7.65 at 1200 °C, and Al4.83Li1.17F0.10O7.78 at 1300 °C. The Rietveld analysis revealed that ALFO and LiAl5O8 were isostructural with respect to the spinel-type lattice and in a disorder–order relationship in the arrangement of Li+ and Al3+. The emission peak of Cr3+ in LiAl5O8 resided at 716 nm, while Cr3+ in ALFO showed a rather broad doublet peak with the tops at 708 nm and 716 nm when prepared at 1200 °C. The broad emission peak indicated that the local environment around Cr3+ in ALFO was distorted, which was also supported by electron spin resonance spectra, suggesting that the local environment around Cr3+ in ALFO was more inhomogeneous than expected from the diffraction-based structural analysis. It was demonstrated that even a small amount of dopant (in this case fluorine) could affect the local environment around luminescent centers, and thus the luminescence properties.


Introduction
Inorganic phosphors have been widely used in display and lighting applications such as cathode-ray tubes and fluorescent lamps.Conventional phosphors used in these applications are typically excited by high-energy radiation such as X-rays and ultraviolet (UV) rays or electron beams.Recent light-emitting diode (LED) lighting achieves a pseudowhite color by combining blue light emitted from LED chips and the emission of the phosphors excited by the blue light, which has lower energy than X-rays and UV rays.
"Red" is an important color in lighting applications because it has a significant emotional effect on humans, as people unconsciously perceive the warmth of an atmosphere, the state of a person's health from their complexion, or the freshness of red meat from the color red.Typical rare-earth ions that emit red light are Eu 3+ and Eu 2+ .The former shows aluminum source was aluminum triisopropoxide (>99.9%,Kanto Chem.Co., Inc.); aluminum hydroxide was precipitated at pH 10 in isopropanol solution by NH 3 aq.Using Cr(NO 3 ) 3 •9H 2 O (98.0-103.0%,Kanto Chem.Co., Inc.) as the starting material, Cr 3+ was coprecipitated with aluminum hydroxide in the isopropanol solution.The coprecipitated hydroxide mixture was rinsed with deionized water, heated in a platinum crucible to 1000 • C at a ramp rate of 10 • C/min, and cooled in the furnace to obtain γ-Al 2 O 3 :Cr 3+ .The concentration of Cr 3+ was defined as Cr 3+ /(Al 3+ + Cr 3+ ), assuming that it replaced Al 3+ in the products.γ-Al 2 O 3 :Cr 3+ and LiF were weighed at (Al + Cr)/Li = 4 (note that γ-Al 2 O 3 contains a certain amount of water in its composition and that (Al + Cr)/Li in the starting composition could be slightly smaller than 4).Typically, 0.2 g of a mixture of LiF and γ-Al 2 O 3 :Cr 3+ was placed in a 5 cm 3 platinum crucible with a lid.The 5 cm 3 platinum crucible was put in a larger platinum crucible (30 cm 3 ) with a lid to suppress volatilization of the components, specifically lithium and fluorine, during heat treatment.Samples were heated at 800-1300 • C at a ramp rate of 20 • C/min.Heating times were 15 min at 1100 • C and above, and 1 h at 800-1000 • C.
For comparison, LiAl 5 O 8 :Cr 3+ was prepared from Li 2 CO 3 (99%, Kanto Chem.Co., Inc.) and α-Al 2 O 3 :Cr 3+ at a ratio of Li:(Al + Cr) = 1:5.α-Al 2 O 3 was prepared by heating the Al(OH) 3 precipitate at 1200 • C for 2 h in air.The Li and Al sources were thoroughly mixed in an alumina mortar and pressed into pellets.The pellets were heated in an alumina crucible with the inner bottom covered with pre-prepared LiAl 5 O 8 powder to suppress the diffusion of the lithium component from the sample to the alumina crucible.
The chemical compositions of the samples were analyzed for Li, Al, and Cr using inductively coupled plasma-optical emission spectroscopy after the samples were decomposed in molten salt.Fluorine contents were analyzed by ion chromatography after thermal hydrolysis.Oxygen contents were estimated from charge neutrality using the analyzed Al 3+ , Li + , Cr 3+ , and F − contents.The morphology of the phosphor particles was observed with scanning electron microscopy (SEM) (T-330, JEOL Ltd., Tokyo, Japan and TM3030 plus, Hitachi High-Tech Co., Tokyo, Japan).Carbon was evaporated for conductive coating on the samples.
The crystallographic phases were identified by X-ray diffraction (XRD) using Cu Kα radiation (MiniFlex, Rigaku Co., Tokyo, Japan) operated at 40 kV and 15 mA.The scan step was 0.01 • .Structural refinement was carried out by the Rietveld method using Rietan-FP [36].The crystal structures were illustrated using VESTA [37].The PL spectra and the photoluminescence excitation (PLE) spectra were measured using a fluorescence spectrometer (FL-6000, Shimadzu Co., Kyoto, Japan) and a multichannel spectral analyzer (PMA-11, Hamamatsu Photonics Co., Hamamatsu, Japan) combined with a lamp and a bandpass filter (MC-570, Asahi Spectra Co. Ltd., Tokyo, Japan) as the excitation light source.Quantum efficiencies (QEs) were evaluated with FL-6000 using an integral sphere.X-band (9.13 GHz) electron spin resonance (ESR) measurements were conducted at room temperature between 0 and 800 mT with 100 kHz magnetic field modulation to detect differences in the local environments around Cr 3+ in the different host materials.As an aid for the discussion of the ESR results, classical molecular dynamics (MD) simulations using the MXDORTO code [38] were performed to reproduce the local structures.One Cr 3+ ion was introduced into an octahedral site in the MD cell to replace the Al 3+ ion, and the MD cell consisted of the crystallographic unit cells with x, y, and z axes of 23-26 Å.The detailed calculation procedure of the MD has already been described in our previous paper [39].

Compositions and Crystal Structure
Figure 1 compares the XRD patterns of ALFO:Cr 3+ and LiAl 5 O 8 :Cr 3+ prepared at 1200 • C with the Cr 3+ concentration of 0.5%.Each pattern is consistent with the single phase of LiAl 5 O 8 (ICDD # 38-1426) and Al 4 LiO 6 F (ICDD #38-610) reported by Belov et al. [33].The peaks marked with ▼ are the extra peaks originating from the cation ordering of Li + and Al 3+ in spinel-type LiAl 5 O 8 , which will be discussed below.

Compositions and Crystal Structure
Figure 1 compares the XRD patterns of ALFO:Cr 3+ and LiAl5O8:Cr 3+ prepared at 1200 °C with the Cr 3+ concentration of 0.5%.Each pattern is consistent with the single phase of LiAl5O8 (ICDD # 38-1426) and Al4LiO6F (ICDD #38-610) reported by Belov et al. [33].The peaks marked with ▼ are the extra peaks originating from the cation ordering of Li + and Al 3+ in spinel-type LiAl5O8, which will be discussed below.Table 1 shows the chemical compositions of the ALFO host material without the luminescent center ion Cr 3+ prepared at 1100 °C, 1200 °C, and 1300 °C.The XRD patterns of these samples are shown in Figure S1 in Supplementary Materials, confirming that each sample was in the single phase.The composition suggested by Belov et al. [33] was Al4LiO6F, but the compositional analysis (Table 1) showed that ALFO is a fluorine-doped form of LiAl5O8 with Li-rich compositions.The Al/Li/F ratios varied depending on the preparation temperature.The fluorine content was 0.28 for the sample prepared at 1100 °C; it decreased to 0.10 for the sample prepared at 1300 °C, indicating that fluorine dissipated during heat treatment at the high temperatures.The lithium content also decreased from Al/Li = 4.69/1.31(=3.58/1) at 1100 °C and 4.73/1.27(=3.72/1) at 1200 °C to 4.83/1.17(=4.13/1) at 1300 °C.The Cr 3+ concentration in the Cr 3+ -doped ALFO samples was consistent with the starting composition; the compositional analysis showed that ALFO:Cr 3+ prepared at 1200 °C with a starting concentration of 0.5% Cr 3+ was 0.48%.
The structural refinement by the Rietveld analysis for ALFO prepared at 1200 °C without Cr 3+ converged to Rwp/Rp = 0.058/0.042,goodness-of-fit (S) = 1.90, and the lattice  Table 1 shows the chemical compositions of the ALFO host material without the luminescent center ion Cr 3+ prepared at 1100 • C, 1200 • C, and 1300 • C. The XRD patterns of these samples are shown in Figure S1 in Supplementary Materials, confirming that each sample was in the single phase.The composition suggested by Belov et al. [33] was Al 4 LiO 6 F, but the compositional analysis (Table 1) showed that ALFO is a fluorine-doped form of LiAl 5 O 8 with Li-rich compositions.The Al/Li/F ratios varied depending on the preparation temperature.The fluorine content was 0.28 for the sample prepared at 1100 • C; it decreased to 0.10 for the sample prepared at 1300 • C, indicating that fluorine dissipated during heat treatment at the high temperatures.The lithium content also decreased from Al/Li = 4.69/1.31(=3.58/1) at 1100 • C and 4.73/1.27(=3.72/1) at 1200 • C to 4.83/1.17(=4.13/1) at 1300 • C. The Cr 3+ concentration in the Cr 3+ -doped ALFO samples was consistent with the starting composition; the compositional analysis showed that ALFO:Cr 3+ prepared at 1200 • C with a starting concentration of 0.5% Cr 3+ was 0.48%.
The structural refinement by the Rietveld analysis for ALFO prepared at 1200 • C without Cr 3+ converged to R wp /R p = 0.058/0.042,goodness-of-fit (S) = 1.90, and the lattice constant a = 7.9222(8) Å. Figure S2 in Supplementary Materials shows the refinement results and the structural parameters are listed in Table S1 in Supplementary Materials.The structural model was based on disordered spinel with the space group Fd−3m.The spinel-type lattice accommodates cations in the tetrahedral and octahedral sites.In the initial model, Al 3+ and Li + were assumed to be randomly distributed in the tetrahedral sites (8a position) and the octahedral sites (16d position).For the random distribution, the occupancies (g) of Li estimated from the chemical composition analysis (Table 1) should be g = 0.211 for both the tetrahedral and octahedral sites.However, the refined structure indicated a slight preference for Li to occupy the octahedral site, with g = 0.183 and 0.226 for the tetrahedral and octahedral sites, respectively.
According to the literature, LiAl 5 O 8 has crystallographic polymorphs, namely I (disordered) and II (ordered) [19,[40][41][42] (Appendix A). Figure 2 compares the crystal structures of ALFO and ordered LiAl 5 O 8 .Figure S3 in Supplementary Materials illustrates the structural differences in the arrangement of the tetrahedra and the octahedra in the range of x = 0.2 to 0.6 for spinel-type ALFO and ordered LiAl 5 O 8 in a perspective view along <100>.The octahedra form diagonal chains by edge sharing, and the chains of the octahedra are linked to each other by the tetrahedra connected by corner sharing.In ordered LiAl 5 O 8 , the tetrahedra consist of only AlO 4 , and the chains of the octahedra consist of one LiO 6 and three AlO 6 in sequence (Figure 2a and Figure S3a in Supplementary Materials).In ALFO, on the other hand, the tetrahedra are statistically AlO 4 or LiO 4 and the octahedra are AlO 6 or LiO 6 (Figure 2b and Figure S3b in Supplementary Materials).
type lattice accommodates cations in the tetrahedral and octahedral sites.In the initial model, Al 3+ and Li + were assumed to be randomly distributed in the tetrahedral sites (8a position) and the octahedral sites (16d position).For the random distribution, the occupancies (g) of Li estimated from the chemical composition analysis (Table 1) should be g = 0.211 for both the tetrahedral and octahedral sites.However, the refined structure indicated a slight preference for Li to occupy the octahedral site, with g = 0.183 and 0.226 for the tetrahedral and octahedral sites, respectively.
According to the literature, LiAl5O8 has crystallographic polymorphs, namely I (disordered) and II (ordered) [19,[40][41][42] (Appendix A). Figure 2 compares the crystal structures of ALFO and ordered LiAl5O8. Figure S3 in Supplementary Materials illustrates the structural differences in the arrangement of the tetrahedra and the octahedra in the range of x = 0.2 to 0.6 for spinel-type ALFO and ordered LiAl5O8 in a perspective view along <100>.The octahedra form diagonal chains by edge sharing, and the chains of the octahedra are linked to each other by the tetrahedra connected by corner sharing.In ordered LiAl5O8, the tetrahedra consist of only AlO4, and the chains of the octahedra consist of one LiO6 and three AlO6 in sequence (Figure 2a and Figure S3a in Supplementary Materials).In ALFO, on the other hand, the tetrahedra are statistically AlO4 or LiO4 and the octahedra are AlO6 or LiO6 (Figure 2b and Figure S3b in Supplementary Materials).The spinel-type framework leads to the unmarked peaks common to ALFO and LiAl5O8 in the XRD patterns in Figure 1.The peaks marked with ▼ in Figure 1 are derived from the periodicity due to the regular arrangement of Al 3+ and Li + on the cation sites in ordered LiAl5O8.The difference in the cation arrangement results in the lattice symmetry of Fd−3m for ALFO and P4332 for ordered LiAl5O8.The doped Cr 3+ was assumed to occupy octahedral sites in both the host materials because of the preference of Cr 3+ with the d 3 configuration to occupy octahedral sites; the ionic radius of Cr 3+ on a tetrahedral site is not defined in Shannon s ionic radii table [43].
As a summary of the compositional and structural analyses, ALFO and ordered LiAl5O8 have a disorder-order relationship with respect to the arrangement of Li + and Al 3+ on the cation sites.ALFO has slightly Li-richer compositions than LiAl5O8, resulting in anion vacancies for the charge neutrality.Fluorine replaces 1.3-3.5% of the oxygen sites.
The solubility limit of Cr 3+ in the ALFO lattice exceeded 10%. Figure S4 in Supplementary Materials shows the XRD patterns of ALFO:Cr 3+ prepared at 1200 °C at different Cr 3+ concentrations from 0.01 to 10%.All the samples were substantially a single phase of ALFO; the six-fold coordinated ionic radii of Al 3+ and Cr 3+ were 0.535 Å and 0.615 Å [43], The spinel-type framework leads to the unmarked peaks common to ALFO and LiAl 5 O 8 in the XRD patterns in Figure 1.The peaks marked with ▼ in Figure 1 are derived from the periodicity due to the regular arrangement of Al 3+ and Li + on the cation sites in ordered LiAl 5 O 8 .The difference in the cation arrangement results in the lattice symmetry of Fd−3m for ALFO and P4 3 32 for ordered LiAl 5 O 8 .The doped Cr 3+ was assumed to occupy octahedral sites in both the host materials because of the preference of Cr 3+ with the d 3 configuration to occupy octahedral sites; the ionic radius of Cr 3+ on a tetrahedral site is not defined in Shannon's ionic radii table [43].
As a summary of the compositional and structural analyses, ALFO and ordered LiAl 5 O 8 have a disorder-order relationship with respect to the arrangement of Li + and Al 3+ on the cation sites.ALFO has slightly Li-richer compositions than LiAl 5 O 8 , resulting in anion vacancies for the charge neutrality.Fluorine replaces 1.3-3.5% of the oxygen sites.
The solubility limit of Cr 3+ in the ALFO lattice exceeded 10%. Figure S4 in Supplementary Materials shows the XRD patterns of ALFO:Cr 3+ prepared at 1200 • C at different Cr 3+ concentrations from 0.01 to 10%.All the samples were substantially a single phase of ALFO; the six-fold coordinated ionic radii of Al 3+ and Cr 3+ were 0.535 Å and 0.615 Å [43], and replacing Al 3+ with larger Cr 3+ shifted the diffraction peaks slightly toward the lower angles.

Morphologies
Figure 3 shows the SEM images of 0.5% Cr 3+ -doped ALFO and LiAl 5 O 8 prepared at 1200 • C and 1300 • C. The effect of treatment temperature did not appear to have a significant effect on the morphology; ALFO:Cr 3+ was composed of angular grains with well-developed crystal faces.The grain size varied from sub-micrometers to several micrometers.On the other hand, the grains of LiAl 5 O 8 :Cr 3+ did not have a definite shape and their size was several hundred nanometers, which formed agglomerates of a few micrometers in size.The difference in morphology between ALFO:Cr 3+ and LiAl 5 O 8 :Cr 3+ was attributed to the difference in the lithium sources.The melting points of LiF and Li 2 CO 3 used to prepare 1200 °C and 1300 °C.The effect of treatment temperature did not appear to have a significant effect on the morphology; ALFO:Cr 3+ was composed of angular grains with welldeveloped crystal faces.The grain size varied from sub-micrometers to several micrometers.On the other hand, the grains of LiAl5O8:Cr 3+ did not have a definite shape and their size was several hundred nanometers, which formed agglomerates of a few micrometers in size.The difference in morphology between ALFO:Cr 3+ and LiAl5O8:Cr 3+ was attributed to the difference in the lithium sources.The melting points of LiF and Li2CO3 used to prepare ALFO and LiAl5O8 are 848 °C and 723 °C, respectively.The development of the crystal faces of the ALFO:Cr 3+ grains strongly suggested that LiF formed the melt at elevated temperatures and acted as a self-flux to grow the crystalline grains.On the other hand, the indistinct shape of the grains observed in Figure 3c,d for LiAl5O8:Cr 3+ indicated that no flux growth effect was expected, and thus Li2CO3 reacted with α-Al2O3:Cr 3+ to form LiAl5O8:Cr 3+ before forming the melt.

Photoluminescence Properties
The emission and excitation spectra of 0.5% Cr 3+ in ALFO, LiAl5O8, and α-Al2O3 prepared at 1200 °C are compared in Figure 4.The excitation spectra were similar to each other regardless of the type of the host crystal, and the excitation bands consisted of two broad peaks at around 420 nm and 570 nm.They corresponded to the transitions from the ground state 4 A2 to the excitation states 4 T1 ( 4 F) and 4 T2 ( 4 F) in Cr 3+ of the d 3 state.The emission peaks depended on the host crystal.α-Al2O3:Cr 3+ showed a typical line spectrum, named "R line", at 694 nm with several small side peaks on the longer wavelength side, as found in the literature [9,44]; LiAl5O8:Cr 3+ showed a sharp peak at 716 nm with smaller

Photoluminescence Properties
The emission and excitation spectra of 0.5% Cr 3+ in ALFO, LiAl 5 O 8 , and α-Al 2 O 3 prepared at 1200 • C are compared in Figure 4.The excitation spectra were similar to each other regardless of the type of the host crystal, and the excitation bands consisted of two broad peaks at around 420 nm and 570 nm.They corresponded to the transitions from the ground state 4 A 2 to the excitation states 4 T 1 ( 4 F) and 4 T 2 ( 4 F) in Cr 3+ of the d 3 state.The emission peaks depended on the host crystal.α-Al 2 O 3 :Cr 3+ showed a typical line spectrum, named "R line", at 694 nm with several small side peaks on the longer wavelength side, as found in the literature [9,44]; LiAl 5 O 8 :Cr 3+ showed a sharp peak at 716 nm with smaller peaks at 703 nm and 730 nm which are also similar to those reported in the previous literature [19,45].On the other hand, Cr 3+ in ALFO characteristically exhibited a doublet peak at 708 and 716 nm, and the profile was broader than those of α-Al 2 O 3 and LiAl 5 O 8 .Although the apparent difference between the crystal structures of ALFO and ordered LiAl 5 O 8 shown by the Rietveld analysis was in the cation arrangement, the broad doublet peak in ALFO:Cr 3+ indicated that the local environment around the luminescent center Cr 3+ in ALFO was even more inhomogeneous than simply considering octahedral coordination by six neighboring oxygens.
hough the apparent difference between the crystal structures of ALFO and o LiAl5O8 shown by the Rietveld analysis was in the cation arrangement, the broad peak in ALFO:Cr 3+ indicated that the local environment around the luminescen Cr 3+ in ALFO was even more inhomogeneous than simply considering octahedral nation by six neighboring oxygens.Figure 5 shows the variation of the emission peaks of ALFO:Cr 3+ with the tre temperature and compares them with those of α-Al2O3:Cr 3+ and LiAl5O8:Cr 3+ .The 696 nm of the ALFO:Cr 3+ sample prepared at 1000 °C was due to the unreacted Al idue (Figure S5 in Supplementary Materials), indicating that the coprecipitated C incorporated into the α-Al2O3 grains to form α-Al2O3:Cr 3+ .The peak at around 715 specific to ALFO:Cr 3 and was observed in ALFO:Cr 3+ prepared at 1000-1300 °C.I ingly, the rather broad doublet peak with peak tops at 708 nm and 716 nm was ch istically observed in the sample prepared at 1200 °C, and the detailed mechanism occurrence of the doublet peak is still unclear.It could be attributed to the differ the local coordination environment around Cr 3+ that did not appear in the averag ture by XRD (Figure S5 in Supplementary Materials). Figure S5 in Supplementary rials shows the XRD patterns of the ALFO:Cr 3+ samples prepared at 1000-1300 °C ual unreacted α-Al2O3 was recognized in the sample prepared at 1000 °C, while th ples prepared at 1100-1300 °C were in the single phase.Figure 5 shows the variation of the emission peaks of ALFO:Cr 3+ with the treatment temperature and compares them with those of α-Al 2 O 3 :Cr 3+ and LiAl 5 O 8 :Cr 3+ .The peak at 696 nm of the ALFO:Cr 3+ sample prepared at 1000 • C was due to the unreacted Al 2 O 3 residue (Figure S5 in Supplementary Materials), indicating that the coprecipitated Cr 3+ was incorporated into the α-Al 2 O 3 grains to form α-Al 2 O 3 :Cr 3+ .The peak at around 715 nm was specific to ALFO:Cr 3 and was observed in ALFO:Cr 3+ prepared at 1000-1300 • C. Interestingly, the rather broad doublet peak with peak tops at 708 nm and 716 nm was characteristically observed in the sample prepared at 1200 • C, and the detailed mechanism of the occurrence of the doublet peak is still unclear.It could be attributed to the difference in the local coordination environment around Cr 3+ that did not appear in the average structure by XRD (Figure S5 in Supplementary Materials). Figure S5 in Supplementary Materials shows the XRD patterns of the ALFO:Cr 3+ samples prepared at 1000-1300 • C. Residual unreacted α-Al 2 O 3 was recognized in the sample prepared at 1000 • C, while the samples prepared at 1100-1300 • C were in the single phase.
erials 2024, 17, x FOR PEER REVIEW Figure 5. Variation of PL spectra of ALFO:Cr 3+ with preparation temperature of α-Al2O3:Cr 3+ and LiAl5O8:Cr 3+ prepared at 1200 °C are also shown for comp Figure 6 compares the PL spectra of ALFO:Cr 3+ (a) and LiAl5O8: Cr 3+ concentrations.The emission intensity of ALFO:Cr 3+ increased f 0.5% and decreased above 0.5%.Above 2.5%, the intensity of the mai creased while the shoulders developed around 780 nm.In the previo been discussed that the broad emission of Cr 3+ in the near-infrared reg to the 4 T2 → 4 A2 transition of Cr 3+ placed in a weak crystal field, or to m between Cr 3+ ions at the neighboring sites.According to the Tanabe-Su 48], the first excited state in the d 3 configuration of octahedral coordina state, depending on the strength of the crystal field.The transition from 4 A2 ground state yields the line spectra typically observed for Cr 3+ aro broad near-infrared emission is reported for the transition from the 4 materials with a weak crystal field [21][22][23][24]27,29,30]. Magnetic interac red-shift of the emission [49], as exemplified by the so-called N-lines of and the broad luminescence due to Cr 3+ -Cr 3+ pairs observed in SrAl11.88Rajendran et al., which was concluded from the decay time measurem shoulder peak around 780 nm in ALFO:Cr 3+ , the origin was attribut interaction between Cr 3+ ions from the fact that the change in the latt Cr 3+ addition, i.e., the change in the strength of the crystal field at the very large (Figure S4 in Supplementary Materials), and that the intera ions was observed in the ESR signal at a high Cr 3+ concentration as d 3.4.Figure 6 compares the PL spectra of ALFO:Cr 3+ (a) and LiAl 5 O 8 :Cr 3+ (b) at different Cr 3+ concentrations.The emission intensity of ALFO:Cr 3+ increased from Cr 3+ = 0.1% to 0.5% and decreased above 0.5%.Above 2.5%, the intensity of the main doublet peak decreased while the shoulders developed around 780 nm.In the previous literature, it has been discussed that the broad emission of Cr 3+ in the near-infrared region was due either to the 4 T 2 → 4 A 2 transition of Cr 3+ placed in a weak crystal field, or to magnetic interactions between Cr 3+ ions at the neighboring sites.According to the Tanabe-Sugano diagram [46][47][48], the first excited state in the d 3 configuration of octahedral coordination is the 4 T 2 or 2 E state, depending on the strength of the crystal field.The transition from the 2 E state to the 4 A 2 ground state yields the line spectra typically observed for Cr 3+ around 700 nm, while broad near-infrared emission is reported for the transition from the 4 T 2 state in the host materials with a weak crystal field [21][22][23][24]27,29,30]. Magnetic interactions also bring the red-shift of the emission [49], as exemplified by the so-called N-lines of Cr 3+ emission [9,15] and the broad luminescence due to Cr 3+ -Cr 3+ pairs observed in SrAl 11.88−x Ga x O 19 :0.12Cr 3+ by Rajendran et al., which was concluded from the decay time measurements [28].As for the shoulder peak around 780 nm in ALFO:Cr 3+ , the origin was attributed to the magnetic interaction between Cr 3+ ions from the fact that the change in the lattice parameter with Cr 3+ addition, i.e., the change in the strength of the crystal field at the Cr 3+ sites, was not very large (Figure S4 in Supplementary Materials), and that the interaction between Cr 3+ ions was observed in the ESR signal at a high Cr 3+ concentration as described in Section 3.4.LiAl5O8:Cr 3+ showed the maximum intensity at Cr 3+ = 1.0%, and the change of the overall peak profile was less distinct than ALFO:Cr 3+ .The broad peak around 780 nm also developed in LiAl5O8:Cr 3+ above 1.0%, but the intensity was relatively low.
Figure 7 shows the variation of the internal and external quantum efficiencies (QEs) of ALFO:Cr 3+ and LiAl5O8:Cr 3+ versus Cr 3+ concentration.The circles and triangles indicate the internal and external quantum efficiencies QEInt and QEExt, respectively; the filled and unfilled marks indicate the QEs of ALFO:Cr 3+ and LiAl5O8:Cr 3+ , respectively.ALFO:Cr 3+ showed higher QEs than LiAl5O8:Cr 3+ .The QEInt for ALFO was 85.6% at Cr 3+ = 0.5%, higher than the maximum QEInt of 58.3% for LiAl5O8 at Cr 3+ = 1.0%.The highest QEExt was 18.6% for ALFO:Cr 3+ and 11.6% for LiAl5O8:Cr 3+ .The enhancement of QEs of ALFO:Cr 3+ was attributed to the high crystallinity suggested by the well-developed grains observed in SEM (Figure 3), the disruption of local symmetry around the luminescent center Cr 3+ from the ideal octahedron with the introduction of F − and vacancies on the O 2− sites, and the increased area of the PL peaks due to peak broadening.

Local Environments around Cr 3+
Cr 3+ prefers octahedral coordination in the host spinel lattice in both ALFO   LiAl 5 O 8 :Cr 3+ showed the maximum intensity at Cr 3+ = 1.0%, and the change of the overall peak profile was less distinct than ALFO:Cr 3+ .The broad peak around 780 nm also developed in LiAl 5 O 8 :Cr 3+ above 1.0%, but the intensity was relatively low.
Figure 7 shows the variation of the internal and external quantum efficiencies (QEs) of ALFO:Cr 3+ and LiAl 5 O 8 :Cr 3+ versus Cr 3+ concentration.The circles and triangles indicate the internal and external quantum efficiencies QE Int and QE Ext , respectively; the filled and unfilled marks indicate the QEs of ALFO:Cr 3+ and LiAl 5 O 8 :Cr 3+ , respectively.ALFO:Cr 3+ showed higher QEs than LiAl 5 O 8 :Cr 3+ .The QE Int for ALFO was 85.6% at Cr 3+ = 0.5%, higher than the maximum QE Int of 58.3% for LiAl 5 O 8 at Cr 3+ = 1.0%.The highest QE Ext was 18.6% for ALFO:Cr 3+ and 11.6% for LiAl 5 O 8 :Cr 3+ .The enhancement of QEs of ALFO:Cr 3+ was attributed to the high crystallinity suggested by the well-developed grains observed in SEM (Figure 3), the disruption of local symmetry around the luminescent center Cr 3+ from the ideal octahedron with the introduction of F − and vacancies on the O 2− sites, and the increased area of the PL peaks due to peak broadening.LiAl5O8:Cr 3+ showed the maximum intensity at Cr 3+ = 1.0%, and the change of the overall peak profile was less distinct than ALFO:Cr 3+ .The broad peak around 780 nm also developed in LiAl5O8:Cr 3+ above 1.0%, but the intensity was relatively low.
Figure 7 shows the variation of the internal and external quantum efficiencies (QEs) of ALFO:Cr 3+ and LiAl5O8:Cr 3+ versus Cr 3+ concentration.The circles and triangles indicate the internal and external quantum efficiencies QEInt and QEExt, respectively; the filled and unfilled marks indicate the QEs of ALFO:Cr 3+ and LiAl5O8:Cr 3+ , respectively.ALFO:Cr 3+ showed higher QEs than LiAl5O8:Cr 3+ .The QEInt for ALFO was 85.6% at Cr 3+ = 0.5%, higher than the maximum QEInt of 58.3% for LiAl5O8 at Cr 3+ = 1.0%.The highest QEExt was 18.6% for ALFO:Cr 3+ and 11.6% for LiAl5O8:Cr 3+ .The enhancement of QEs of ALFO:Cr 3+ was attributed to the high crystallinity suggested by the well-developed grains observed in SEM (Figure 3), the disruption of local symmetry around the luminescent center Cr 3+ from the ideal octahedron with the introduction of F − and vacancies on the O 2− sites, and the increased area of the PL peaks due to peak broadening.

Local Environments around Cr 3+
Cr 3+ prefers octahedral coordination in the host spinel lattice in both ALFO and LiAl5O8, whereas the different spectra suggested different local environments around Cr 3+

Local Environments around Cr 3+
Cr 3+ prefers octahedral coordination in the host spinel lattice in both ALFO and LiAl 5 O 8 , whereas the different spectra suggested different local environments around Cr 3+ in ALFO and LiAl 5 O 8 .Such aperiodic local structures are difficult to investigate using diffraction-based structural analysis, particularly for low-concentration dopants.In fact, the ALFO:Cr 3+ samples prepared at the different temperatures showed substantially the same XRD patterns (Figure S5 in Supplementary Materials), but the emission peaks varied as shown in Figure 5.
Cr 3+ in the d 3 configuration is an ESR active ion, and the ESR technique was expected to effectively detect the differences in the electronic structures affected by the coordination environment.Figure 8 compares the ESR spectra of 0.5% Cr 3+ doped α-Al 2 O 3 (a), LiAl 5 O 8 (b), and ALFO (c) prepared at 1200 • C. Cr 3+ in α-Al 2 O 3 was focused on in the 1960s as a good model example for ESR measurements [44,[50][51][52][53][54][55][56][57][58].The ESR signal of Cr 3+ in α-Al 2 O 3 (Figure 8a) was consistent with those reported for Cr 3+ in polycrystalline α-Al 2 O 3 in the previous literature [50] and consisted of several peaks corresponding to g = 3.79, 2.26, 1.72, and 1.46.For octahedrally coordinated d 3 ions placed in a magnetic field, the lowest energy state is the spin quartet, with the spin quantum number s = −3/2 along the direction of the magnetic field.The signal of a polycrystalline sample is the average of the spectra of individual crystallites, and the angular dependence of Cr 3+ in single crystalline Al 2 O 3 [59] indicated that the peaks at g = 3.79 and 2.26 were assigned to the s = −3/2 to −1/2 transition, and the peaks at g = 1.72 and 1.46 to the −1/2 to +1/2 transition and +1/2 to +3/2 transition, respectively (Appendix B) [50,51]. in ALFO and LiAl5O8.Such aperiodic structures are difficult to investigate using diffraction-based structural analysis, particularly for low-concentration dopants.In fact, the ALFO:Cr 3+ samples prepared at the different temperatures substantially the same XRD patterns (Figure S5 in Supplementary Materials), but the emission peaks varied as shown in Figure 5. Cr 3+ in the d 3 configuration is an ESR active ion, and the ESR technique was expected to effectively detect the differences in the electronic structures affected by the coordination environment.Figure 8 compares the ESR spectra of 0.5% Cr 3+ doped α-Al2O3 (a), LiAl5O8 (b), and ALFO (c) prepared at 1200 °C.Cr 3+ in α-Al2O3 was focused on in the 1960s as a good model example for ESR measurements [44,[50][51][52][53][54][55][56][57][58].The ESR signal of Cr 3+ in α-Al2O3 (Figure 8a) was consistent with those reported for Cr 3+ in polycrystalline α-Al2O3 in the previous literature [50] and consisted of several peaks corresponding to g = 3.79, 2.26, 1.72, and 1.46.For coordinated d 3 ions placed in a magnetic field, the lowest energy state is the spin quartet, with the spin quantum number s = −3/2 along the direction of the magnetic field.The signal of a polycrystalline sample is the average of the spectra of individual crystallites, and the angular dependence of Cr 3+ in single crystalline Al2O3 [59] indicated that the peaks at g = 3.79 and 2.26 were assigned to the s = −3/2 to −1/2 transition, and the peaks at g = 1.72 and 1.46 to the −1/2 to +1/2 transition and +1/2 to +3/2 transition, respectively (Appendix B) [50,51].Cr 3+ in LiAl5O8 showed essentially the same ESR signal (Figure 8b) as reported by Singh et al. [45] for LiAl5O8:Cr 3+ .Singh et al. assigned distinct peaks at g = 4.03 and 3.27 and smaller peaks at g = 5.44, 4.89, and 4.51 to the isolated Cr 3+ ions and the resonance signal at g = 1.97 to the magnetic interaction between the Cr 3+ ions, based on the literature [60,61].Cr 3+ in LiAl 5 O 8 showed essentially the same ESR signal (Figure 8b) as reported by Singh et al. [45] for LiAl 5 O 8 :Cr 3+ .Singh et al. assigned distinct peaks at g = 4.03 and 3.27 and smaller peaks at g = 5.44, 4.89, and 4.51 to the isolated Cr 3+ ions and the resonance signal at g = 1.97 to the magnetic interaction between the Cr 3+ ions, based on the literature [60,61].
The basic features of the ESR signal of Cr 3+ in ALFO (Figure 8c) were similar to those of Cr 3+ in LiAl 5 O 8 (Figure 8b), but the profiles became broad and the signal positions shifted toward the low magnetic field side.The broadening of the ESR signal was attributed to an inhomogeneous local environment around the Cr 3+ ions in ALFO.The CrO 6 octahedra in ALFO are expected to be disturbed by the statistical distribution of Al 3+ and Li + on the adjacent cation sites.The shift of the ESR signal of ALFO toward the lower field side than LiAl 5 O 8 indicated the increased zero-field splitting.
The PL spectra of Cr 3+ in ALFO prepared at 1100 • C and 1300 • C were apparently similar to that of Cr 3+ in LiAl 5 O 8 , but the ERS signal of Cr 3+ in ALFO prepared at 1300 • C (Figure 8d) was different from that in LiAl 5 O 8 prepared at 1200 • C (Figure 8b), indicating that the similarity in the PL spectra was not due to similar local environments around the luminescent center Cr 3+ in these host materials.
Increasing the Cr 3+ concentration to 2.5% resulted in broadening and enhancement of the ESR peak around g ~2.3 (Figure 8e), which was considered to reflect the magnetic interaction between Cr 3+ ions, as Singh et al. discussed that g = 1.95 was due to exchange coupling of Cr 3+ -Cr 3+ pairs [45].The broadening of the peak at g ~1.96 with increasing Cr 3+ concentration was also observed in Cr 3+ -containing phosphate glasses in the literature [62].The presence of the magnetic interactions between Cr 3+ ions at high concentrations was consistent with the discussion of the PL spectra, where the broad shoulder peak developed around 780 nm with increasing Cr 3+ concentration (Figure 6a).S2 in Materials.
In α-Al 2 O 3 , Cr 3+ was considered to be placed in a trigonal symmetry as suggested by McClure experimentally [63], which was also reproduced in our [39].The previous literature referred to the rhombic distortion for CrO 6 in LiAl 5 O 8 with C 2 symmetry [19,45].Our MD result indicated that Cr 3+ was arranged in a monoclinic symmetry that retained a single two-fold axis passing through the midpoints of O1 and O3, and of O5 and O6 (Figure 9b), and the two-fold axis is illustrated on the O1-O3-O6-O5 plane (c).Because of the complexity of the structure, including the disordered arrangement of cations and the presence of vacancies on the anion sites, MD for ALFO has not yet been performed; the local environment around Cr 3+ in ALFO was inferred to be similar to that in LiAl 5 O 8 , but even more disordered.
The basic features of the ESR of Cr 3+ in ALFO (Figure 8c) were similar to those of Cr 3+ in LiAl5O8 (Figure 8b), but the profiles became broad and the signal positions shifted toward the low magnetic field side.The broadening of the ESR signal was attributed to an inhomogeneous local environment around the Cr 3+ ions in ALFO.The CrO6 octahedra in ALFO are expected to be disturbed by the statistical distribution of Al 3+ and Li + on the adjacent cation sites.The shift of the ESR signal of ALFO toward the lower field side than LiAl5O8 indicated the increased zero-field splitting.
The PL spectra of Cr 3+ in ALFO prepared at 1100 °C and 1300 °C were apparently similar to that of Cr 3+ in LiAl5O8, but the ERS signal of Cr 3+ in ALFO prepared at 1300 °C (Figure 8d) was different from that in LiAl5O8 prepared at 1200 °C (Figure 8b), indicating that the similarity in the PL spectra was not due to similar local environments around the luminescent center Cr 3+ in these host materials.
Increasing the Cr 3+ concentration to 2.5% resulted in broadening and enhancement of the ESR peak around g ~ 2.3 (Figure 8e), which was considered to reflect the magnetic interaction between Cr 3+ ions, as Singh et al. discussed that g = 1.95 was due to exchange coupling of Cr 3+ -Cr 3+ pairs [45].The broadening of the peak at g ~ 1.96 with increasing Cr 3+ concentration was also observed in Cr 3+ -containing phosphate glasses in the literature [62].The presence of the magnetic interactions between Cr 3+ ions at high concentrations was consistent with the discussion of the PL spectra, where the broad shoulder peak developed around 780 nm with increasing Cr 3+ concentration (Figure 6a).
Since it was difficult to directly deduce the detailed local environment from the ESR signals, the local environment of isolated Cr 3+ ions is discussed here using MD.The O-Cr-O bond angles of CrO6 octahedra in α-Al2O3 and ordered LiAl5O8 obtained in MD are tabulated in Table S2 in Supplementary Materials.
In α-Al2O3, Cr 3+ was considered to be placed in a trigonal symmetry as suggested by McClure experimentally [63], which was also reproduced in our MD [39].The previous literature referred to the rhombic distortion for CrO6 in LiAl5O8 with C2 symmetry [19,45].Our MD result indicated that Cr 3+ was arranged in a monoclinic symmetry that retained a single two-fold axis passing through the midpoints of O1 and O3, and of O5 and O6 (Figure 9b), and the two-fold axis is illustrated on the O1-O3-O6-O5 plane (c).Because of the complexity of the structure, including the disordered arrangement of cations and the presence of vacancies on the anion sites, MD for ALFO has not yet been performed; the local environment around Cr 3+ in ALFO was inferred to be similar to that in LiAl5O8, but even more disordered.

Figure 1 .
Figure 1.Comparison of XRD patterns of ALFO:Cr 3+ and LiAl5O8:Cr 3+ prepared at 1200 °C.The peaks marked with ▼ are the extra peaks originating in the cation ordering of Li + and Al 3+ in the spineltype lattice of LiAl5O8 (see text for details).

Figure 1 .
Figure 1.Comparison of XRD patterns of ALFO:Cr 3+ and LiAl 5 O 8 :Cr 3+ prepared at 1200 • C. The peaks marked with ▼ are the extra peaks originating in the cation ordering of Li + and Al 3+ in the spinel-type lattice of LiAl 5 O 8 (see text for details).
ALFO and LiAl 5 O 8 are 848 • C and 723 • C, respectively.The development of the crystal faces of the ALFO:Cr 3+ grains strongly suggested that LiF formed the melt at elevated temperatures and acted as a self-flux to grow the crystalline grains.On the other hand, the indistinct shape of the grains observed in Figure3c,d for LiAl 5 O 8 :Cr 3+ indicated that no flux growth effect was expected, and thus Li 2 CO 3 reacted with α-Al 2 O 3 :Cr 3+ to form LiAl 5 O 8 :Cr 3+ before forming the melt.

Figure 7 .
Figure 7. Internal and external quantum efficiencies of ALFO:Cr 3+ and LiAl 5 O 8 :Cr 3+ with different Cr 3+ concentrations.QE Int and QE Ext represent internal and external quantum efficiencies, respectively.
Since it was difficult to directly deduce the detailed local environment from the ESR signals, the local environment of isolated Cr 3+ ions is discussed here using MD.The O-Cr-O bond angles of CrO 6 octahedra in α-Al 2 O 3 and ordered LiAl 5 O 8 obtained in MD are tabulated in Table
Fluorine-doped lithium aluminate (ALFO) was prepared from LiF and Al 2 O 3 at a ratio of 1:2.ALFO was a fluorine-doped form of LiAl 5 O 8 with slightly lithium-rich compositions.The concentration of the incorporated fluorine was dependent on the preparation conditions; the chemical composition varied from Al 4.69 Li 1.31 F 0.28 O 7.55 prepared at 1100 • C to Al 4.83 Li 1.17 F 0.10 O 7.78 prepared at 1300 • C. ALFO and LiAl 5 O 8 were isostructural with

Table 1 .
Chemical compositions of the fluorine-doped lithium aluminate (ALFO) host material prepared at different temperatures.

Table 1 .
Chemical compositions of the fluorine-doped lithium aluminate (ALFO) host material prepared at different temperatures.
and LiAl5O8, whereas the different spectra suggested different local environments around Cr 3+