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

Abstract

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 Al_4.69Li_1.31F_0.28O_7.55 at 1100 °C, Al_4.73Li_1.27F_0.17O_7.65 at 1200 °C, and Al_4.83Li_1.17F_0.10O_7.78 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.

1. 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 pseudo-white 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 Eu³⁺ and Eu²⁺. The former shows orange-red luminescence with a sharp peak around 610 nm due to the 4f-4f transition in some host materials, while the latter, e.g., CaAlSiN₃:Eu²⁺ [1,2,3] and K₂CaPO₄F:Eu²⁺ [4], shows a rather broad emission between 600 and 700 nm due to the 5d-4f transition. In recent years, fluorescent materials other than inorganic phosphors, such as polymethine dye molecules, their aggregates and nanoparticles, conjugated polymer nanoparticles, single-walled carbon nanotubes, quantum dots, and lanthanide complex molecules, have also been extensively investigated for emerging applications utilizing near-infrared emission from 700 to 2000 nm, including medical diagnosis, health monitoring, agriculture, and night vision [5,6,7,8].

The 3d transition metal ions, Cr³⁺, Mn⁴⁺, and Fe³⁺, can act as luminescent centers emitting in the red to deep red region from 640 to 800 nm in suitable host materials due to 3d-3d transitions [9]. The luminescence properties are highly dependent on the combination of the luminescent center ions and the host materials. For example, Mn⁴⁺ in K₂SiF₆ has sharp excitation lines around 450 nm, leading it to be a candidate for a red phosphor for LED lighting [10]. On the other hand, Mn⁴⁺ in SrLaAlO₄ [11] and SrGdAlO₄ [12] is not excited by blue light as it shows the excitation peak in the UV region around 365 nm.

Cr³⁺ has specifically attracted attention as a luminescent center for near-infrared luminescence. Various compounds, whether natural or synthetic, have been reported as host materials, in which Cr³⁺ often replaces Al³⁺ or Ga³⁺ in the host compounds. The sharp deep red emission around 694 nm of Cr³⁺ in α-Al₂O₃ has been utilized for solid-state lasers. Emission of Cr³⁺ has also been observed in, for example, β-Ga₂O₃ [13], spinel-type MgAl₂O₄ [14,15,16], ZnAl₂O₄ [15,16,17,18], LiAl₅O₈ [19,20], garnet-type Y₃Al₅O₁₂ [21,22,23], Gd₃Ga₅O₁₂ [24], and Gd₃Sc₂Ga₃O₁₂ [22], beryl Be₃Al₂Si₆O₁₈ [25,26], aluminosilicate CaSc_1–xAl_1+xSiO₆ [27], SrAl_11.88−xGa_xO₁₉ [28], Bi₂Ga₄O₉ [29], La₃GaGe₅O₁₆ [30], and phosphate KMP₂O₇ (M = Ga, Sc, In, and Lu) [31]. Cr³⁺ in these host materials has been considered to occupy octahedral sites. When the strength of the crystal filed is relatively large, such as α-Al₂O₃ and the spinel-type hosts [14,15,16,17,18,19,20], sharp emission spectra are observed near 700 nm due to the electronic transition from the excited state ²E to the ground state ⁴A₂, while in the host materials with a weak crystal field, such as the garnet-type hosts [21,22,23,24], CaSc_1−xAl_1+xSiO₆ [27], Bi₂Ga₄O₉ [29], and La₃GaGe₅O₁₆ [30], a red-shifted broad emission is observed due to the spin-allowed ⁴T₂ → ⁴A₂ transition.

We have demonstrated different types of deep red luminescence from 650 to 800 nm achieved with a single host material doped with either Mn⁴⁺, Fe³⁺, or Cr³⁺ [32]. The host material was prepared from LiF and Al₂O₃ at 1:2, which first appeared in the report by Belov et al. [33], described as “Al₄LiO₆F” with a spinel-type lattice, assuming a stoichiometric reaction between LiF and two Al₂O₃. Our previous paper and other researchers referred to this material as Al₄LiO₆F [34] or LiAl₄O₆F [35], following Belov’s suggestion. However, the composition of Al₄LiO₆F does not match the spinel-type composition of AB₂O₄. In normal spinel, the divalent cation A²⁺ and the trivalent cation B³⁺ occupy the tetrahedral and octahedral cation sites in the spinel-type lattice.

In our previous paper [32], we showed that Mn⁴⁺ in the host material prepared from LiF and Al₂O₃ significantly enhanced photoluminescence (PL) compared to spinel-type lithium aluminate LiAl₅O₈, indicating its potential as a new host material for deep red phosphors. However, the details of the composition and crystal structure had not been clarified. In this paper, we denote this host material as aluminum lithium fluoride oxide (ALFO) and discuss the characteristics of ALFO and the luminescent properties of Cr³⁺ in ALFO, the luminescent center ion for deep red phosphors which can be excited by visible radiation; this finding will help provide new insights into the design of new phosphor host materials.

2. Materials and Methods

ALFO samples were prepared from LiF and γ-Al₂O₃; LiF was prepared from lithium hydroxide monohydrate (99%, Kanto Chem. Co., Inc., Tokyo, Japan) and ammonium fluoride (99%, Kanto Chem. Co., Inc.) by precipitation in aqueous solution. The starting aluminum source was aluminum triisopropoxide (>99.9%, Kanto Chem. Co., Inc.); aluminum hydroxide was precipitated at pH 10 in isopropanol solution by NH₃ aq. Using Cr(NO₃)₃·9H₂O (98.0–103.0%, Kanto Chem. Co., Inc.) as the starting material, Cr³⁺ 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-Al₂O₃:Cr³⁺. The concentration of Cr³⁺ was defined as Cr³⁺/(Al³⁺ + Cr³⁺), assuming that it replaced Al³⁺ in the products. γ-Al₂O₃:Cr³⁺ and LiF were weighed at (Al + Cr)/Li = 4 (note that γ-Al₂O₃ 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-Al₂O₃:Cr³⁺ was placed in a 5 cm³ platinum crucible with a lid. The 5 cm³ platinum crucible was put in a larger platinum crucible (30 cm³) 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, LiAl₅O₈:Cr³⁺ was prepared from Li₂CO₃ (99%, Kanto Chem. Co., Inc.) and α-Al₂O₃:Cr³⁺ at a ratio of Li:(Al + Cr) = 1:5. α-Al₂O₃ 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 LiAl₅O₈ 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 Al³⁺, Li⁺, Cr³⁺, 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 Cr³⁺ 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 Cr³⁺ ion was introduced into an octahedral site in the MD cell to replace the Al³⁺ 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].

3. Results and Discussion

3.1. Compositions and Crystal Structure

Figure 1 compares the XRD patterns of ALFO:Cr³⁺ and LiAl₅O₈:Cr³⁺ prepared at 1200 °C with the Cr³⁺ 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 Al³⁺ in spinel-type LiAl₅O₈, which will be discussed below.

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

Preparation Temperature	Chemical Composition
1100 °C	Al4.69 Li1.31 F0.28 O7.55
1200 °C	Al4.73 Li1.27 F0.17 O7.65
1300 °C	Al4.83 Li1.17 F0.10 O7.78

shows the chemical compositions of the ALFO host material without the luminescent center ion Cr³⁺ 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 Cr³⁺ concentration in the Cr³⁺-doped ALFO samples was consistent with the starting composition; the compositional analysis showed that ALFO:Cr³⁺ prepared at 1200 °C with a starting concentration of 0.5% Cr³⁺ was 0.48%.

The structural refinement by the Rietveld analysis for ALFO prepared at 1200 °C without Cr³⁺ 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, Al³⁺ 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 Al³⁺ 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 Cr³⁺ was assumed to occupy octahedral sites in both the host materials because of the preference of Cr³⁺ with the d³ configuration to occupy octahedral sites; the ionic radius of Cr³⁺ 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 Al³⁺ 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 Cr³⁺ in the ALFO lattice exceeded 10%. Figure S4 in Supplementary Materials shows the XRD patterns of ALFO:Cr³⁺ prepared at 1200 °C at different Cr³⁺ concentrations from 0.01 to 10%. All the samples were substantially a single phase of ALFO; the six-fold coordinated ionic radii of Al³⁺ and Cr³⁺ were 0.535 Å and 0.615 Å [43], and replacing Al³⁺ with larger Cr³⁺ shifted the diffraction peaks slightly toward the lower angles.

3.2. Morphologies

Figure 3 shows the SEM images of 0.5% Cr³⁺-doped ALFO and LiAl₅O₈ prepared at 1200 °C and 1300 °C. The effect of treatment temperature did not appear to have a significant effect on the morphology; ALFO:Cr³⁺ 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 LiAl₅O₈:Cr³⁺ 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:Cr³⁺ and LiAl₅O₈:Cr³⁺ 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:Cr³⁺ 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:Cr³⁺ indicated that no flux growth effect was expected, and thus Li₂CO₃ reacted with α-Al₂O₃:Cr³⁺ to form LiAl₅O₈:Cr³⁺ before forming the melt.

3.3. Photoluminescence Properties

The emission and excitation spectra of 0.5% Cr³⁺ 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 ⁴A₂ to the excitation states ⁴T₁ (⁴F) and ⁴T₂ (⁴F) in Cr³⁺ of the d³ state. The emission peaks depended on the host crystal. α-Al₂O₃:Cr³⁺ 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:Cr³⁺ 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, Cr³⁺ in ALFO characteristically exhibited a doublet peak at 708 and 716 nm, and the profile was broader than those of α-Al₂O₃ and LiAl₅O₈. Although the apparent difference between the crystal structures of ALFO and ordered LiAl₅O₈ shown by the Rietveld analysis was in the cation arrangement, the broad doublet peak in ALFO:Cr³⁺ indicated that the local environment around the luminescent center Cr³⁺ in ALFO was even more inhomogeneous than simply considering octahedral coordination by six neighboring oxygens.

Figure 5 shows the variation of the emission peaks of ALFO:Cr³⁺ with the treatment temperature and compares them with those of α-Al₂O₃:Cr³⁺ and LiAl₅O₈:Cr³⁺. The peak at 696 nm of the ALFO:Cr³⁺ sample prepared at 1000 °C was due to the unreacted Al₂O₃ residue (Figure S5 in Supplementary Materials), indicating that the coprecipitated Cr³⁺ was incorporated into the α-Al₂O₃ grains to form α-Al₂O₃:Cr³⁺. The peak at around 715 nm was specific to ALFO:Cr³ and was observed in ALFO:Cr³⁺ 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 Cr³⁺ 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:Cr³⁺ samples prepared at 1000–1300 °C. Residual unreacted α-Al₂O₃ was recognized in the sample prepared at 1000 °C, while the samples prepared at 1100–1300 °C were in the single phase.

Figure 6 compares the PL spectra of ALFO:Cr³⁺ (a) and LiAl₅O₈:Cr³⁺ (b) at different Cr³⁺ concentrations. The emission intensity of ALFO:Cr³⁺ increased from Cr³⁺ = 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 Cr³⁺ in the near-infrared region was due either to the ⁴T₂ → ⁴A₂ transition of Cr³⁺ placed in a weak crystal field, or to magnetic interactions between Cr³⁺ ions at the neighboring sites. According to the Tanabe–Sugano diagram [46,47,48], the first excited state in the d³ configuration of octahedral coordination is the ⁴T₂ or ²E state, depending on the strength of the crystal field. The transition from the ²E state to the ⁴A₂ ground state yields the line spectra typically observed for Cr³⁺ around 700 nm, while broad near-infrared emission is reported for the transition from the ⁴T₂ 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 Cr³⁺ emission [9,15] and the broad luminescence due to Cr³⁺-Cr³⁺ pairs observed in SrAl_11.88−xGa_xO₁₉:0.12Cr³⁺ by Rajendran et al., which was concluded from the decay time measurements [28]. As for the shoulder peak around 780 nm in ALFO:Cr³⁺, the origin was attributed to the magnetic interaction between Cr³⁺ ions from the fact that the change in the lattice parameter with Cr³⁺ addition, i.e., the change in the strength of the crystal field at the Cr³⁺ sites, was not very large (Figure S4 in Supplementary Materials), and that the interaction between Cr³⁺ ions was observed in the ESR signal at a high Cr³⁺ concentration as described in Section 3.4.

LiAl₅O₈:Cr³⁺ showed the maximum intensity at Cr³⁺ = 1.0%, and the change of the overall peak profile was less distinct than ALFO:Cr³⁺. The broad peak around 780 nm also developed in LiAl₅O₈:Cr³⁺ above 1.0%, but the intensity was relatively low.

Figure 7 shows the variation of the internal and external quantum efficiencies (QEs) of ALFO:Cr³⁺ and LiAl₅O₈:Cr³⁺ versus Cr³⁺ 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:Cr³⁺ and LiAl₅O₈:Cr³⁺, respectively. ALFO:Cr³⁺ showed higher QEs than LiAl₅O₈:Cr³⁺. The QE_Int for ALFO was 85.6% at Cr³⁺ = 0.5%, higher than the maximum QE_Int of 58.3% for LiAl₅O₈ at Cr³⁺ = 1.0%. The highest QE_Ext was 18.6% for ALFO:Cr³⁺ and 11.6% for LiAl₅O₈:Cr³⁺. The enhancement of QEs of ALFO:Cr³⁺ 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 Cr³⁺ from the ideal octahedron with the introduction of F⁻ and vacancies on the O²⁻ sites, and the increased area of the PL peaks due to peak broadening.

3.4. Local Environments around Cr³⁺

Cr³⁺ prefers octahedral coordination in the host spinel lattice in both ALFO and LiAl₅O₈, whereas the different spectra suggested different local environments around Cr³⁺ in ALFO and LiAl₅O₈. Such aperiodic local structures are difficult to investigate using diffraction-based structural analysis, particularly for low-concentration dopants. In fact, the ALFO:Cr³⁺ 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.

Cr³⁺ in the d³ 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% Cr³⁺ doped α-Al₂O₃ (a), LiAl₅O₈ (b), and ALFO (c) prepared at 1200 °C. Cr³⁺ 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 Cr³⁺ in α-Al₂O₃ (Figure 8a) was consistent with those reported for Cr³⁺ 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 octahedrally coordinated d³ 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 Cr³⁺ 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].

Cr³⁺ in LiAl₅O₈ showed essentially the same ESR signal (Figure 8b) as reported by Singh et al. [45] for LiAl₅O₈:Cr³⁺. 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 Cr³⁺ ions and the resonance signal at g = 1.97 to the magnetic interaction between the Cr³⁺ ions, based on the literature [60,61].

The basic features of the ESR signal of Cr³⁺ in ALFO (Figure 8c) were similar to those of Cr³⁺ 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 Cr³⁺ ions in ALFO. The CrO₆ octahedra in ALFO are expected to be disturbed by the statistical distribution of Al³⁺ 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 Cr³⁺ in ALFO prepared at 1100 °C and 1300 °C were apparently similar to that of Cr³⁺ in LiAl₅O₈, but the ERS signal of Cr³⁺ 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 Cr³⁺ in these host materials.

Increasing the Cr³⁺ 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 Cr³⁺ ions, as Singh et al. discussed that g = 1.95 was due to exchange coupling of Cr³⁺-Cr³⁺ pairs [45]. The broadening of the peak at g ~ 1.96 with increasing Cr³⁺ concentration was also observed in Cr³⁺-containing phosphate glasses in the literature [62]. The presence of the magnetic interactions between Cr³⁺ ions at high concentrations was consistent with the discussion of the PL spectra, where the broad shoulder peak developed around 780 nm with increasing Cr³⁺ concentration (Figure 6a).

Since it was difficult to directly deduce the detailed local environment from the ESR signals, the local environment of isolated Cr³⁺ 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, Cr³⁺ 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 Cr³⁺ 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 Cr³⁺ in ALFO was inferred to be similar to that in LiAl₅O₈, but even more disordered.

4. Conclusions

Fluorine-doped lithium aluminate (ALFO) was prepared from LiF and Al₂O₃ at a ratio of 1:2. ALFO was a fluorine-doped form of LiAl₅O₈ 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 LiAl₅O₈ were isostructural with respect to having a spinel-type lattice and they were in a disorder–order relationship, where the regular arrangement of Li⁺ and Al³⁺ in ordered LiAl₅O₈ was disordered to a random arrangement with a slight preference of Li⁺ on the octahedral sites. The structural difference between ALFO and LiAl₅O₈ resulted in the different PL spectra for Cr³⁺. Cr³⁺ in ALFO showed deep red emission at around 700 nm, as did Cr³⁺ in LiAl₅O₈, but the peaks broadened. This was attributed to the disordered arrangement of Li⁺ and Al³⁺ on the adjacent cation sites, which led to ununiform local distortion of the CrO₆ octahedra.

In conclusion, a new host material was proposed for deep red phosphors with the luminescent center Cr³⁺ excited by visible rays of blue (~410 nm) and green (~570 nm) light. This work revealed that the structural modification with a small amount of dopant (in this case, fluorine) led to variations in the luminescence properties and enhancement of PL even for the common lattice structure.