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## Creator

Zhiyuan Pan, Sihan Feng, Yun Wang, Xuejiao Wang, Qi Zhu, [Ji-Guang Li](https://orcid.org/0000-0002-5625-7361)

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[High Efficiency NIR Luminescence Beyond 900 nm Through Promoting Cr3+ Pair Generation in Sr3Zr2O7 Layered Perovskite](https://mdr.nims.go.jp/datasets/90f0c9c9-d7e7-45ca-901a-e082674bc571)

## Fulltext

1  High Efficiency NIR Luminescence Beyond 900 nm Through Promoting Cr3+ Pair Generation in Sr3Zr2O7 Layered Perovskite Zhiyuan Pana, Sihan Fenga, Yun Wanga, Xuejiao Wangb,*, Qi Zhua, Ji-Guang Lic,∗ a Key Laboratory for Anisotropy and Texture of Materials (Ministry of Education), School of Materials Science and Engineering, Northeastern University, Shenyang, Liaoning 110819, China b School of Materials and Environmental Engineering, Bohai University, Jinzhou, Liaoning 121007, China c Research Center for Electronic and Optical Materials, National Institute for Materials Science, Tsukuba, Ibaraki 305-0044, Japan       *Corresponding author Dr. Xuejiao Wang Bohai University Tel: +86-416-3400708 E-mail: wangxuejiao@bhu.edu.cn  Dr. Ji-Guang Li National Institute for Materials Science Tel: +81-29-860-4394 E-mail: li.jiguang@nims.go.jp mailto:wangxuejiao@bhu.edu.cnmailto:li.jiguang@nims.go.jp2  Abstract The near-infrared (NIR) light of 900-1100 nm exhibits enormous application potential in fields such as optical communication and biological detection. However, high-efficiency phosphors emitting beyond 900 nm remain extremely limited due to non-radiative deactivation. In this study, a 455 nm blue light excitable NIR phosphor that efficiently emits in the wide range of 700-1200 nm (peaking at ~960 nm) was obtained through Cr3+/La3+ codoping of Ruddlesden-Popper type Sr3Zr2O7 perovskite, whose internal/external quantum efficiencies (%) and thermal stability of luminescence (I423/I298, %) reached ~92.4/57.5 and 73, respectively. Experimental and theoretical analysis found the decisive role of the La3+ codopant in Cr3+ luminescence: it not only inhibited Cr3+ from oxidation but also attracted the Cr3+ ions to aggregate around it to form Cr3+-Cr3+ pairs for the aforesaid excellent luminescence. Combining the NIR phosphor with a Si-PIN photodetector created a highly effective blue-to-NIR conversion unit, demonstrating application potential of the phosphor for underwater wireless optical communication (UWOC).   Keywords: Near-infrared phosphor, Sr3Zr2O7 perovskite, Cr3+-pair luminescence, La3+ doping    3  1 Introduction With the wide application of near-infrared (NIR) technology in night vision, biological imaging, optical communication and other technical fields, developing high performance NIR light sources has become an urgent task.1-5 Compared with other NIR light sources, blue-light excited NIR pc-LEDs have drawn wide attention because the technology to fabricate the blue LED chips for excitation has matured and NIR phosphors are relatively easy to synthesize and their excitation/emission can be adjusted through materials design.6,7 Therefore, in recent years, developing high quality NIR phosphors has become an important research direction in the field of optics.  As one of the most intensely studied NIR activator ions, Cr3+ exhibits tunable luminescence by its crystal-field sensitive 3d3 energy level. By designing host materials and doping strategies, flexible NIR luminescence in the range of 700-1200 nm can be achieved. Up to now, many Cr3+-activated NIR phosphors have been developed,8,9 but most of them suffer from unsatisfactory efficiency of blue-light absorption by the parity-forbidden d-d transition of Cr3+, which reduces the matching between the phosphor and commercial blue LED chip. To overcome this, researchers have proposed to enhance orbital mixing and reduce the degree of forbidden by constructing low-symmetry coordination octahedra for Cr3+.10 Typical examples may include Ga1.6Sc0.4O3:Cr3+ (λem = 784 nm, λex = 442 nm, IQE = 99%),11 Ca2.7Na0.3Mg0.7Sb2Al2.3O12:Cr3+ (λem = 760 nm, λex = 460 nm, IQE = 90.6%)12 and Ca3Sc2Si3O12:Cr3+ (λem = 770 nm, λex = 460 nm, IQE = 92.3%) phosphors.13 For luminescence longer than 850 nm, however, high IQE and good thermal stability are 4  frequently hard to attain since the probability of non-radiative transitions would significantly increase by the reduced energy gap between the lowest-lying excited state (emitting state) and ground state of Cr3+,8 such as in the cases of LiScP2O7:Cr3+ (λem = 880 nm/λex = 470 nm, IQE = 38%, I423 K/I298 K ≈ 20%),14 NaScSi2O6:Cr3+ (λem = 845 nm/λex = 460 nm, IQE = 64.4%, I423 K/I298 K = 75%)15 and LiInGe2O6:Cr3+ (λem = 880 nm/λex = 460 nm, IQE = 81.2%, I423 K/I298 K ≈ 25%).16 This restricts application in areas like biological detection and optical communication, where NIR lights of longer wavelengths are preferred. Therefore, the development of blue-light excitable NIR phosphors of high efficiency, long emitting wavelength and high thermal stability has significant practical importance. The novel emission features of Cr3+-Cr3+ pairs are drawing wide attention during recent years,17-22 and the study of which can be dated back to 1961 by Gill et al.23 In 2024, Wang et al.24 successfully achieved 913 nm emission of Cr3+ pairs with high efficiency (IQE/EQE = 83.9%/35.7%) and good thermal stability (I423 K/I298 K = 75.8%) through high concentration Cr3+ doping of Mg0.8Zn0.2Ga2O4 spinel. Despite this success, over-doping of Cr3+ activators inevitably induces concentration quenching, thereby reducing the overall intensity of luminescence. Regarding this issue, You et al.25 designed and synthesized a BaO(Al2O3)4(ZnAl2O4):xCr3+ phosphor (BAZO:xCr3+, λem = 750 nm, λex = 460 nm, x = 0.06-0.3), where the layered crystal structure effectively alleviated the concentration quenching problem and allowed for high concentration Cr3+ doping (x = 0.28). The Ruddlesden-Popper (R-P) type perovskite compounds of (AO)(ABO3)n general formula not only have a layered crystal structure but also provide 5  a [BO6] octahedral site suitable for Cr3+ to occupy, which make them valuable hosts for Cr3+ luminescence. In this study, Sr3Zr2O7 (SZO), which belongs to the above R-P family, was selected for the first time as the host lattice for Cr3+. By allowing Cr3+ to occupy the large-sized and highly distorted [ZrO6] octahedral sites, we successfully developed in this work a series of blue-light excitable high-performance Sr3Zr2O7:Cr3+,La3+ (SZCL) NIR phosphors. Theoretical and experimental analyses revealed that the La3+ co-dopant may effectively inhibit Cr3+ from oxidation, induce the Cr3+ ions to redistribute, and promote the creation of Cr3+-Cr3+ pairs, with which excellent NIR luminescence was achieved. The SZO:8%Cr,16%La optimal phosphor is not only excitable with 455 nm blue light but also emits at a long wavelength of 960 nm with high quantum efficiency (IQE/EQE = 92.4%/57.5%) and good thermal stability (I423 K/I298 K = 73%). The phosphor was also demonstrated to have the potential for application in underwater wireless optical communication (UWOC) technology.  2 Experimental Section 2.1 Materials and Synthesis The phosphors were synthesized through high temperature solid reaction, using SrCO3 (99.99% pure; Aladdin Bio-Chem. Tech. Co. Ltd, Shanghai, China), ZrO2 (99.99% pure; Aladdin), La2O3 (99.5% pure; Huizhou Ruier Rare-Chem. Co. Ltd, Huizhou, China), and Cr2O3 (99.99% pure; Aladdin) as raw materials. The La2O3 was dried at 200 ℃ for 2 h before use. The reactants were weighed according to the formulae of Sr3Zr2O7, Sr3Zr1.97O7:0.03Cr, Sr3-xZr2-xO7:xCr,xLa (x = 0.01-0.1) and Sr3-yZr1.92O7:0.08Cr,yLa (y = 0-0.2), respectively, and then thoroughly mixed via grounding 6  with a mortar and pestle for 40 min. The reactant mixture was then transferred into a corundum crucible for solid reaction at 1500 ºC for 6 h in air, with heating rates of 5 ºC /min up to 400 ºC and then 8 ºC /min up to 1500 ℃. All the samples containing Cr ions were subjected to a reduction treatment in a 20 vol% H2/80 vol% Ar gas mixture (flow rate: 150 mL min−1), where the heating rates are the same as above and the holding time is 2 h. To make a fluorescent film, 0.05 g of the SZO:8%Cr,16%La phosphor was mixed with 0.4g PDMS (Polydimethylsiloxane) glue, followed by spin casting and baking at 60 ºC for 3 h.  2.2 Characterization X-ray diffractometry (XRD) was performed with a Model SmartLab instrument (Rigaku, Japan) operating at 40 kV/200 mA, using nickel-filtered Cu-Kα radiation as the X-ray source (λ = 0.15406 nm). The XRD data for phase identification were collected over the 2θ range of 20-70° via continuous scan at 10° 2θ/min and those for Rietveld structure refinement were acquired for 2θ = 10-110° via step scan, using a step size of 0.02° and a counting time of 2 s per step. Structure refinement was conducted with the TOPAS version 4.2 software suite. Morphology and elemental distribution were analyzed via field-emission scanning electron microscopy (JSM-7800F, JEOL, Japan; X-max20, OXFORD, UK). Diffuse reflectance spectroscopy (DRS) was performed with a UV-vis spectrophotometer (Model UV-3600 Plus, Shimadzu, Japan) equipped with a 150 mm diameter integrating sphere. Room temperature photoluminescence was analyzed with a Model Fluorolog-3 spectrophotometer (HORIBA, Japan) and all the measurements were carried out with a scan speed of 200 7  nm min-1 and slit widths of 10 nm for excitation/emission. Temperature-dependent photoluminescence and quantum efficiency were measured by a Model FLS1000 fluorospectrometer equipped with an integrating sphere (150 mm in diameter) and a TAP-02 temperature controller (Edinburgh Instruments Ltd., UK). Fluorescence decay was analyzed with a DeltaFlex modular fluorescence lifetime system (HORIBA, Japan), using NanoLED-455 (λ = 450 nm, 1.4 ns pulse duration) as the excitation source. X-ray photoelectron spectroscopy (XPS) was performed with a Model Axis Supra instrument (Kratos Analytical Ltd., UK). Electron paramagnetic resonance (EPR) analysis was conducted at the X-band frequency (9.85 GHz) on a Model Bruker Emxplus instrument (Karlsruhe, Germany). 2.3 Computational Methodology All the first-principles calculations were carried out using the Castep tool in Materials studio 2020 software, and the details of which can be found in the Supporting Information file. 2.4 Experimental setup of the underwater wireless optical communication (UWOC) system The signal light source is a blue light laser (450 nm, Laserland, China). Si-PIN (Model S1223-01, Hamamatsu, Japan) was used as photodetector. Electrical signal acquisition was accomplished with an oscilloscope (B2902B, Keysight, USA). A long-pass filter membrane (T80LGM, Anford, China) was employed to block the visible light. Its optical transmission is <2%, ~40% and ~90% in the 400-700 nm, 700-800 nm and 800-1600 nm (near infrared) spectral regions, respectively. A water-filled glass tank of 8  10 cm × 10 cm × 15 cm was used as the water environment along the signal transmission path. 3 Results and Discussion 3.1 Structure and Luminescence Properties of SZCL  Fig. 1. (a) Schematic crystal structure of SZO; (b) XRD patterns of the SZO, SZO:3%Cr and SZO:3%La,3%Cr powders; (c) enlarged view of the 29.5°-31.5° region of the XRD patterns; Rietveld refinement of the XRD patterns for SZO (d), SZO:3%Cr (e), SZO:3%Cr,3%La(1) (f) and  SZO:3%Cr,3%La(2) (g). 9  Sr3Zr2O7 (SZO) crystallizes in the tetragonal system (space group: I4/mmm) and is featured by a typical R-P type perovskite structure, where the [SrZrO3]2 double perovskite layer and [SrO] rock-salt layer alternatively stacks along the c axis. In such a structure, Zr4+ ions exclusively form [ZrO6] octahedra whereas Sr2+ ions form [SrO9] and [SrO12] polyhedrons,26 as illustrated in Fig. 1a. It is generally accepted that a dopant ion prefers to replace the host ion of a similar ionic radius. Thus, when Cr3+ (0.61 Å for CN= 6, CN: coordination number) was introduced into SZO, it tends to replace Zr4+ (0.72 Å for CN = 6) to reside in octahedron. Since the valence mismatch between Cr3+ and Zr4+ may induce the oxidation of Cr3+,27 La3+ was co-doped for charge compensation given its similar radius to Sr2+ (1.36 Å for La3+ and 1.44 Å for Sr2+, CN = 12). We first synthesized the three samples of SZO, Sr3Zr1.97O7:0.03Cr (SZO:3%Cr) and Sr2.97Zr1.97O7:0.03Cr,0.03La (SZO:3%Cr,3%La) for comparative studies, whose XRD patterns are shown in Fig. 1b. It is seen that all the samples exhibited diffraction profiles essentially identical to that of the SZO standard (PDF No.23-0559). Close observation (Fig. 1c) found that the (105) and (110) peaks of SZO:3%Cr and SZO:3%Cr,3%La slightly shifted to higher angles, indicating some extent of lattice contraction. Rietveld refinement of XRD pattern was conducted for the three samples to derive structure details, and the results are shown Fig. 1d-g and Tables S1, S2. From the structure of SZO (Fig. 1a) and ionic size, it is considered that only the [ZrO6] site is suitable for Cr3+ to occupy. For La3+, however, the two sites of [SrO9] and [SrO12] are available. Therefore, La3+ was separately set at the two sites of [SrO9] (Fig. 1f, La(1)) 10  and [SrO12] (Fig. 1g, La(2)) for the refinement of SZO: 3%Cr,3%La. From the better R and χ2 factor values shown in the Fig. 1f, it can be concluded that La3+ inclines to occupy the [SrO9] site. This is likely because the loose interlayer structure allows for greater local distortion. Table S1 lists the detailed lattice parameters, atomic positions, atomic occupancy, and atomic displacement parameters of the three samples. It can be found that, for either SZO:3%Cr or SZO:3%Cr,3%La, the lattice parameters decreased relative to those of SZO, which is consistent with the higher-angle shifting of diffraction peaks (Fig. 1c). According to the refinement results, Cr3+ and La3+ preferentially occupy the Zr4+ and Sr2+ sites with an occupancy of 3%, respectively. Table S2 shows the (Zr,Cr)-O bond lengths, from which the Zr-O bonds in SZO were assayed to have an average length (dav) of ~2.1634 Å. Besides, the [ZrO6] octahedron was calculated to have a distortion index (Di) of ~0.0632,28 which is a relatively high value.10 Therefore, when Cr3+ occupies [ZrO6] octahedron in SZO, a high blue light absorption efficiency may be achieved.  Fig. 2. (a) XPS survey spectra of the SZO:3%Cr, SZO:3%La,3%Cr, SZO:8%La,8%Cr and SZO:8%La,16%Cr samples; (b-e) high resolution XPS spectra for the Cr 2p3/2 and Cr 2p1/2 core levels; (f) high resolution XPS spectra of the Cr2O3, CrO2 and CrO3 reference standards. 11  After Cr3+ doping to form SZO:3%Cr, the dav of (Zr,Cr)-O was shortened to 2.0629 Å, indicating contracted [(Zr,Cr)O6] octahedra. When La3+ and Cr3+ were co-doped into SZO to form SZO:3%Cr,3%La, however, the dav of (Zr,Cr)-O was elongated to 2.0941 Å and at the same time the lattice constants of SZO:3%Cr,3%La became slightly larger than those of SZO:3%Cr (Table S1), implying lattice expansion. This expansion can also be perceived from the enlarged view of the XRD patterns in Fig. 1c, where the diffraction peaks of SZO:3%Cr,3%La shifted to smaller angles relative to those of SZO:3%Cr. Such a lattice expansion is abnormal since La3+ is smaller than Sr2+. In view of the significant difference in ionic radii among the different valence states of Cr (rCr3+ = 0.61 Å, rCr4+ = 0.55 Å and rCr6+ = 0.44 Å for CN = 6) and the invariable oxidation states of Sr2+, La3+ and Zr4+, the abnormal lattice change was supposed to be due to La3+-induced valence change of Cr. To verify this, comparative XPS analysis was performed for the SZO:3%Cr and SZO:3%Cr,3%La samples, and the results are shown in Fig. 2. The survey spectra (Fig. 2a) clearly showed the presence of the constituent elements as well as ubiquitous carbon. Fig. 2b and c shows the high resolution XPS spectra of the 2p core level of Cr in SZO:3%Cr and SZO:3%Cr,3%La, where two asymmetric peaks were well resolved. Each of the peaks can be deconvoluted into three sub-peaks via Gaussian fitting, with those at ~572/585 eV (in yellow), ~576/587 eV (in green) and ~580/589 eV (in blue) well assignable to Cr4+, Cr3+ and Cr6+, respectively, according to the high-resolution XPS results reported by Wang et al.29 It is also seen that these peaks well conform to those of the CrO2, Cr2O3 and CrO3 reference standards (Fig. 2f), respectively, further confirming the coexistence of Cr4+, Cr3+ and Cr6+ ions in 12  either of the samples. From the integral areas of the sub-peaks, the portions of Cr4+, Cr3+ and Cr6+ were estimated to be ~21.1%, 18.3% and 60.6% for SZO:3%Cr, respectively. Although the SZO:3%Cr sample has been treated in a reducing atmosphere, the low concentration of Cr3+ indicates that severe oxidation has occurred. This suggests that hydrogen reduction alone is insufficient to stabilize Cr3+ in the SZO lattice. In SZO:3%Cr,3%La, however, the proportion of Cr3+ significantly increased to 64.1% while those of Cr6+ and Cr4+ decreased to 18.1% and 19.8%, respectively. Such results thus clearly indicate that doping La3+ at the Sr2+ site is an effective strategy to stabilize the valence state of Cr3+ through charge compensation. The valence change also proves that the abnormal lattice expansion of SZO:3%Cr,3%La was caused by the increased content of larger Cr3+ ions.  Fig. 3. FE-SEM morphology (a) and EDS elemental mapping (b) for SZO:3%Cr,3%La.  FE-SEM observation (Fig. 3a) found that the SZO:3%Cr,3%La sample contains clusters of micro-sized primary particles, where the tending to be cuboidal morphology may correspond to the tetragonal crystal structure of the material. Elemental mapping 13  via EDS (Fig. 3b) indicated that the constituent elements are evenly distributed within the particles, confirming the formation of solid solution.     Fig. 4. (a) Schematic of Models 1-4 (substitution model); (b) The formation energy (Ef) for Models 1-4; (c) Calculated band structure, total DOS, and partial density of states (PDOS) of the constituent elements for SZO. Density functional theory (DFT) was used to evaluate the energetics of La3+ incorporation into SZO:Cr. To further confirm lattice occupancy, computation was conducted by assuming that La3+ occupies [SrO9] (Sr1, Model 1; Fig. 4a) and [SrO12] (Sr2, Model 2; Fig. 4a) sites, respectively. Detailed calculation process can be found in the Supporting Information file. From the results shown in Fig. 4b, it can be concluded that, conforming to the finding of Rietveld refinement, La3+ indeed prefers [SrO9] since the computed formation energy (Ef) is lower. With this site preference, the spatial 14  correlation between La3+ and Cr3+ was examined, which is the key to understand the mutual interaction of these two types of dopant ions.30 To determine the preferred relative position, a Cr3+ ion was placed at either a Zr site adjacent to La (Zra, Model 3; Fig. 4a) or a Zr site distant from La (Zri, Model 4; Fig. 4a). Comparison of the Ef for these two configurations (Fig. 4b) revealed a clear energetic preference for the former case (Zra site). This indicates that La3+ and Cr3+ tend to be neighbors in the SZO lattice, which is complying with the case revealed for the SrTiO3:La,Cr analogous system.31 Such a scenario could be favored by the ease of local charge compensation, since replacing Sr2+ with La3+ and replacing Zr4+ with Cr3+ bring about positive and negative charges, respectively. DFT calculation showed that the SZO host has an indirect bandgap of ~3.18 eV, with the conduction band (CB) and valence band (VB) being primarily composed of Zr 4d and O 2p orbitals, respectively (Fig. 4c). In the band structure of SZO:3%Cr,3%La, the d-orbitals of Cr3+ introduce energy states near the Fermi level (Fig. 5a-c). SZO:3%Cr,3%La also shows an anisotropic band structure, since the dispersion from the G to X point, which determines the effective mass of electrons along the [100] direction, spans an energy range of ~1 eV (Fig. 5a) while that from the G to Z point (along [001]) is nearly negligible (Fig. 5a). The effective mass (m*) of charge carriers is inversely proportional to the second derivative of the E-k curve, as follows:32,33 m* = ℏ2(d2Edk2)-1(1) The above formula implies a low effective mass and high carrier mobility for a steep 15  and highly dispersive band and vice versa for a flat band. It can thus be inferred from the results of band analysis that the electron mobility along [100] is much higher than that along [001] in SZO:3%Cr,3%La, and this holds for other in-plane directions such as [110] (G–R). The strongly suppressed electron transport along the [001] direction may effectively isolate the Cr3+ ions residing in different layers. This would in turn alleviate concentration quenching even at high doping levels by spatial isolation, as reported for other layer-structured phosphor systems.24,34,35   Fig. 5. Calculated band structure (a), total density of states (DOS, b), and partial density of states (PDOS) of the constituent elements (c) for SZO:3%Cr,3%La. Diffuse reflectance spectroscopy (DRS) of SZO, SZO:3%Cr and SZO:3%Cr,3%La 16  revealed that SZO mainly absorbs the ultraviolet (UV) light up to ~280 nm (Fig. S1). The optical bandgap of SZO was determined to be ~3.45 eV from the Tauc plot (Fig. S1, the inset),36-38 which is close to that (3.18 eV) derived via DFT calculation. SZO:3%Cr showed a new absorption band in the 400-750 nm region that is attributable to Cr6+ ions.39 SZO:3%Cr,3%La exhibited two new absorption bands centered at ~461 and 647 nm, which are assignable to the 4A2 → 4T1 (4F) and 4A2 → 4T2 (4F) transitions of Cr3+ ions, respectively. The 4A2 → 4T1 (4P) transition of Cr3+ overlaps with host absorption, resulting in an intensified composite band at ~275 nm. The results may thus provide optical evidence for the aforesaid effective stabilization of Cr3+ ions by La3+ incorporation. Particularly, the strong absorption at ~460 nm implies that the SZO:3%Cr,3%La phosphor can be efficiently excited by blue light.   Fig. 6. (a) PL and PLE spectra of the SZO:3%Cr,3%La sample; (b) Tanabe–Sugano energy level diagram for Cr3+ under octahedral coordination; (c) Fluorescence decay curves of SZO:3%Cr,3%La; (d) Configurational coordinate diagrams of an isolated Cr3+ ion and Cr3+−Cr3+ ion pair; (e) PL spectra of SZO:xCr,xLa (x = 1-10%); 17  Fig. 6a shows the photoluminescence spectra of SZO:3%Cr,3%La. The excitation spectrum (PLE, in light-blue color) contains three bands centered at ~279, 455 (the strongest) and 625 nm, respectively, which are closely matching those found in the DRS spectrum. Under 455 nm excitation, the phosphor outputs a strong emission ranging from ~700 to 1200 nm (PL, in brick red), which can be deconvoluted into two bands centered at ~780 and 930 nm, respectively. The emission behavior of Cr3+ is well known to heavily depend on the strength (Dq/B ratio) of its surrounding crystal field, and the values of crystal field splitting energy Dq and Racah parameter B can be derived from the following equations:40,41 Dq = E( A2 4  → T2 4 )10(2) DqB = 15(x - 8)(x2 - 10x)(3) x = E( A2 4  → T1 4 ) - E( A2 4  → T2 4 )Dq(4) For the 780 nm emission, E(4A2 → 4T2) = 1.968 eV and E(4A2 → 4T1) = 2.725 eV, thus Dq/B was calculated to be ~2.54, larger than 2.3. This indicates that the Cr3+ ions are in a strong octahedral field in SZO, as seen from the Tanabe-Sugano energy level diagram for octahedrally coordinated for Cr3+ (Fig. 6b). Such a field strength would give rise to a sharp emission from the 2E → 4A2 transition of Cr3+,8 which corresponds to the ~780 nm main peak in this work (Fig. 6a). The PL spectrum, however, clearly presents a broad band peaking at ~930 nm. As the PLE acquired by monitoring this emission did not show new spectral features, as compared in Fig. S2, it can be concluded that such an emission did not arise from another separate emitting site or 18  from Cr in a different valence state but is instead associated with the same Cr3+ centers. A number of studies have shown that the Cr3+ ions under a high enough concentration tend to form Cr3+-Cr3+ pairs by shortened interionic distance, and the ground states of the paired Cr3+ ions will couple to form a combined ground state (4A2, 4A2).25,42 Under such a circumstance, when one Cr3+ ion (Cr1) absorbs energy and becomes excited, energy exchange interaction may occur with the ground state of the other Cr3+ ion in the pair (Cr2) to form multi-coupled states such as (4A2, 2E) or (4A2, 4T2) (Fig. 6d). A key spectroscopic signature of such pairs is that their PLE spectrum is virtually indistinguishable from that of a single Cr3+ ion, since the initial excitation is localized on a single ion within the pair.24 Fig. 6c gives the fluorescence decay curves for the 780 and 930 nm emissions of the SZO:3%Cr3+,3%La3+ phosphor (λex = 450 nm), which can be fitted with the following exponential equation: I(t) = I0 × A exp ( -t/τ)                      (5)  where I0 and I(t) are initial emission intensity and the intensity at decay time t, respectively, A is a pre-exponential constant, and τ is fluorescence lifetime. It is seen from Fig. 6c that the 780 nm emission have a millisecond-level lifetime, characteristic of the spin-forbidden 2E → 4A2 transition.6,8 The broad 930 nm emission also has a millisecond-level lifetime, implying that it does not arise from the spin-allowed 4T2 → 4A2 transition of Cr3+, which has a microsecond-scale lifetime though typically featured by a wide emission.43 Noteworthy is that broadband emission of a relatively long lifetime is common for Cr3+ pairs. The above results may indicate that the broadband emission near 930 nm originates from Cr3+ pairs. 19  The luminescence properties of Cr3+ pairs can be explained by analyzing the coupling effect. When the distance between two Cr3+ ions meets the condition for coupling to occur, the spin interaction between them can be described by the following equation:25,41,44 HAB = -J(SASB) + j(SASB)2                     (6) in which HAB is the Heisenberg Hamiltonian for exchange interaction between two metal ions with spins of SA and SB, J is an isotropic bilinear parameter giving the strength of exchange coupling interaction, and j is the exchange parameter for biquadratic exchange. Theoretically, exchange interaction is expected to occur in transition metal ion pairs, including both 3dn and 4fn ions. Given that the contribution of the second term is negligible, the energy eigenvalue E(S) of HAB can be determined by the following equation:25,41,44 E(S) = -J[S(S + 1) - SA(SA + 1) - SB(SB + 1)]            (7) where S represents the total spin, taking all integer values between the sum (SA + SB) and the difference |SA - SB| of the spin values SA and SB of the two ions. For transition metal ion Cr3+ (ground state: 4A2, spin S = 3/2), the coupling energy levels (4A2, 4A2) of the two ions’ ground states yield four possible spin values of S = 0, 1, 2, 3. When one of the Cr3+ ions is in the 2E excited state (spin S = 1/2) and the other remains in the ground state, the total spin of the pair’s excited state energy level is S = 1, 2 (Fig. 6d). Therefore, when two Cr3+ ions couple together, the excited and ground state energy levels of the pair will no longer separate from each other but will instead be composed of a series of new levels with small energy differences. When excited, the electrons will 20  undergo transitions between these densely distributed energy levels, thereby generating a continuous emission band covering a wide energy range. This explains why the 930 nm emission appeared as a broadband in this work. As pair formation is strongly dependent on the concentration of Cr3+, a series of phase-pure SZO:xCr,xLa samples (x = 1%-10%; Fig. S3) were synthesized with equimolar of Cr3+ and La3+ for investigation. PL analysis (λex = 455 nm, Fig. 6e) showed that the intensity of Cr3+ emission increases with increasing x up to 8%, together with a gradual shifting of the main emission from 780 to 960 nm. This indicates that a suitably high Cr3+ content may not only provide more luminescent centers but also dramatically increase the number of Cr3+ pairs. Noteworthy is that the emission from Cr3+ pairs appeared at a Cr3+ concentration (x = 2%, Fig. 5e) well below the onset (x = 8%) of concentration quenching. Generally, when Cr3+ ions are randomly distributed in the host, a doping level close to that for quenching concentration to occur is required to meet the distance condition for Cr3+ pair formation.19-22 Therefore, the early emergence of Cr3+ pair emission in the SZO:xCr,xLa phosphors is highly unusual. 3.2 Luminescence optimization  Fig. 7. (a) PL spectra of SZO:8%Cr,yLa; (b) Emission intensities at 780 and 960 nm as a function of y for SZO:8%Cr,yLa; (c) EPR spectra of the SZO:8%Cr,y%La samples (y = 4%, 8% and 16%).  Since La3+ and Cr3+ tend to be neighbors, as revealed via DFT calculation (Fig. 4a 21  and b), we speculated that La3+ doping disrupted the random distribution of Cr3+ and thus promoted the early formation of Cr3+ pairs. To verify this, a series of phase-pure SZO:8%Cr,yLa (y = 0-20%) phosphors were prepared under the fixed optimal Cr3+ content of 8%, whose XRD patterns are shown in Fig. S4. PL analysis (Fig. 7a) revealed the dramatic influence of La3+ content on Cr3+ luminescence. The y = 0 sample did not exhibit detectable emission, while La3+ doping produced two bands corresponding to 2E → 4A2 transition of Cr3+ and Cr3+ pairs at ~780 and 960 nm, respectively. Furthermore, both the emissions gradually gained intensity up to y = 16%. When the concentration of isolated Cr3+ and Cr3+ pair increased further (y = 20%), concentration quenching of luminescence occurred, as seen from the luminescence intensity drop in Fig. 7a. This further confirms that La3+ doping may effectively inhibit Cr3+ oxidation and the effect of inhibition is dependent on La3+ content. While the luminescence remained weak up to 8% of La3+ (equimolar with Cr), significant intensity enhancement was observed under excessive La3+ doping (y > 8%). This suggests that super-stoichiometric La3+ is required for better suppression of Cr3+ oxidation, which was confirmed by the results of XPS analysis shown in Fig. 2d and e. It is seen there that Cr3+ comprises only 31.7% of the total Cr species in the SZO:8%Cr,8%La sample whereas nearly all the Cr ions are in the 3+ valence state in SZO:8%Cr,16%La. We also found that the SZO:8%Cr (y = 0) and SZO:8%Cr,16%La (y = 16%) samples appear brownish and greenish under natural light (Fig. S5), respectively, implying that the dominant Cr species are Cr6+ in the former and Cr3+ in the latter according to the report of Zhou et al.45 This further confirms that La3+ doping inhibits Cr3+oxidation. It should 22  be noted that excessive doping of La3+ may produce unbalanced point defects such as oxygen interstitials. To determine whether the luminescence enhancement is related to such defects, Al3+ ion, whose radius (0.53 Å under CN = 6) is close to that of Cr3+ (0.61 Å, CN = 6), was used to replace Cr to form a Sr3Zr2O7:8%Al3+,16%La3+ reference sample under the same synthesis conditions. XRD and PL analyses indicated that the product is a pure perovskite phase but did not show luminescence under 455 nm excitation (Fig. S6). Such a result thus ruled out the possibility of emission from defect-related energy levels.   Fig. 8. Schematics for Models Ⅰ-Ⅴ, where Models Ⅰ-Ⅱ illustrate configurations with two Cr3+ ions introduced into the lattice while Models Ⅲ-Ⅴ represent three possible configurations for an added La3+, which are based on Models Ⅰ-Ⅱ. The Ef of each configuration is displayed in the bar chart. Although the actual fraction (31.7%) of Cr3+ in SZO:8%Cr,8%La is not high, the emission from Cr3+ pairs (λem =960 nm) has become dominant (Fig. 7a). It is also seen from Fig. 7b that the 960 nm emission gains intensity much faster than the 780 nm one with increasing La3+ content, further demonstrating that La3+ not only suppresses Cr3+ oxidation but also promotes the formation of Cr3+ pairs. To explore the promotion 23  mechanism of La3+, we constructed five configurational models to simulate the distribution of Cr3+ and La3+ and the formation energy (Ef) of Cr3+ pairs was evaluated through DFT calculation, as shown in Fig. 8. Models Ⅰ and Ⅱ present two Cr3+ ions introduced into SZO as separate ones and adjacent ones, which were calculated to have inter-atomic distances of 10.316 and 4.290 Å after structure optimization, respectively. Model I may thus preclude interaction between the ions due to the large separation distance, while Model II may satisfy the distance prerequisite for coupling and allow the formation of a Cr³⁺-Cr³⁺ pair. The calculated Ef values (Fig. 8), however, showed that Model I is more stable, suggesting that Cr3+ pairs are difficult to form in SZO without La3+ doping. By introducing a La3+ ion into Model I and Model II as a neighbor of Cr3+, the structures shown in Model Ⅲ and Model Ⅳ were obtained, respectively. It is seen from the results of calculation that the Ef of Cr3+ pair formation can be significantly reduced from -1.31 to -1.98 eV for Model Ⅳ, a value even lower than that of Model Ⅲ for random distribution of Cr3+. Interestingly, Ef reduction only occurs when Cr3+ pair forms around La3+ (Model Ⅳ). As seen from the result of Model Ⅴ (Fig. 8), the Ef will significantly increase if Cr3+ pair is formed far away from the La3+ ion. These results demonstrate that La3+ does not uniformly lower the Ef of Cr3+ pair globally but instead acts as a center to attract the Cr3+ ions to aggregate around it and thus induce Cr3+ pairs to form. The driving force behind the phenomenon could be the ease of local charge compensation as mentioned earlier. For Model Ⅳ, it is also seen that the separation distance of the two Cr3+ ions in a pair formed around La3+ is 4.166 Å (Fig. 8), which is smaller than that without La3+ doping (Model II, 4.290 Å). This might be because La3+ is smaller than Sr2+, which makes the local structure more compact. A 24  shorter distance will further enhance the coupling of Cr3+ ions, thereby making the emission of Cr3+ pairs stronger. Electron paramagnetic resonance (EPR) is used to analyze the local coordination of Cr3+ and the quantity of Cr3+ pair. Fig. 7c shows the results obtained with SZO:8%Cr,yLa (y = 4%, 8% and 16%). It is seen that all the samples exhibited two main resonance signals, with the g-values of 1.983 and 4.801 attributable to Cr3+ pair and isolated Cr3+, respectively.46 The absence of other signals implies that Cr3+ ions do not occupy additional sites in the SZO structure. It is also seen from the figure that the signals of isolated Cr3+ and Cr3+ pair successively gain intensity with increasing La3+ incorporation, manifesting the crucial role La3+ in inhibiting Cr3+ oxidation and meanwhile promoting Cr3+ pair formation. Additionally, the similar linewidths of the g = 1.983 signals indicate that Cr3+ pairs occupy the same crystallographic sites in both the samples.47  Fig. 9. (a) Performance comparison of the SZO:8%Cr,16%La optimal sample with other reported NIR phosphors, with the reference given as the number in brackets. (b) Temperature dependent emission spectra of SZO:8%Cr,16%La under 455 nm excitation, with the inset showing normalized intensities of the 780 and 960 nm emissions as a function of the measurement temperature, and (c) ln(I0/IT − 1) vs. 1/(kT) relation and linear fitting. The SZO:8%Cr,16%La optimal phosphor was analyzed to have IQE, EQE and excitation absorption efficiency (AE) of 92.4%, 57.5% and 62.2%, respectively (Fig. S7). As compared in Fig. 9a, the IQE of our phosphor is among the best for various 25  Cr3+-doped NIR phosphors,10-16,24,48-50 especially those emitting longer than 850 nm. Such a highly efficient broadband NIR emission (700-1200 nm) is benefiting from the inhibited Cr3+ oxidation and promoted Cr3+ pair formation by La3+ doping. From the temperature-dependent PL spectra (Fig. 9b), it is seen that the luminescence of SZO:8%Cr,16%La was successively weakened by a higher temperature, but the 780 and 960 nm emissions retained ~73 and 70% of their room temperature intensities at 423 K, respectively (Fig. 9b, inset), indicating an excellent thermal stability for phosphors emitting beyond 900 nm.14-16 Noteworthy is that the different instruments for room temperature (Fluorolog-3) and varying temperature (FLS1000) measurements led to slightly different spectral shapes but no significant difference in the position and relative intensity of the main peaks. The activation energy (ΔE) of thermal quenching can be derived from the equation:51,52 IT = I01 + Aexp(- ΔEkT)(8) Where IT and I0 represent the emission intensities at temperature T and room temperature, respectively, A is a preexponential factor and K is the Boltzmann constant. Linear fitting of the ln[(I0/IT)−1] versus 1/T transformation of experimental data found ΔE to be ~0.275 eV (Fig. 9c). Generally, a larger ΔE implies that it is harder for electrons to reach the intersection point in the configurational coordinate diagram and thus a better thermal stability of the phosphor. The ΔE value of SZO:8%Cr,16%La is larger than those of Cr3+ emission in many other hosts, such as Mg0.8Zn0.2GaO4 (ΔE = 0.265 eV),24 NaScSiO6 (ΔE = 0.22 eV),15 LiInGe2O6 (ΔE = 0.253 eV),53 and Ga2GeO5 (ΔE = 0.254 eV).54 The good thermal stability makes our phosphor a promising candidate for applications demanding reliable performance in fluctuating thermal 26  environments. 3.3 Application of the SZO:8%Cr,16%La phosphor in UWOC UWOC (underwater wireless optical communication) transmits information by converting optical signals into electrical ones using photodiodes. The mainstream incident signal mostly adopts blue light because of its low loss in seawater.55 At the receiving end, however, blue light signal is not within the optimal response range (700-1000 nm) of the most commonly used Si-PIN photodetector.56,57 This low response level will seriously affect the strength of the converted electrical signal, thereby influencing the quality of signal transmission. As demonstrated earlier, the SZO:8%Cr,16%La phosphor of this work exhibited excellent performance for blue-to-NIR conversion: it can be efficiently excited by 455 nm blue light and then emit a broad NIR spectrum spanning 700-1200 nm, with a peak wavelength of ~960 nm and high IQE/EQE values of 92.4%/57.5%. More importantly, the PL spectrum of this phosphor strongly overlaps with the optimal response range of the Si-PIN photodetector (700-1000 nm, Fig. 10a).58 In view of these, we proposed in this work a device (referred to as “coated photodiode” Fig. 10b) for blue light signal conversion, which consists of an NIR fluorescent film (~0.4 mm thick) of the SZO:8%Cr,16%La phosphor, a visible light filter film and a Si-PIN photodiode. The fluorescent film can convert the incident blue light into NIR light to enhance the responsivity of Si-PIN, while the filter can block the original blue light passing through the film, as aforementioned in the Experimental Section, to prevent the two from overlapping and interference. To verify the feasibility of this design, we constructed a laboratory-level prototype UWOC system (Fig. 10c), which consists of a blue light laser (450 nm) as signal source, a water-filled glass tank (10 cm × 10 cm × 15 cm) for water environment, the coated photodiode as signal 27  receiving end, an oscilloscope for display, and a computer for data processing. In this system, ASCII codes (representing binary numbers) were employed for signal transmission, and an uncoated Si-PIN photodiode of the same type was tested in parallel to serve as a control. The signal was simulated by on/off (1/0) keying of the light source, and the transmitted signal was received by the coated and bare Si-PIN, respectively, and the resulting current waveform was displayed on an oscilloscope and recorded. Fig. 10d shows the waveforms obtained for letters “NEU”. It is seen that both the Si-PIN detectors successfully received light signals, but the average intensity of the coated one is approximately 1.58 times that of bare one. The results thus demonstrated that the phosphor of this work may have the potential for application in high performance UWOC systems.    Fig. 10. (a) Comparison of the spectral response of the Si-PIN photodiode with the PL spectra of blue LED source and SZO:8%Cr,16%La phosphor, (b) Schematic illustration of the blue light converter; (c) Schematic illustration of the experimental setup of the UWOC system; (d) Demonstration of transmitting “NEU” ASCII codes in the UWOC system. Conclusion 28  This study successfully prepared a series of Sr3Zr2O7(SZO):xCr,yLa (x = 0-10%, y = 0-20%) NIR phosphors, where the optimal x and y values are 8% and 16%, respectively. The SZO:8%Cr,16%La optimal phosphor emits NIR light in the wide range of ~700-1200 nm (peak wavelength ~960 nm) under 455 nm blue light excitation, with high internal/external quantum efficiencies (92.4%/57.5%) and good thermal stability (I423/I298 ~73%). The La3+ co-dopant may inhibit Cr3+ from oxidation and meanwhile attract the Cr3+ ions to aggregate around it to form pairs, thus playing a decisive role in the luminescence of Cr3+. The phosphor was also demonstrated to have the potential for application in underwater wireless optical communication (UWOC). Acknowledgements  This work was partially supported by the National Natural Science Foundation of China (Grant No. 52371057)  References  [1] X. Huang, S. Han, W. Huang, X. Liu, Enhancing solar cell efficiency: the search for luminescent materials as spectral converters, Chem. Soc. Rev. 2013, 42, 173. [2] Y. Xie, W. Liu, W. Deng, H. Wu, W. Wang, Y. Si, X. Zhan, C. Gao, X.-K. Chen, H. Wu, J. Peng, Y. Cao, Bright short-wavelength infrared organic light-emitting devices, Nat. Photonics 2022, 16, 752-761. [3] J. Ming, Y. Chen, H. Miao, Y. Fan, S. Wang, Z. Chen, Z. Guo, Z. Guo, L. Qi, X. Wang, B. Yun, P. Pei, H. He, H. Zhang, Y. Tang, D. Zhao, G. K.-L. Wong, J.-C. G. Bünzli, F. Zhang, High-brightness transition metal-sensitized lanthanide near-infrared luminescent nanoparticles, Nat. Photon. 2024, 18, 1254. 29  [4] Y. Wang, Z. Pan, S. Feng, L. Gao, X. Wang, Q. Zhu, and J.-G. Li, Ca2+/Si4+ Modification of the (Gd,Lu)AG Garnet for Enhanced Broadband Cr3+ Luminescence of High Thermal Stability, Inorg. Chem. 2024, 63, 24971-24983. [5] F. Zhu, Y. Gao, J. Qiu, Sr2AlTaO6: Ni2+ phosphors with excellent IQE and thermal stability as NIR-II source for night vision, nonvisual detection, and far-field imaging, Chem. Eng. J. 2025, 505, 159559. [6] G. Liu and Z. Xia, Modulation of Thermally Stable Photoluminescence in Cr3+-Based Near-Infrared Phosphors, J. Phys. Chem. Lett. 2022, 13, 5001-5008. [7] H. Zeng, T. Zhou, L. Wang, R.-J. Xie, Two-Site Occupation for Exploring Ultra-Broadband Near-Infrared Phosphor-Double-Perovskite La2MgZrO6:Cr3+, Chem. Mater. 2019, 31, 5245-5253. [8] F. Zhao, Z. Song, Q. Liu, Advances in Chromium-Activated Phosphors for Near-Infrared Light Sources, Laser Photonics Rev. 2022, 16, 2200380. [9] S. C. Lal, I. N. Jawahar, S. Ganesanpotti, Enhancing the inherent NIR photoluminescence in SrLaLiTeO6 through Cr3+-Yb3+ co-substitution for high performance pc-LEDs, Dalton Trans., 2024, 53, 1230–1244. [10] L. Zhong, Y. Xiang, S. Liu, Z. Chen, Q. Kong, Z. Bai, Y. Ren, F. Xie, C. Jiang, L. Zhou, J.-C. G. Bünzli, M. Wu, Valence and Site Engineering Enable Efficient Broadband Near-Infrared Emission at 960 nm in Cr3+-Activated Forsterite, Adv. Mater. 2025, 37, 2508768. [11] M.-H. Fang, K.-C. Chen, N. Majewska, T. Lesniewski, S. Mahlik, G. Leniec, S. M. Kaczmarek, C.-W. Yang, K.-M. Lu, H.-S. Sheu, R.-S. Liu, Hidden Structural Evolution 30  and Bond Valence Control in Near-Infrared Phosphors for Light-Emitting Diodes, ACS Energy Lett. 2021, 6, 109-114. [12] Z. Li, G. Zhu, S. Li, W. Xu, Q. Bian, Y. Cong, M. He, X. Luo, S. Xin, B. Dong, High-Performance NIR Emission in Chromium-Doped Garnet Phosphors Enabled by Structure and Excitation Regulation, Laser Photonics Rev. 2024, 18, 2300732. [13] Z. Jia, C. Yuan, Y. Liu, X.-J. Wang, P. Sun, L. Wang, H. Jiang, J. Jiang, Strategies to approach high performance in Cr3+-doped phosphors for high-power NIR-LED light sources, Light Sci. Appl. 2020, 9, 86. [14] L. Yao, Q. Shao, S. Han, C. Liang, J. He, J. Jiang, Enhancing Near-Infrared Photoluminescence Intensity and Spectral Properties in Yb3+ Codoped LiScP2O7:Cr3+, Chem. Mater. 2020, 32, 430-2439. [15] Y. Yan, M. Shang, S. Huang, Y. Wang, Y. Sun, P. Dang, J. Lin, Photoluminescence Properties of AScSi2O6:Cr3+ (A = Na and Li) Phosphors with High Efficiency and Thermal Stability for Near-Infrared Phosphor-Converted Light-Emitting Diode Light Sources, ACS Appl. Mater. Interfaces 2022, 14, 8179-8190. [16] T. Liu, H. Cai, N. Mao, Z. Song, Q. Liu, Efficient near-infrared pyroxene phosphor LiInGe2O6:Cr3+ for NIR spectroscopy application, J. Am. Ceram. Soc. 2021, 104, 4577-4584. [17] M. Huang, K. Chen, N. Majewska, M. Kamiński, G. Leniec, E. Mijowska, W. Pang, V. K. Peterson, D.-H. Cherng, K.-M. Lu, S. Mahlik, R.-S. Liu. Spinel-Type Structured Phosphor Near-Infrared-II Emission: Intervalence Charge Transfer and Hetero-Valent Chromium Pairs, Angew. Chem. Int. Ed. 2024, 63, e202412815. https://www.iikx.com/sci/physics/18780.htmlhttps://www.x-mol.com/paper/1384259150721081344?advhttps://www.x-mol.com/paper/1384259150721081344?advhttps://www.x-mol.com/paper/1384259150721081344?adv31  [18] Q. Pang, Y. Wang, L. Yan, G. Zhu, S. Xu, J. Zhang, X. Zhang, Y. Cao, B. Chen, Cr3+-Cr3+ Ion Pair Induced Fast Energy Migration in Cr3+ Doped Na-β-Al2O3 Ultra-Wide Near-Infrared Phosphors for NIR Spectroscopy Application, Laser Photonics Rev. 2024, 18, 2301039. [19] X. Chen, X. Huang, Highly Efficient and Thermally Stable NIR-Emitting Phosphor with Largely Tunable Peak Wavelength and Bandwidth Toward NIR Spectroscopy Applications, Laser Photonics Rev. 2025, 19, 2402226. [20] M. Szymczak, A. Antuzevics, P. Rodionovs, M. Runowski, U. R. R.-Mendoza, D. Szymanski, V. Kinzhybalo, L. Marciniak, Bifunctional Luminescent Thermometer-Manometer Based on Cr3+-Cr3+ Pair Emission, ACS Appl. Mater. Interfaces 2024, 16, 64976-64987. [21] S. Liu, J. Du1, Z. Song, C. Ma, Q. Liu, Intervalence charge transfer of Cr3+-Cr3+ aggregation for NIR-II luminescence, Light Sci. Appl. 2023, 12, 181. [22] H. Zhu, Y. Li, Y. Xi, C. Xin, C. Zhou, Z. Yang, L. Ruan, Y. Li, Y. Peng, M. S. Molokeev, A. Zolotov, J. Wang, Z. Zhou, M. Xia, Abnormal Lattice Shrinkage, Site Occupation, and Luminescent Properties of Cr3+-Activated β-Al2O3 Structure Phosphors, Laser Photonics Rev. 2024, 19, 2401089. [23] J. C. Gill, Spin-Lattice Relaxation of Pairs of Chromium Ions in Ruby, Nature 1961, 190, 619. [24] Ge. Chen, Y. Jin, L. Yuan, B. Wang, J. Huo, H. Suo, H. Wu, Y. Hu, F. Wang, Unlocking Cr3+-Cr3+ Coupling in Spinel: Ultrabroadband Near-Infrared Emission beyond 900 nm with High Efficiency and Thermal Stability, ACS Appl. Mater. 32  Interfaces 2024, 16, 30185-30195. [25] X. Zhang, L. Zhou, H. You, Stable and Highly Efficient Near-Infrared Emission Achieved in Spinel Blocks, Adv. Mater. 2025, 37, 2419897. [26] S. Yoshida, K. Fujita, H. Akamatsu, O. Hernandez, A.S. Gupta, F. G. Brown, H. Padmanabhan, A. S. Gibbs, T. Kuge, R. Tsuji, S. Murai, J. M. Rondinelli, V. Gopalan, K. Tanaka, Ferroelectric Sr3Zr2O7: Competition between Hybrid Improper Ferroelectric and Antiferroelectric Mechanisms, Adv. Funct. Mater. 2018, 28, 18101856. [27] Y. Xu, G. Abulipizi, Y. Wang, Y. Fang, Z. Yu, J. Zhou, Z. Li, Near-Infrared Persistent Luminescence of CaTiO3:Cr,Y for Imaging of Bone Implants Using Red-Light Illumination Instead of X‑ray, ACS Appl. Mater. Interfaces. 2024, 16, 55823-55831. [28] W. H. Baur, The geometry of polyhedral distortions. Predictive relationships for the phosphate group, Acta Cryst. 1974, B30, 1195. [29] Z. Wang, L. Xi, Y. Yang, Y. Li, X. Han, Y. Zuo, J. Wang, Spin-dependent Transport Properties of CrO2 Micro Rod, Nano-Micro Lett. 2014, 6, 365-371. [30] M. T. Buscaglia, V. Buscaglia, M. Viviani, P. Nanni, Influence of Foreign Ions on the Crystal Structure of BaTiO3, J. Eur. Ceram. Soc., 2000, 20, 1997-2007. [31] R.B. Comes, P.V. Sushko, S.M. Heald, R.J. Colby, M.E. Bowden, S.A. Chambers, Band-Gap Reduction and Dopant Interaction in Epitaxial La,Cr Co-doped SrTiO3 Thin Films, Chem. Mater. 2014, 26, 7073-7082. [32] X. Sun, X. Xu, Efficient photocatalytic hydrogen production over La/Rh co-doped Ruddlesden-Popper compound Sr2TiO4, Appl. Catal. B-Environ. 2017, 210, 149-159.  https://advanced.onlinelibrary.wiley.com/authored-by/Yoshida/Suguruhttps://advanced.onlinelibrary.wiley.com/authored-by/Fujita/Kojihttps://advanced.onlinelibrary.wiley.com/authored-by/Akamatsu/Hirofumihttps://advanced.onlinelibrary.wiley.com/authored-by/Hernandez/Olivierhttps://advanced.onlinelibrary.wiley.com/authored-by/Sen+Gupta/Arnabhttps://advanced.onlinelibrary.wiley.com/authored-by/Brown/Forrest+G.https://advanced.onlinelibrary.wiley.com/authored-by/Padmanabhan/Haricharanhttps://advanced.onlinelibrary.wiley.com/authored-by/Padmanabhan/Haricharanhttps://advanced.onlinelibrary.wiley.com/authored-by/Gibbs/Alexandra+S.https://advanced.onlinelibrary.wiley.com/authored-by/Kuge/Toshihirohttps://advanced.onlinelibrary.wiley.com/authored-by/Tsuji/Ryosukehttps://advanced.onlinelibrary.wiley.com/authored-by/Murai/Shunsukehttps://advanced.onlinelibrary.wiley.com/authored-by/Rondinelli/James+M.https://advanced.onlinelibrary.wiley.com/authored-by/Gopalan/Venkatramanhttps://advanced.onlinelibrary.wiley.com/authored-by/Tanaka/Katsuhisahttps://onlinelibrary.wiley.com/authored-by/Baur/W.+H.33  [33] H. Mizoguchi, P.M. Woodward, Electronic Structure Studies of Main Group Oxides Possessing Edge-Sharing Octahedra:  Implications for the Design of Transparent Conducting Oxides, Chem. Mater. 2004, 16, 5233-5248. [34] P. A. M. Berdowski, G. Blasse, Luminescence and energy migration in a two-dimensional system: NaEuTiO4, J. Lumin. 1984, 29, 243.  [35] L. Tu, X. Liu, F. Wu, H. Zhang, Excitation energy migration dynamics in upconversion nanomaterials, Chem. Soc. Rev. 2015, 44, 1331. [36] Y. Li, B. Yu, H. Wang, Y. Wang, Structural and optical characteristics of novel rare-earth-free red-emitting BaSn(PO4)2:Mn4+ phosphor, J. Mol. Struct. 2021, 1299, 129839. [37] L. Fang, L. Zhang, H. Wu, H. Wu, G. Pan, Z. Hao, F. Liu, J. Zhang, Efficient Broadband Near-Infrared CaMgGe2O6:Cr3+ Phosphor for pc-LED, Inorg. Chem. 2022, 61, 8815. [38] P. K. Varriam and S. Ganesanpotti, Harnessing the Dual-Mode Luminescence of Er/Yb Co-Doped SrLaLiTeO6 Double Perovskite Phosphors for Remarkably Wide Range Temperature Sensing and NIR pc-LEDs, Laser Photonics Rev. 2024, 18, 2400245. [39] X. Sun, Y. Xie, F. Wu, H. Chen, M. Lv, S. Ni, G. Liu, X. Xu, Photocatalytic Hydrogen Production over Chromium Doped Layered Perovskite Sr2TiO4, Inorg. Chem. 2015, 54, 7445-7453. [40] Y. Tanabe, S. Sugano, On the Absorption Spectra of Complex Ions II, J. Phys. Soc. Jpn. 1954, 9, 766-779. [41] G.C. Liu, M.S. Molokeev, Z.G. Xia, Structural Rigidity Control toward Cr3+-Based 34  Broadband Near-Infrared Luminescence with Enhanced Thermal Stability, Chem. Mater. 2022, 34, 1376-1384. [42] E. Song, M. Chen, Z. Chen, Y. Zhou, W. Zhou, H.-T. Sun, X. Yang, J. Gan, S. Ye, Q. Zhang, Mn2+-activated dual-wavelength emitting materials toward wearable optical fibre temperature sensor, Nat. Commun. 2022, 13, 2166. [43] P. Luo, D. Sun, Z. Lyu, M. You, Z. Lu, X. Zhang, L. Zhou, H. You, Achieving tunable ultra-broadband NIR emission originating from the two-site occupation of Cr3+ ions in Mg3Ga2SnO8:Cr3+, Inorg. Chem. Front. 2025, 12, 3663-3671. [44] A.P. Vink, M.A. de Bruin, S. Roke, P. S. Peijzel, A. Meijerink, Luminescence of Exchange Coupled Pairs of Transition Metal Ions, J. Electrochem. Soc. 2001,148, E313-E320. [45] Y. Zhou, Q. Cao, Y. Han, Z. Qiu, Jilin Zhang, W. Zhou, S. Lian, Achieving a Cr6+-free Cr3+-activated spinel phosphor by a one-step solid-state reaction, Inorg. Chem. Front., 2024, 11, 6127–6134. [46] V. Singh, R.P.S. Chakradhar, J.L. Rao, H.-Y. Kwak, EPR and photoluminescence properties of combustion-synthesized ZnAl2O4:Cr3+ phosphors, J. Mater. Sci. 2010, 46, 2331-2337. [47] A. A. Prokhorov, L. F. Chernush, T. N. Melnik, R. Minikayev, A. Mazur, V. Babin, M. Nikl, J. Lancok, A. D. Prokhorov, Optical and magnetic properties of the ground state of Cr3+ doping ions in REM3(BO3)4 single crystals, Sci. Rep. 2019, 9, 12787. [48] F. Zhu, Y. Gao, J. Qiu, High performance NIR-I to NIR-II emission of a Cr3+-doped Cs2NaLuCl6 phosphor with an IQE and EQE of up to 92.9% and 60.75%, Inorg. Chem. javascript:void(0);javascript:void(0);javascript:void(0);javascript:void(0);javascript:void(0);javascript:void(0);javascript:void(0);javascript:void(0);35  Front. 2024, 11, 7098-7109. [49] G.C. Liu, M.S. Molokeev, Z.G. Xia, Structural Rigidity Control toward Cr3+-Based Broadband Near-Infrared Luminescence with Enhanced Thermal Stability, Chem. Mater. 2022, 34, 1376−1384. [50] Y. Xiao, L. Han, Z. Xiao, J. Song, T. Li, J. Liu, T. Liu, D. Huang, W. You, X. Han, X. Sun, X. Ye, Near-Unity and Near-Zero-Thermal-Quenching Luminescent GAGG–Al2O3:Cr3+ Ceramic via Containerless Solidification and Glass Crystallization Methods for NIR Spectroscopy Application, Laser Photonics Rev. 2025, 19, 2500165. [51] Y. Zhou, C. Li, Y. Wang, Crystal-Field Engineering Control of an Ultraviolet–Visible-Responsive Near-Infrared-Emitting Phosphor and Its Applications in Plant Growth, Night Vision, and NIR Spectroscopy Detection, Adv. Opt. Mater. 2022, 10, 2102246. [52] L. Chen, S. Yu, Gu. Shen, S. Tang, T. Zhang, J.-G. Li, Q. Zhu, Large-scale irrigation of Cr3+ into different octahedra of zinc aluminate toward continual broadband near-infrared emission, Ceram. Int. 2024, 50, 1956-1969. [53] X. H. Chen, E. H. Song, Y. Y. Zhou, F. Q. He, J. Q. Yang, Q. Y. Zhang, Distorted octahedral site occupation-induced high-efficiency broadband near-infrared emission in LiScGe2O6:Cr3+ phosphor, J. Mater. Chem. C 2021, 9, 13640-13646. [54] C.-Y. Peng, B. Wang, L.-F. Yuan, K.-G. Hu, G. Chen, H.-Y. Wu, Y.-H. Hu, Y.-H. Jin, Six- and five-coordinated Cr3+ in Ga2GeO5 invokes tunable broadband near-infrared emission toward night-vision applications, Rare Met. 2023, 42, 3787-3796. [55] S. A. Sullivan, Experimental Study of the Absorption in Distilled Water, Artificial Sea Water, and Heavy Water in the Visible Region of the Spectrum, J. Opt. Soc. Am. 1963, 53, 962-968. [56] P. Buzhan, B. Dolgoshein, L. Filatov, A. Ilyin, V. Kantzerov, V. Kaplin, A. Karakash, F. Kayumov, S. Klemin, E. Popova, S. Smirnov, Silicon photomultiplier and its possible applications, Nucl. Instrum. Meth. A, 2003, 504, 48-52. 36  [57] S.J. Lange, T.O. Buchmann, M. Sebek, M.L. Welsch, E.J.R. Kelleher, N. Kawai, H. Takahashi, K. Katsuyama, P.U. Jepsen, Lightwave-Driven Long-Wavelength Photomultipliers, Laser Photonics Rev. 2024, 18, 2300417. [58] HAMAMATSU, SI-PIN photodiodes datasheet [EB].2023.