Novel combustion synthesis and spectroscopic investigation of YInO₃:Cr³⁺ polymorphs

Abstract

Identifying synthesis pathways for stabilizing metastable phases of materials and understanding the relationship between optical properties and local symmetry are important for advancing optical materials. In this work, we report a novel combustion synthesis route for two polymorphic structures of Cr³⁺-doped YInO₃ perovskite, namely the hexagonal and cubic (C-type) phases, along with their structural and optical properties. The use of tryptophan enabled the stabilization of the less explored metastable C-type YInO₃ compound. The structural differences between the polymorphs were identified using X-ray diffraction. Raman spectroscopy confirmed the hexagonal phase through characteristic vibrational bands with peaks at 379 and 611 cm⁻¹, while the predominant band in the C-type phase appears at 386 cm⁻¹. Scanning electron microscopy analysis revealed that the cubic YInO₃ polymorph exhibits a smaller grain size compared with the hexagonal phase. Due to the limited crystallographic data available for the cubic phase, its structure was refined using HighScore software, and the corresponding local symmetry parameters were analyzed. Novel excitation and photoluminescence data for YInO₃:Cr³⁺ were obtained in near-infrared (NIR) spectral regions. The Cr³⁺ NIR emission observed at 921 nm in the hexagonal phase is blue-shifted relative to the cubic phase, where the emission maximum occurs at 938 nm. This difference of 17 nm could be correlated with structural difference in local symmetry, such as polyhedral volume Δ_Cub-Hex 4.1926–5.1527 ų. Supplementary visible absorption/excitation, emission, and theoretical calculations of Cr³⁺ energy levels, particularly for the hexagonal phase, were done, and excitation/emission mapping under VUV to elucidate the contribution of potential defects is discussed.

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Introduction

The increasing demand for efficient and tunable optical materials has motivated extensive research on oxide-based phosphors and functional ceramic materials. Among these, metastable and low-symmetry crystalline phases have attracted particular interest due to their unique structure–property relationships, which often give rise to novel optical phenomena. In particular, metastable oxide phases with reduced or altered local symmetry represent a rapidly expanding class of functional materials, offering optical properties that cannot be accessed in their thermodynamically stable counterparts. In such systems, subtle changes in coordination geometry and crystal-field strength may drastically modify electronic transitions of optically active dopants, particularly transition-metal ions with strong electron–lattice coupling. Establishing reliable synthesis routes for such materials, along with a fundamental understanding of how crystal chemistry influences their optical response, remains a key challenge in materials science. Perovskite crystals constitute a large group of solids with various crystal structures, including cubic, orthorhombic, monoclinic and other space groups. Their general chemical formula can be written as ABO₃, where A and B can be either two trivalent cations or a divalent and a tetravalent cation, respectively. Such compounds can be easily doped with various transition metal and rare earth ions and can find numerous applications in lighting, sensing, etc.

Within oxide-based phosphors, a distinct subcategory is represented by metastable perovskite oxides. Their intrinsic structural characteristics often involve unusual atomic configurations and structurally integrated defects, including lattice distortions associated with local symmetry breaking (e.g., octahedral tilting, bond-angle variations, and bond-length asymmetry within the BO₆ framework), a high concentration of oxygen vacancies and interface-induced structural modifications. Metastable perovskite structures are characterized by non-equilibrium, high-energy states which, compared with thermodynamically stable phases, provide greater structural diversity and consequently enable tunable and optimized functional properties. The non-equilibrium nature of metastable systems requires synthesis through non-equilibrium pathways, typically involving controlled or energetically demanding processing conditions. Under such conditions, the metastable phase can become kinetically trapped, preventing transformation into the thermodynamically stable phase.¹ By carefully tailoring synthesis parameters, various synthesis techniques can be employed to prepare these materials. In particular, high-energy synthesis routes such as pulsed laser deposition, sputtering, high-pressure methods, and combustion synthesis are inherently suitable strategies, as the large amount of energy introduced into the system can stabilize metastable phases and make them energetically competitive with their equilibrium counterparts.^1–3

Within this framework, YInO₃ represents a versatile system, as it can be stabilized in crystallographically distinct hexagonal and cubic polymorphs while preserving an identical chemical composition. Under specific synthesis conditions, some of the perovskites could also adopt a metastable phase, where stability and structure of non-centrosymmetric hexagonal phases transform to a cubic C-type structure, as in the case of some RInO₃ (R, rare-earth elements), including HoInO₃ and YInO₃.^4,5 This polymorphism enables a direct comparison of optical properties governed primarily by differences in local coordination geometry and site symmetry. Several perovskites, such as RGaO₃ and RInO₃, can adopt either a high-symmetry P63/mmc phase or a low-symmetry P63cm phase, with minor differences in their XRD patterns. Despite this, doping strategies and dopants as probes allow the establishment of correlations between local symmetry and optical properties. These techniques, as in the case of photoluminescence, are sensitive enough and may provide valuable insights into the evolution of local structure environments.⁶

The luminescence properties of Cr³⁺ ion in various host materials and the influence of the crystal field on optical properties have been previously discussed in a number of publications.^7,8 Despite extensive studies on the Cr³⁺-activated luminescence in oxide hosts, most reports focus on thermodynamically stable crystal structures and excitation in the visible or near-ultraviolet range. The combined influence of metastable crystal phases, reduced or unusual coordination environments, and surface-localized Cr³⁺ centers, particularly under high-energy UV/VUV excitation, remains largely unexplored. Addressing this gap is essential for understanding how local symmetry and excitation pathways jointly govern luminescence mechanisms in Cr³⁺-doped oxides. Understanding the optical behavior of Cr³⁺ ion as a dopant in YInO₃ materials and mostly their hexagonal and other stable or metastable phases is essential, taking into consideration the versatility of structure for the emission and absorption properties and their correlations with local symmetry characteristics.^5,9 The emission characteristics of Cr³⁺ in an unusual five-fold coordinated local environment have been analyzed, but in a different oxide system.¹⁰ Under specific combustion synthesis conditions, the orthorhombic phase of YInO₃ was obtained and doped with Cr³⁺ ions.⁹

However, the derived structure of the cubic C-type YInO₃ polymorph may exhibit similarities in local symmetries with cubic Sc₂O₃ (or In₂O₃) oxides, which have also been studied as Cr³⁺ doped systems.^11,12 Other solid solutions of indates like YIn_1−xFe_xO₃, YMn_1−xIn_xO₃, Y_1−xGd_xInO₃, and Eu³⁺ doped YInO₃ with metastable C-type were partially studied.^5,13–16 It is worth noting that structural modifications in the YMn_1−xIn_xO₃ solid solutions, analyzed using Raman spectroscopy, have highlighted peaks in the 350–450 cm⁻¹ range, distinguishing the hexagonal and cubic phases¹⁷ Other perovskites investigations include experimental and density functional theory (DFT) studies on the electronic structures of LnInO₃ (Ln = La, Pr, Nd, Sm) or (Dy, Er, Ho).^18,19 Furthermore, the compounds XInO₃ (X – Rb, Cs, Fr) with a cubic structure, along with their electronic properties, have also been explored by first-principles DFT-based calculations.²⁰ DFT calculations have also been conducted to further understand some optoelectronic and thermoelectric properties and the cubic structure of YInO₃ or GaInO₃.²¹ Despite these extensive studies, the correlation between synthesis conditions, phase stabilization and the resulting local symmetry around Cr³⁺ ions in perovskite-derived systems remains an open area of investigation. In particular, exploring non-equilibrium synthesis approaches that enable access to metastable structures offers new opportunities to tailor optical properties. Therefore, a systematic examination of Cr³⁺ doped perovskite materials with controlled symmetry environments can provide valuable insights into structure–luminescence relationships and support the design of next generation optical and multifunctional materials.

Thus, in order to establish a direct correlation between crystal structure, local symmetry, and optical response, a set of complementary experimental techniques should be intentionally combined. X-ray diffraction provides phase identification and average crystallographic symmetry, while Raman spectroscopy probes local structural distortions and phase-related vibrational fingerprints. Scanning electron microscopy yields information on morphology and particle size, which is essential for understanding surface-related optical effects. Photoluminescence spectroscopy under visible excitation selectively probes bulk Cr³⁺ centers, whereas synchrotron-based UV/VUV excitation enables access to surface-dominated and host-mediated excitation pathways. Finally, crystal-field analysis within the Tanabe–Sugano framework allows quantitative interpretation of the experimental spectra in terms of local coordination and symmetry.

Therefore, in this work, we report an adapted and unique gel-combustion synthesis route that enables the selective stabilization of hexagonal and metastable cubic C-type YInO₃:Cr³⁺ polymorphs. The main objectives are: (i) to elucidate a novel combustion synthesis approach for obtaining the metastable C-type YInO₃:Cr³⁺ phase; (ii) to correlate differences in structure, local coordination symmetry, and optical properties of Cr³⁺ ions in both polymorphs; and (iii) to interpret, using data provided in the SI, the hexagonal phase visible-range spectroscopy, Cr³⁺ energy level calculations, and VUV excitation–emission mapping, analyzing contributions from potential defects or impurities.

Materials and methods

Hexagonal and cubic samples of YIn_1−xCr_xO₃ (x = 0.005) were prepared by a novel combustion method in air. The reagents used were: Y(NO₃)₃·4H₂O (Sigma Aldrich), metallic indium, HNO₃ (69%, ρ = 1,41 g cm⁻³, Merck KGaA), Cr(NO₃)₃·9H₂O (Sigma Aldrich), NH₃ (25%, Chimreactiv SRL), Leucine (AppliChem GmbH) and Tryptophan (AppliChem GmbH). The precursor solutions for both cubic and hexagonal samples were: indium nitrate with concentrations of 1 mol L⁻¹ for YInO₃, and 0.995 mol L⁻¹ for YIn_1−xCr_xO₃, respectively, yttrium nitrate of concentration 1 mol L⁻¹ and chromium nitrate of concentration 0.005 mol L⁻¹. Indium nitrate solutions were obtained by dissolving appropriate amounts of metallic indium in HNO₃. In order to obtain samples with hexagonal crystallographic symmetry, 300 µL of precursors were mixed in an alumina crucible and left in air for 1 hour for better diffusion. Subsequently, to each composition, 0.39 g of Leucine and 900 µL of NH₃ were added. The as prepared samples were heated in air at 1120 °C for 1 hour, with a heating rate of 5° per min, then naturally cooled to room temperature. In order to obtain samples with cubic crystallographic symmetry, 300 µL of precursors were mixed on a watch glass, and left in the air for 1 hour for better diffusion. Subsequently, to each composition, 300 µL of HNO₃ and 0.0876 g of Tryptophan were added. The gel obtained was placed on a hot plate and heated freely up to approximately 300 °C. As soon as the gel is dry, a blowtorch is used to initiate the combustion reaction. The resulting powders were ground and transferred to an alumina crucible. Finally, the samples were heated in air at 1250 °C for 1 hour, with a heating rate of 10° per min, then naturally cooled to room temperature.

The crystalline structure of the samples was examined by long-time X-ray diffraction using a PanAnalytical X’Pert Pro MPD diffractometer PW (Netherlands) with CuKα radiation, at room temperature. The Rietveld refinement was performed using a pseudo-Voigt profile function and the X’Pert HighScore Plus software (PANalytical).²² The Endeavour software by H. Putz and K. Brandenburg (Crystal Impact, Germany) was used for crystal structure solution based on XRD patterns, unit-cell parameters and chemical composition data.²³ The program employs a direct-space global optimization approach, using simulated annealing and Monte Carlo algorithms to explore possible atomic arrangements. The resulting structural models, which achieve an R-factor of up to 20%, are considered optimal for solutions obtained from powder diffraction data.

Raman spectroscopy measurements were carried out using a µRaman module (Nanonics Imaging Ltd., Israel) coupled with a Shamrock 500i spectrograph (ANDOR, UK). The measurements were taken at room temperature with a 50x objective, employing a 514.5 nm laser as the excitation source and an exposure time of 20 seconds. The room temperature photoluminescence measurements in the UV-VIS domain were performed using the FLS 980, Edinburgh Instruments spectrometer. A Xe lamp was used as the excitation source, and for a typical scan, the monochromator slits for both excitation and emission were set to 1 nm. The PMT Hamamatsu R928P detector was used. Photoluminescence experiments under ultraviolet (UV) and vacuum ultraviolet (VUV) excitation were carried out at the P66 beamline of the PETRA III storage ring at DESY (the beamline and the SUPERLUMI end station).^24,25 The excitation radiation at P66 is generated by a bending magnet source and is therefore relatively moderate, not inducing radiation-related defects in the studied samples. The corresponding photon flux is approximately two orders of magnitude lower than that at the undulator-based FinEstBeAMS beamline^26–28 of the MAX IV synchrotron, to avoid radiation damage due to the high excitation intensity.

Microstructural and compositional analyses were performed using a Helios 5 UX scanning electron microscope (Thermo Fisher Scientific, Netherlands). For image acquisition, the powder samples were prepared by lightly distributing the powders onto a conductive carbon tape and removing unbound particles with a controlled nitrogen stream. To acquire micrographs, the microscope was operated in immersion mode at 2 kV and 13 pA using a through-the-lens detector (TLD) and a short-dwell multi-frame acquisition scheme (50 ns dwell, 30-frame integration) to suppress charging in the insulating oxide powders. For quantitative energy-dispersive X-ray spectroscopy (EDX), the measurements were performed on 5 mm pellets pressed at 2 tons using a Specac Atlas 15T hydraulic press. The pellets were then coated with a thin carbon film using a Quorum Q150R ES Plus sputter coater (UK; 3 pulses, 35 A, 30 s intervals, 4 × 10⁻² Pa) and the EDX spectra were acquired at 20 kV with a 60 s live time. Quantitative analysis was performed using PathFinder 2.8 software (Thermo Fisher Scientific, USA). To calculate the Cr³⁺ energy levels in this work, we used a simplified approach, using equations derived from the Tanabe-Sugano matrices for the energy levels of the d³ electron configuration – E(⁴T_2g), E(⁴T_1g(⁴F)), and E(²E_g) of the ⁴T_2g, ⁴T_1g(⁴F), and ²E_g states.²⁹

Results and discussion

Structure analysis

The crystalline phase formation and structural evolution of YInO₃ are strongly influenced by synthesis parameters, particularly the choice of chelating agents in combustion synthesis. Variations in the combustion chemistry can modify reaction kinetics, local temperature profiles and cation diffusion, thereby stabilizing distinct crystal structures. To elucidate these effects, X-ray diffraction analysis was employed to examine the phase composition and structural characteristics of YInO₃ powders synthesized under different chelation conditions. It is worth emphasizing that the final powders were prepared using two distinct combustion synthesis routes, each employing a different amino acid—Leucine or Tryptophan. These compounds are regarded as novel fuels in combustion synthesis and only limited studies have documented their utilization, particularly that of Tryptophan.^30–33 In this study, the amino acids served a dual function, acting as chelating agents during the gel formation stage and as fuels in the combustion stage. Moreover, the combustion stage and the thermal treatment were carried out using different approaches.

The XRD patterns of the calcined powders obtained using leucine as a chelating agent (Fig. 1a and b) reveal the formation of the hexagonal crystalline YInO₃ phase, identified according to the ICDD reference 01-070-0122 (Fig. 1d), corresponding to the space group P63cm. Due to the highly exothermic and rapid reaction inherent to combustion synthesis, a minor secondary phase, of cubic C-type YInO₃ (indicated by red arrows), is also observed in all hexagonal samples. The hexagonal structure is characterized by long-range ordering of Y³⁺ and In³⁺ cations, which is favored under conditions of high local temperature and sufficient cation diffusion.

Fig. 1 X-ray diffraction patterns of hexagonal (a) and (b) and C-type YInO₃ (c), correlated with ICDD reference data (d).

In contrast, when tryptophan is used as the chelating agent, the powders exhibit the cubic C-type bixbyite structure of YInO₃, corresponding to ICDD reference 00-025-1172 and crystallizing in the Ia3̄ space group (Fig. 1d). This cubic structure, with composition (Y_0.5In_0.5)₂O₃, features random occupancy of Y³⁺ and In³⁺ cations across the two crystallographically distinct sites, resulting in complete cation disorder. The diffraction peak indicated by the black arrow may correspond to the (123)/(213) reflection characteristic of the cubic phase or traces of minor hexagonal YInO₃.

Unlike the hexagonal phase, which is thermodynamically stable at ambient conditions, the cubic C-type phase is metastable and typically forms under non-equilibrium conditions, converting to the hexagonal phase upon sufficient thermal treatment (∼1175–1250 °C), as reported in previous studies.³⁴

The observed phase selectivity can be rationalized by differences in the molecular structure, combustion behavior and coordination chemistry of the chelating agents. Leucine is a small aliphatic amino acid with relatively simple coordination ability and a thermal decomposition activation energy of approximately 160 kJ mol⁻¹.

Its combustion is rapid and highly exothermic (heat of combustion ≈27.3 kJ g⁻¹), generating higher local temperatures that favor cation ordering and the formation of the thermodynamically stable hexagonal YInO₃. In contrast, tryptophan contains a bulky aromatic indole group, exhibits a slightly higher heat of combustion (≈31.0 kJ g⁻¹) and has a significantly higher decomposition activation energy (≈195 kJ mol⁻¹). This higher energy barrier for decomposition, combined with the structural complexity of the indole ring, results in a distinct combustion kinetic profile characterized by enhanced gas evolution. The substantial variation in activation energy critically influences both the ignition characteristics and the combustion mode, thereby affecting the thermal evolution of the system. This environment suppresses long-range cation ordering and favors retention of the metastable cubic C-type phase. The indole ring further enhances coordination with metal cations, which increases steric hindrance and contributes to kinetic stabilization of the cubic structure. These findings highlight the microscopic role of chelating agents in regulating phase formation in YInO₃. The combination of thermochemical properties, molecular structure and combustion kinetics explains why leucine promotes the ordered hexagonal phase, whereas tryptophan stabilizes the metastable cubic C-type structure. Rapid nucleation, limited cation diffusion and enhanced gas evolution in tryptophan-assisted combustion collectively prevent the system from reaching the equilibrium hexagonal configuration, providing a mechanistic understanding of the experimentally observed phase selectivity.

Rietveld refinement has been performed using XDR HighScore software. The refinement pattern results for C-type YInO₃:Cr³⁺ are shown in Fig. 2 and for the hexagonal phase in SI Fig. S1. The determined structural parameters are listed in Table 1.

Fig. 2 HighScore Rietveld refinement of Cr³⁺-doped C-type YInO₃.

Table 1 Structural refinement results for YInO₃ polymorphs

Compound	Space group (No.)	a (Å)	b (Å)	c (Å)	V (10^6 pm^3)	α, γ angles
YInO3 hex	P63cm (185)	6.2716 (6)	6.2716 (6)	12.254 (1)	417.4258	90°
						120°
YInO3:Cr0.5% hex	P63cm (185)	6.273 (1)	6.273 (1)	12.256 (2)	417.7448	90°
						120°
YInO3:Cr0.5% cub	Ia3̄ (206)	10.3876 (5)	10.3876 (5)	10.3876 (5)	1120.833	90°
						90°

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The SEM measurements were performed to characterize the morphology of YInO₃:Cr³⁺ samples. Fig. 3 displays the surface morphology of all samples, along with EDAX measurements, which prove that within the error margins of the electron microscope, the anticipated compositions were obtained. In the case of cubic YInO₃:0.5% Cr, the use of the combustion synthesis with Tryptophan resulted in reduced crystallite size (around 200 nm), as the highly exothermic and short-duration reaction promotes rapid nucleation and limits grain growth.

Fig. 3 SEM images and EDAX results for YInO₃ hexagonal (a), Cr³⁺-doped YInO₃:0.5% hexagonal (b), and Cr³⁺-doped YInO₃:0.5% cubic (c) phases.

The combustion synthesis technique also promotes rapid gas evolution and fast reaction kinetics, resulting in a highly porous crystalline network with relatively large pore sizes. Moreover, it can be noticed that the C-type particles exhibit a spherical prolonged shape and a more uniform morphology.

The Raman measurements were performed on a hexagonal and cubic YInO₃ samples under 514 nm laser excitation. The registered peaks are listed in Table 2 and Fig. 4.

Table 2 Observed Raman peaks for hexagonal and C-type YInO₃ phases

Ref.	Raman shift (cm^−1)
This work hex	301	326	364	379	435	611
5	297	323	364	378	431	612
15	300	328	—	381	437	612
16	299	325	364	379	432	611
This work cub		386	461	484	580	610

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Fig. 4 Raman spectra of the synthesized YInO₃ phases: (a) hexagonal and (b) cubic (C-type).

A good agreement was obtained with published data on a similar hexagonal host reported by N. Arai and other references in Table 2. The observed peak at ∼611 cm⁻¹ could also be common for the cubic and the hexagonal phases, as confirmed in the reported data.⁵

Additionally, the peak at 402 cm⁻¹ is reported by Shukla et al., to be a component of the C-type phase, which in Fig. 3b can be seen as a component of a shoulder with a peak at 386 cm⁻¹.⁵

Local symmetry

The structural properties of the synthesized YInO₃ samples were analyzed and compared, showing the difference in local symmetry of the cationic sites. The resulting polyhedra of local symmetry of both phases are shown in Fig. 5. The substitution of Cr³⁺ during doping in hexagonal YInO₃ favors occupation of the In³⁺ 6c site with structural parameters shown in Table 3 (row 1). The following observations can be made: the hexagonal YInO₃ phase exhibits fivefold coordination at the 6c, In³⁺ site, in contrast, the cubic phase of YInO₃ has two six-coordinated sites, 24d and 8a, as shown in Table 3 (row 2). Based on the determined cubic structural model, obtained using the structure-solution tool Endeavour,²³ the In³⁺ cations occupy a single 24d site in the cubic phase, whereas the 8a site is occupied by Y³⁺ ions. The corresponding local polyhedral parameters are presented in Fig. 5 (b) and Table 3.

Fig. 5 Local coordination environments in YInO₃, (a) hexagonal phase showing InO₅ and two YO₇ polyhedra, and (b) cubic (C-type) phase showing InO₆ and YO₆ polyhedra.

Table 3 Site symmetries of hexagonal and cubic YInO₃ phases, along with those of related cubic oxides

Nr.	Compound	Space group number	Site symbol (Wickoff)	CN	Avg. bond length (Å)	Min. bond length (Å)	Max. bond length (Å)	Polyhedral volume (Å^3)
a Data from materials project crystallographic files repository. CN stays for coordination number.
1	YInO3 (hex), Y0 site	185	4b	7	2.3149	2.07858	2.62456	18.7113
	YInO3 (hex), Y1 site	185	2a	7	2.3719	2.16980	2.62281	19.6016
	YInO3 (hex), In1 site	185	6c	5	2.1608	1.89730	2.33779	8.6172
2	YInO3 (cub), In0 site	206	24d	6	2.1692	2.16925	2.16925	12.8098
	YInO3 (cub), Y0 site	206	8a	6	2.2623	2.16698	2.37320	13.7699
3	In2O3 (cub), In0 site^a	206	24d	6	2.1941	2.14047	2.23125	12.7408
	In2O3 (cub), In1 site^a	206	8b	6	2.1859	2.18590	2.18590	13.3142
4	Y2O3 (cub), Y0 site^a	206	24d	6	2.2838	2.26972	2.24709	14.2879
	Y2O3 (cub), Y1 site^a	206	8a	6	2.2841	2.28413	2.28413	15.0069
5	Sc2O3(cub), Sc0 site^a	206	24d	6	2.1260	2.08750	2.16696	11.6201
	Sc2O3 (cub), Sc1 site^a	206	8a	6	2.1234	2.12344	2.12344	12.2046

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A comparison of the In³⁺ local environments in the cubic (24d site) and hexagonal (6c site) phases shows an increase in coordination number from 5 to 6, which results also in the increase of 0.0084 Å in the average In–O bond length and expands the polyhedral volume with 4.1926 ų for the C-type 24d site.

However, in the C-type structure, of A₂O₃ type, such as In₂O₃, dopants as Sn⁴⁺ prefers the 8b site.³⁵ This could depend on A cation but also on a dopant like in Sc₂O₃:Cr³⁺ prefer mostly C_3i symmetry (8a) but in Y₂O₃ host the Cr³⁺ is mentioned to occupy the C₂ site (24d).^11,12,36

These crystallographic differences directly determine the local coordination and symmetry of the cation sites, which are correlated with optical properties in the following section.

Spectroscopic consideration

Diffuse reflectance spectra of both hexagonal and cubic YInO₃ phases were measured and converted using the Kubelka–Munk function to visualize the UV absorption edge of the samples (Fig. 6a). The optical bandgap for the hexagonal phase was determined to be E_g = 3.63 eV, while for the cubic phase it was E_g = 3.63 eV. Both values were calculated using Tauc plots, as shown in the inset of Fig. 6a.

Fig. 6 (a) Kubelka–Munk absorption spectra and the corresponding bandgap determination (inset) for Cr³⁺-doped hexagonal and cubic YInO₃ phases. (b) Photoluminescence excitation in the UV region and Cr³⁺ NIR emission of the samples.

The use of synchrotron-based UV/VUV radiation enables a substantial extension of the luminescence excitation energy range, beyond the bandgap. Spectroscopy with tunable VUV excitation allows a direct comparison of impurity-centre luminescence under both direct intracentre excitation and excitation mediated by energy transfer from the intrinsic electronic states of the host lattice, including electron–hole pairs, excitons, and defects. This approach is particularly important for wide-bandgap materials,^37–47 including perovskites.^48,49

Fig. 6b presents the photoluminescence excitation and emission results obtained for YInO₃:Cr³⁺(0.5%) with hexagonal and cubic crystal structures. Under UV/VUV excitation, all doped samples exhibited broad luminescence bands in the near-infrared spectral range. It can be observed that the emission at 921 nm in the hexagonal phase is blue shifted compared with the cubic phase 938 nm emission, being attributed to Cr³⁺ dopant emission. Comparing with cubic Sc₂O₃:Cr, the emission in NIR at 870 nm is substantially blue shifted, possibly due to Cr³⁺ preference mostly of C_3i symmetry (8a) in this host.¹¹

Because the absorption coefficient of YInO₃ above 3.6 eV is extremely high, the penetration depth becomes very small, meaning that the excitation is localized almost entirely within the near-surface region of the particles. The Cr³⁺ ions at or near the surface experience reduced coordination, lower crystal-field strength, and enhanced structural disorder compared to bulk Cr³⁺ sites.

According to the Tanabe–Sugano diagram, these surface environments correspond to a deeper weak-field regime in which the ⁴T₂ level lies below or very close to ²E. In this case, relaxation into the ²E state becomes highly inefficient, and the population remains in the vibronically active ⁴T₂ manifold. Radiative decay ⁴T₂ →⁴A₂ therefore becomes dominant and produces the broad, featureless NIR band centered at 900–950 nm, as seen in Fig. 6b.

Importantly, the observed emission band at 920–940 nm falls well within the characteristic NIR range of Cr³⁺ broadband emission (⁴T₂ →⁴A₂) in weak crystal fields. In contrast, Cr⁴⁺ centers in oxide hosts are known to exhibit broadband emission predominantly in the longer-wavelength NIR region (typically above 1100 nm and often peaking near 1200 nm), accompanied by distinct excitation and absorption features in the visible–NIR range.^50,51 The absence of any pronounced emission beyond 1100 nm, as well as the lack of characteristic Cr⁴⁺-related excitation signatures, makes a significant contribution from Cr⁴⁺ centers unlikely.

Supplementary aspects related to visible absorption/excitation, emission, and theoretical calculations of Cr³⁺ energy levels, particularly for the hexagonal phase, are detailed in the (SI), Chapter 2. Additionally, excitation/emission mapping under VUV excitation and the contribution of potential defects are discussed in Chapter 3.

Differences in the structure of Cr-doped YInO₃ between the hexagonal and cubic phases influence the local symmetry, including variations in polyhedral volume (_Cub-Hex 4.1926–5.1527 ų) and consequently lead to differences in optical properties, such as an emission maximum shift of approximately 17 nm, as shown in Table 4.

Table 4 Correlation between structural characteristics and optical properties

Nr.	Compound	Space group number	Site symbol (Wickoff)	CN	Avg. bond (Å)	Polyh. volume (Å^3)	Bandgap (eV)	PLE (nm)	PL (nm)
1	YInO3 (hex), In1 site	185	6c	5	2.1608	8.6172	3.63	315	921
2	YInO3 (cub), In0 site	206	24d	6	2.1692	12.8098	3.68	293	938
	YInO3 (cub), Y0 site		8a		2.2623	13.7699

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Conclusions

In this study, we successfully synthesized and investigated the optical properties of Cr³⁺-doped YInO₃ perovskite powders. A novel combustion method was developed to obtain both the cubic and hexagonal phases. A gel-combustion synthesis employing tryptophan enabled the stabilization of the less explored metastable C-type YInO₃ phase. The study demonstrates that the choice of chelating agent governs the phase selectivity in YInO₃, with leucine favoring the thermodynamically stable hexagonal structure and tryptophan stabilizing the metastable cubic C-type phase through differences in combustion kinetics, thermal decomposition and cation mobility. Both structural polymorphs were confirmed by XRD. Raman spectroscopy confirmed the hexagonal phase through characteristic vibrational bands with peaks at 379 and 611 cm⁻¹, while in C-type prevail band has a 386 cm⁻¹ peak. SEM imaging indicated that the cubic YInO₃ phase exhibits a smaller grain size than the hexagonal one. Owing to the limited crystallographic data available for the cubic phase its structure was refined and further analyzed in terms of local symmetry and coordination environments. Importantly, near-infrared photoluminescence showed a clear dependence on crystal structure: the emission maximum at 921 nm for the hexagonal phase is blue-shifted relative to 938 nm for the cubic phase. This ∼17 nm shift is correlated with differences in local symmetry and polyhedral volume _Cub-Hex 4.1926–5.1527 ų, highlighting the strong relationship between structural characteristics and optical properties. Supplementary visible absorption/excitation and emission measurements, along with theoretical calculations of Cr³⁺ energy levels, especially for the hexagonal phase were conducted. Furthermore, excitation/emission mapping under VUV irradiation was performed to clarify the role of potential defects. These findings contribute to a deeper understanding of the optical properties and structure–property relationships of Cr³⁺-doped YInO₃ polymorphs and demonstrate the potential of combustion synthesis for obtaining metastable phases, providing valuable insights for their application in optoelectronic materials.

Author contributions

R. A. B: methodology, investigation, writing – original draft, writing – review & editing, resources. A. I. B: investigation, writing – review & editing, formal analysis, resources. M. I: formal analysis, writing – review & editing, resources. A. V. R: conceptualization, investigation, writing – original draft, writing – review & editing. M. B: writing – review & editing investigation, formal analysis. M. G. B: conceptualization, investigation, writing – original draft, writing – review & editing, validation. Y. S.: investigation, formal analysis, writing – review & editing. E. V: formal analysis, investigation. E. A. K: investigation, formal analysis, resources. V. P: methodology, conceptualization, investigation, writing – original draft, writing – review & editing.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Visible absorption/excitation, emission, and theoretical calculations of Cr³⁺ energy levels, particularly for the hexagonal phase, as well as excitation/emission mapping under VUV to elucidate the contribution of potential defects, are presented in the supplementary information (SI). See DOI: https://doi.org/10.1039/d6ma00420b.

Acknowledgements

M. G. B. thanks the National Science Center (NCN) Poland under project no. 2023/49/B/ST5/03384; Ministry of Science, Technological Development, and Innovation of the Republic of Serbia under contract 451-03-47/2023-01/200017; Program for the Foreign Experts (Grant No. W2017011) offered by Chongqing University of Posts and Telecommunications; The National Foreign Experts Program for “Belt and Road Initiative” Innovative Talent Exchange (Grant No. DL2021035001L); Estonian Research Council grant PRG2031; E. A. K and V. P acknowledges LZP grant 2023/1-0063 for the support; Nucleu Program within the National Research Development and Innovation Plan 2022–2027, with the support of MCID, project no PN 23 27 02 01, contract no. 29N/2023; A. V. R, M. I., M. G. B. acknowledge the support of Romania's National Recovery and Resilience Plan – NRRP (PNRR), Project C9-I8-C28, and Contract 760107/2023; We acknowledge DESY (Hamburg, Germany), a member of the Helmholtz Association HGF, for the provision of experimental facilities. This research was carried out at P66 beamline at PETRA III and we would like to thank Aleksei Kotlov for his assistance in using of SUPERLUMI setup. Beamtime was allocated for proposal I-20250639 EC

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