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.