AB INITIO STUDY OF STRUCTURAL AND OPTOELECTRONIC PROPERTIES OF LEAD-FREE DOUBLE PEROVSKITES RB2BCUF6 (B = SC, Y)

Abstract

This study investigates the double perovskite family Rb₂BCuF₆ (B = Sc, Y) using density functional theory (DFT) within the CASTEP framework. The compounds crystallize in a cubic structure (space group Fm-3m) and were systematically evaluated for structural stability mechanical behavior electronic structure and optical response using the GGA-PBE functional. Stability criteria including Goldschmidt tolerance factor values of 0.92 for Rb₂ScCuF₆ and 0.91 for Rb₂YCuF₆, along with negative formation energies confirm thermodynamic stability for both compositions. Mechanical property analysis using elastic constants and derived moduli values composition-dependent behavior where Rb₂ScCuF₆ exhibits a bulk modulus of 34.26 GPa, a shear modulus of 5.33 GPa, and a Young's modulus of 15.21 GPa, whereas Rb₂YCuF₆ shows a bulk modulus of 22.80 GPa, a shear modulus of 8.58 GPa, and a Young's modulus of 22.88 GPa. Electronic-structure calculations indicate that both materials are semiconductors with direct band gaps of 1.43 eV (Rb₂ScCuF₆) and 0.91 eV (Rb₂YCuF₆), Provide them suitable for optoelectronic applications. Optical properties, including dielectric functions, refractive indices absorption coefficients and reflectivity are characterized across the 0-40 eV energy range to assess the material's potential for optoelectronic applications. Thermoelectric properties are also evaluated. Collectively this study establishes Rb₂BCuF₆ (B = Sc, Y) as promising, composition-tunable materials for photovoltaic applications and thermoelectric technologies based on theoretical predictions.