Impact of Halide Ion Occupancy on Thermodynamic, Mechanical, Electro-optic, and Electron Transport Characteristics of Rb₂CuAsX₆ (X = F, Cl, Br) Double Perovskites Using Density Functional Theory

This work utilizes density functional theory (DFT) to analyze the structural, thermodynamic, mechanical, electro-optic, and electron transport characteristics of Rb₂CuAsX₆ (X = F, Cl, Br) perovskites. The impact of occupancy of different halide ions at the X-site on the thermodynamic, mechanical, optical, and thermoelectric response of studied compounds has also been evaluated. The investigation of the elastic parameters and formation energy has confirmed that the examined perovskites are cubic in structure, stable, and ductile. The thermodynamic characteristics that rely on temperature are estimated using the quasi-harmonic Debye approach. The thermal features such as entropy, heat capacity, and Debye temperature are calculated and analyzed to assess the stability at elevated temperatures and the suitability of compounds for industrial applications. The band structure computations identified Rb₂CuAsF_6, Rb₂CuAsCl_6, and Rb₂CuAsBr₆ are p-type semiconductors with indirect band gaps of 1.25, 1.10, and 0.83 eV, respectively, which decreases while substituting F with Cl and Br. The optical characteristics such as strong optical absorption (> 10⁵ cm⁻¹) and minimal dispersion in the visible and ultraviolet spectrum highlight their suitability for solar energy conversion and optoelectronics. Additionally, thermoelectric characteristics have been determined, showing a higher thermoelectric figure of merit (ZT) value of 0.80, 0.79, and 0.78, respectively, at room temperature. Our research anticipates these perovskite combinations exhibit superior photoelectric and thermoelectric efficiency, rendering them potential prospects for advanced photovoltaic and thermoelectric devices.