Exploring Rb2YCuCl6 and Cs2YCuCl6 double perovskites: Structural, electronic, optical, elastic, and thermoelectric properties via density functional theory

Double perovskites have gathered momentous attention due to their structural, optoelectronics, and thermal properties. FP-LAPW + lo technique was employed along with the PBEsol-GGA and TB-mBJ potential. The optimized parameters and E-V parabolic curve were computed employing Birch Murnaghan's equation of state (BM-EOS). The negative formation energy E_f for Rb₂YCuCl₆ and Cs₂YCuCl₆ confirms the thermodynamic stability of these double perovskites. The mechanical stability of both compounds was confirmed by their elastic constants $C_{ij}$, Pugh's ratio, and anisotropy. The indirect band plots of Rb₂YCuCl₆ (2.57eV) and Cs₂YCuCl₆ (2.39eV) double perovskites confirm the semiconductor nature. The optical properties like dielectric function, reflectivity, optical conductivity, absorption coefficient, and energy loss function were determined within an energy range of up to 18 eV. The high absorption spectra for the under-study compounds 147.21 × 10⁴ cm⁻¹ at 14.60 eV for Rb₂YCuCl₆ and 261.30 × 10⁴ cm⁻¹ at 15.57 eV for Cs₂YCuCl₆ lies in the far UV region determines its potential use in the high-frequency devices. The thermoelectric properties such as the Seebeck coefficient, ZT, and power factor, are investigated using the semi-classical Boltzmann theory implemented in the BoltzTrap code. The peak value of ZT for Rb₂YCuCl₆ is 0.38 and for Cs₂YCuCl₆ is 0.33. The suggested findings indicate that Rb₂YCuCl₆ and Cs₂YCuCl₆ are potential candidates for thermoelectric and photovoltaic applications.