First principle investigation of pressure-dependent structural stability of double perovskite Cs2GeMnM6 (M = Cl, Br, I) at elevated temperatures

Double perovskites (DPs) are widely used in a range of commercial and industrial fields due to their exceptional structural stability in thermoelectric devices. To address this performance, we explored the structural stability of Cs₂GeMnM₆ (M = Cl, Br, I) DPs compound under different pressure and temperature conditions through first-principles simulations. The structure, mechanical and optical properties were optimized by using the PBE-GGA potential technique, and the thermal stability was evaluated using the Gibbs2 package implemented in the WIEN2K code. The computed results regarding ground state energy and bulk modulus demonstrate that the Cs₂GeMnCl₆ DP compound structure exhibits greater stability compared to the Cs₂GeMnBr₆ and Cs₂GeMnI₆ DPs, however thermodynamic analyses involving Gibbs free energy and enthalpy suggest that the Cs₂GeMnI₆ DPs compound possess enhanced structural stability under specific pressure and temperature conditions. The results reveal that Cs₂GeMnCl₆ DP compound exhibit higher stability based on electronic behavior, while Cs₂GeMnI₆ DP compound display greater stability in terms of vibrational motion, and making them more appropriate for thermoelectric applications under elevated temperature and pressure conditions.