Ab Initio Study of Lead-Free Double Halide Perovskite X₂GeSnCl₆ (X = Na, K) Compounds for Energy Conversion System

In this article, the physical properties of Ge-based lead-free halide double perovskite compounds X₂GeSnCl₆ (X = Na, K) are studied in the framework of density functional theory by using the Wien2k code. Compounds show stability with negative values of ground state energy and formation energy. The band structure in electronic properties exhibits the semiconducting nature with 2.24 eV and 2.21 eV direct band gaps by using a modified Becke Johnson approximation which gives clear results of the band gap. On the other hand, electronic charge density exhibits the covalent band of Cl with Ge and Sn while the ionic bond between Cl and (Na, K). Optical conductivity is high and maximum output in the visible region of the solar energy spectrum along with maximum absorbance for both compounds while reflectivity is lower in the visible region which makes the compounds suitable for solar cell and opto-electronic applications. Using BoltzTraP classical theory in the thermoelectric property valuable results are observed with higher ZT values of K₂GeSnCl₆ with 0.99 which makes it a good candidate for thermoelectric applications. Both compounds are mechanically and dynamically stable with brittle nature; also covalent bonding nature is confirmed by Cauchy pressure with negative values.