<i>Ab Initio</i> Study of the Physical Properties of Cs-Based Double Perovskites Cs2AX6 (A = Ge, Mn; X = Cl, I)

Device applications in magnetic media, spintronics, oxygen membranes, sensors, etc., are some of the uses of ferrite materials. In this work, we have studied the structural, electronic, magnetic, mechanical, and thermoelectric properties of Cs-based double perovskites Cs2AX6 (A = Ge, Mn; X = Cl, I), using Quantum Espresso with generalized gradient approximation Perdew–Burke–Ernzerhof and Perdew–Burke–Ernzerhof in solids exchange–correlation functionals. The band structure results show that Cs2GeCl6 and Cs2MnCl6 are semiconductors with direct band gaps. However, there are bands crossing observed for Cs2GeI6 from the conduction band minimum to the valence band maximum, indicating the metallic nature of the material. Moreover, Cs2MnI6 has magnetic properties; it exhibits a metallic nature in the spin-up state and a semiconductor nature in the spin-down state, suggesting that it can be used in spintronics applications. The calculated total magnetic moment of Cs2MnCl6 is 3.0µB (for both Perdew–Burke–Ernzerhof and Perdew–Burke–Ernzerhof in solids), while for Cs2MnI6, the calculated total magnetic moments are 3.02µB and 3.06µB, for Perdew–Burke–Ernzerhof and Perdew–Burke–Ernzerhof in solids exchange–correlation functionals, respectively. The results of the mechanical properties calculations show that Cs-based double perovskites Cs2AX6 (A = Ge, Mn; X = Cl, I) are mechanically stable. Cauchy's pressure and Poisson's, Frantsevich's, and Pugh's ratios of the studied materials confirm that Cs2MnCl6 is brittle, while the remaining studied double perovskite materials are ductile. Electrical conductivity, thermal conductivity, Seebeck coefficients, power factor, and figure of merit are the thermoelectric parameters analyzed in this study. Seebeck coefficients, electrical conductivity, and power factor increase with the rise in temperature, and Cs2MnX6 (X = Cl, I) double perovskite materials have higher values of electrical conductivity than Cs2GeX6 (X = Cl, I). All the studied materials have positive type conductivity due to their positive Seebeck coefficient values.