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

Abstract

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 3.0µ_B (for both Perdew–Burke–Ernzerhof and Perdew–Burke–Ernzerhof in solids), while for Cs₂MnI₆, the calculated total magnetic moments are 3.02µ_B and 3.06µ_B, 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.