Influence of the spin-orbit coupling effect on the electronic and thermoelectric properties of Cs2MI6 (M = Zr, Hf) variant perovskites

Physical properties of Cs2MI6 (M = Zr, Hf) variant perovskites were investigated using full-potential linearized-augmented-plane-wave method to solve Kohn-Sham equation of density functional theory. Tolerance factors of 0.57 and 0.58 and positive phonon frequencies show cubic stability for Cs(2)Hfl(6) and Cs2ZrI6. The spin-orbit coupling (SOC) effect was taken into account while calculating electronic and thermoelectric properties. Cs(2)Hfl(6) was found with a direct band gap of 2.81 eV, this value decreases to 1.83 eV when SOC is applied. Where, Cs2ZrI6 showed an indirect band gap of 2.22 eV which turns out to be 1.66 eV with SOC effect. The d-states of M (Zr, Hf) were found responsible for down shift in the band gap when SOC is included. Furthermore, electronic conductivity, Seebeck coefficient and figure of merit were calculated with and without SOC. The excellent thermoelectric properties of Cs2MI6 pose these as promising materials for energy conversion.