First-principles study of the structural, mechanical, dynamical, and transport properties of Cs2NaInX6[X = Br, I] for thermoelectric applications

Thermoelectricity generation techniques utilized for conversion of waste heat into electric power are currently gaining prominence. It is necessary that for improved efficiency, the materials used for such applications should have a high thermoelectric figure of merit (ZT) and appropriate stability. In this study, the properties of Cs2NaInX6 double perovskites (DPs) were examined using a first-principles approach to ascertain their suitability as thermoelectric materials. The findings revealed that the materials are mechanically stable according to the Born stability criteria for cubic crystals. They were also observed to be anisotropic given isotropic values of 0.702 and 0.662 for Cs2NaInBr6 and Cs2NaInI6 DPs respectively. Phonon calculations indicate soft modes at the Brillouin zone boundary and center (F) for Cs2NaInI6 DP, which suggests ferroelectric and anti-ferroelectric distortions. Similarly for Cs2NaInBr6 DP, the soft modes were exclusively found at the BZ center (F), suggesting ferroelectric distortion. The transport coefficient properties calculated are used for the determination of the ZT values. The calculated values of ZT at ambient temperature are of the order 1.0 for all the DPs which is high enough for thermoelectric applications suggesting that they may be potential thermoelectric materials