Theoretical Study of the Transport Properties of 〖Cs〗_2 NaBiX_6 [X=Br,I] Double Perovskite and its Stability for Thermoelectric Applications

Perovskite materials are very useful in photovoltaic applications, and their application can also be extended to thermoelectric devices. Double perovskites have lately attracted interest due to research into their characteristics and prospective applications in thermoelectric devices. Double Perovskites (DPs) are considered in this study to cross-examine their transport, mechanical, and vibrational properties from an ab-initio method to determine their potential for thermoelectric application. The transport coefficients evaluated in this study include the Seebeck coefficient, electrical conductivity, thermal conductivity, power factor, and the thermoelectric figure of merit. The properties are calculated as a function of chemical potential at a temperature range of 300 K to 800 K. The findings show a high figure of merit of a near unity at all temperature ranges. The mechanical properties suggest that the materials satisfy the Born stability criteria for cubic crystals and are thus stable mechanically. The vibrational properties through phonon dispersion curves suggest the materials lack dynamical stability as a result of imaginary frequencies present at the Brillouin zone boundary and center () of the DP materials. The materials can, therefore, be useful for thermoelectric applications, allowing for the operation of their devices even at high temperatures.