Rhombohedral Ba2NbBO6 (B = As, Sb, and Bi) Double Perovskites’ Elastic, Thermodynamic, and Directional Thermoelectric Properties

We systematically analyzed the elastic, thermodynamic, and directional thermoelectric properties of a set of double perovskites, specifically Ba₂NbAsO₆, Ba₂NbSbO₆, and Ba₂NbBiO₆. We investigated the dependence of volume, bulk modulus, thermal expansion coefficient, Debye temperature, entropy, isobaric and isochoric heat capacities, and lattice thermal conductivity on temperature and pressure using the quasi-harmonic Debye model. We analyzed the thermoelectric parameters, including the Seebeck coefficient, thermal and electrical conductivity of holes and electrons, figure of merit, and power factor, in the xx, yy, and zz directions, through the quasi-classical Boltzmann model. The findings demonstrate a figure of merit exceeding 0.8 across a wide range of charge carrier concentrations and a Seebeck coefficient greater than 0.9 mV/K, making Ba₂NbAsO₆, Ba₂NbBiO₆, and Ba₂NbSbO₆ compounds promising candidates for heat-to-electricity conversion applications. We assessed the mechanical stability and characteristics, including bulk modulus, shear modulus, Young’s modulus, Poisson’s ratio, Debye temperature, and the velocities of longitudinal, transverse, and average sound propagation, derived from the single-crystal elastic constants C_ij, which were evaluated numerically using the strain-stress technique.