Computational Exploration of Ultralow Lattice Thermal Conductivity and High Figure of Merit in p-Type Bulk RbX2Sb (X = K, Na)

Abstract

Using density functional theory and the Boltzmann transport equation, we have herein studied the electronic structure and thermoelectric behavior of two bulk bialkali antimonides RbK2Sb and RbNa2Sb. Our calculation reveals that both the antimonides exhibit ultralow lattice thermal conductivities resulting from the intrinsic phonon scattering. The lattice thermal conductivities of RbK2Sb and RbNa2Sb at 700 K (300 K) are found to be 0.150 (0.350) and 0.228 (0.532) W m–1 K–1, respectively. Thermodynamically, mechanically, dynamically, and thermally stable p-type RbK2Sb proclaims high values of the Seebeck coefficient due to the large band effective masses and thus the DOS effective masses of the holes, while p-type RbNa2Sb exhibits a moderate Seebeck coefficient and optimum electrical conductivity due to the relatively lower band effective mass and the high relaxation time (corresponding to high mobility) of the holes. Estimated maximum values of the figure of merit at 700 K (300 K) are 4.40 (1.91) and 5.84 (2.76) for the p-type RbK2Sb and RbNa2Sb, respectively.