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

Using density functional theory and the Boltzmann transport equation, we have herein studied the electronic structure and thermoelectric behavior of two bulk bialkali antimonides RbK 2 Sb and RbNa 2 Sb. Our calculation reveals that both the antimonides exhibit ultralow lattice thermal conductivities resulting from the intrinsic phonon scattering. The lattice thermal conductivities of RbK 2 Sb and RbNa 2 Sb 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 RbK 2 Sb 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 RbNa 2 Sb 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 RbK 2 Sb and RbNa 2 Sb, respectively.