First-principles study of anisotropic thermoelectric properties of hexagonal KBaBi

Abstract

Fast-track computational prediction of efficient thermoelectric materials is crucial for designing future thermoelectric generators. In the present study, an attempt has been made to estimate the thermoelectric figure of merit (ZT) of hexagonal KBaBi, a topological insulator by employing first-principles calculations based on Density Functional Theory (DFT) and Boltzmann transport theory. KBaBi, a member of ternary honeycomb compounds has been chosen due to its high band valley degeneracy and hence high density of states effective mass which may lead to possible high-power factor (S2σ). Therefore, to estimate the ZT, properties such as mechanical, phononic, electronic, and transport phenomena of KBaBi have been evaluated. It is observed that KBaBi exhibits low average sound velocity (va) and Debye temperature (θD) resulting from soft acoustic phonon modes. The study reveals larger Seebeck coefficients than the minimum requirement of 200 ​μV/K along a-axis (Sa) and c-axis (Sc) for n and p-type KBaBi respectively, over a broad range of temperature and carrier concentration. Moreover, the calculated relaxation time (τ) based on deformation potential theory suggests the higher p-type conductivity (σ) along c-axis. It also unveils reasonably low lattice thermal conductivities (klat) along a (2.09 Wm−1K−1) and c-axes (2.99 Wm−1K−1) at room temperature. Finally, we report a promising ZT of 1.04 for p-type KBaBi along c-axis despite having strong bipolar effect.