Ab initio study on anisotropic thermoelectric transport in ternary pnictide KZnP

Strongly anisotropic bands near the Fermi level via band structure engineering have been proposed to enhance thermoelectric performance in functional materials. Recent works exhibit the presence of flat-and-dispersive-band-like strong anisotropy in a class of ternary transition metal pnictides. Taking KZnP as a representative example, here we investigate the thermoelectric properties of this class of materials based on first-principles calculations and semiclassical Boltzmann transport theory. Strikingly, the calculated lattice thermal conductivity k_L shows a strong anisotropy with a small value of about 5.24 (2.58) W m⁻¹K⁻¹ along the in-plane (out-of-plane) lattice direction at room temperature. Based on the electron relaxation time calculated from intensive ab initio electron-phonon interactions, a thermoelectric figure of merit zT of 0.32 is predicted for n-type doped KZnP, about two times lower than the value estimated by the constant relaxation time approximation from deformation potential theory. Nanostructures with the characteristic length shorter than 13 nm can reduce the k_L by 40%, enhancing zT to 0.52 along the c axis direction. This work supports that KZnP is a potential candidate for thermoelectric applications.