Directional dependence of thermoelectric properties in Na2AgAs: Contributions of in-plane rattling and multiple energy valleys

Abstract

The thermal transport and thermoelectric properties of Na2AgAs are rigorously explored through first-principles calculations, incorporating phonon self-consistent theory and Boltzmann transport equation. The impact of anharmonic phonon renormalization on κL is comprehensively analyzed, alongside the contributions of three-phonon (3ph) and four-phonon (4ph) scattering mechanisms. It is particularly significant that Na2AgAs exhibits an in-plane rattling vibration mode, which markedly suppresses its κL. Crystallographic anisotropy in bond-length distributions and different bond energies along distinct directions primarily leads to this rattling behavior. Additionally, the valence band of Na2AgAs features multiple energy valleys, which enhances its power factor and optimizes its electronic transport properties. The coexistence of intrinsically low κL and outstanding charge carrier transport makes Na2AgAs a promising p-type thermoelectric material with efficient heat-to-electricity conversion capacity at elevated temperatures. A peak ZT value of 2.6 is achieved under p-type doping at a carrier concentration of 1×1020cm−3 and a temperature of 800 K along the c axis.