Conserved axis-dependent conduction polarity in NaSnAs polycrystalline bulk sample for transverse thermoelectric application

Abstract

Layered semiconductor NaSnAs has a distinct property of large anisotropy in effective mass, resulting in axis-dependent conduction polarity. This property of NaSnAs is useful for transverse thermoelectric (TE) cooling and thermal energy harvesting with a single TE leg. In this study, we used a Sn flux method followed by hot pressing to synthesize NaSnAs polycrystalline bulk samples. Seebeck coefficient measurements revealed n-type polarity along in-plane and p-type polarity along cross-plane directions over a temperature range of 300–520 K. This finding indicates that axis-dependent conduction polarity is conserved in polycrystalline NaSnAs bulk samples with a preferred orientation. A significantly low lattice thermal conductivity of 0.8 W/mK at 520 K was obtained because of phonon scattering caused by strong anharmonicity in the lattice vibration. First-principles calculation suggests that axis-dependent conduction polarity occurs where the Fermi level lies within the bandgap. Furthermore, we predict that high-TE performance can be achieved by optimizing the carrier concentration because of their characteristic pudding mold type band structure, which leads to the coexistence of a high density of states and group velocity.