Anisotropic Anomalies of Thermoelectric Transport Properties and Electronic Structures in Layered Complex Nitrides AMN2 (A = Na, Cu; M = Ta, Nb)

Abstract

Layered compounds have several advantages, with respect to high-performance thermoelectric properties. In this study, we focused on the d0-electron layered complex nitrides AMN2 (A = Na, Cu; M = Ta, Nb), with α-NaFeO2-type and delafossite-type crystal structures. Density functional theory and Boltzmann theory calculations were performed to determine the electronic structures and electronic transport coefficients of the AMN2 compounds, which were compared with that of the three-dimensional (3D) d0-electron perovskite oxide KTaO3. All the AMN2 materials were semiconductors with energy band gaps of 0.6–2.2 eV, and KTaO3 exhibited a 3D electronic structure and isotropic thermoelectric transport properties. In contrast, the α-NaFeO2-type Na(Ta,Nb)N2 (n-type semiconductors) exhibited anisotropic thermoelectric transport properties, and the electronic structures of their conduction bands appeared to be 3D. The thermoelectric transport properties of NaTaN2 are potentially superior to those of KTaO3. The delafossi...