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

Layered compounds have several advantages, with respect to high-performance thermoelectric properties. In this study, we focused on the d 0 -electron layered complex nitrides AMN 2 (A = Na, Cu; M = Ta, Nb), with α-NaFeO 2 -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 AMN 2 compounds, which were compared with that of the three-dimensional (3D) d 0 -electron perovskite oxide KTaO 3 . All the AMN 2 materials were semiconductors with energy band gaps of 0.6–2.2 eV, and KTaO 3 exhibited a 3D electronic structure and isotropic thermoelectric transport properties. In contrast, the α-NaFeO 2 -type Na(Ta,Nb)N 2 ( 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 NaTaN 2 are potentially superior to those of KTaO 3 . The delafossite-type Cu(Ta,Nb)N 2, which have the potential to be p -type semiconductors, show unusual thermoelectric transport properties due to their valence band structures. Thermoelectric transport properties of Cu(Ta,Nb)N 2, calculated using generalized gradient approximation–Perdew–Burke–Ernzerhof functional potential (GGA-PBE), show higher in-plane and lower out-of-plane electronic conductivities. On the other hand, modified Becke–Johnson (mBJ)-calculated electronic transport properties show no anisotropies, and Seebeck coefficients are almost unaffected by temperature.