Mechanism of the low thermal conductivity in novel two-dimensional NaCuSe

Low lattice thermal conductivity is crucial to obtain an excellent thermoelectric figure of merit (ZT) in thermoelectric (TE) materials. Herein, we study the phonon transport properties of two-dimensional (2D) NaCuSe using first-principles calculations. NaCuSe has an intrinsically low lattice thermal conductivity, 2.46 W/mK at 300 K, which originates from its low mean sound velocity (vm) and strong phonon anharmonicity. By utilizing the crystal orbital Hamilton population (COHP) analysis, we attribute low vm to the filling of anti-bonding orbitals between Cu-3d and Se-4p states, giving rise to the weak chemical bonds. Also, this research investigates the scattering processes (the out-of-plan acoustic mode (ZA) + optical mode(O) -> O(ZA + O -> O), the in-plane transverse acoustic mode (TA) + O -> O(TA + O -> O), and the in-plane longitudinal acoustic mode (LA) + O -> O(LA + O -> O). The results demonstrate that NaCuSe is of strong phonon anharmonicity, which could guide to discover and design of new TE materials.