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 (v_m) and strong phonon anharmonicity. By utilizing the crystal orbital Hamilton population (COHP) analysis, we attribute low v_m 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.