Ultralow thermal conductivity and negative thermal expansion of CuSCN

Copper thiocyanate (CuSCN) has recently received considerable attention because of its high hole mobility and applications in solar cells [Science 358(2017)768]. In this work, by performing state-of-the-art theoretical calculations, for the first time we find that the thermal conductivities of both alpha- and beta-CuSCN are ultralow with the values of 1.2 and 2.4 W/mK at room temperature, respectively. Based on detailed analyses of the phonon dispersion, Gruneisen parameters, three phonon scattering rates and atomic displacement parameters, we further demonstrate that the underlying reasons for the ultralow thermal conductivities are due to the avoided crossing between the longitudinal acoustic (LA) phonons and the low-lying optical branches as well as the weak bonding and strong anharmonicity. The low lattice thermal conductivities lead to high ZT values of 1.7 and 2.1 at 800 K for alpha- and beta-CuSCN, respectively. In addition, both materials exhibit large negative thermal expansion (NTE) coefficients originated from the transverse vibrations in Cu-N-C-S chains. These features endow CuSCN with the potential for thermal barrier coating and thermal devices going beyond the reported photovoltaic applications.