Thermal diffusivity and its lower bound in orthorhombic SnSe

The orthorhombic monochalcogenide SnSe has attracted much attention in recent years as a promising high-temperature thermoelectric material. We present a study of its thermal conductivity and specific heat of SnSe between 2 K and 300 K and quantify its anisotropic thermal diffusivity . For both crystallographic orientations, thermal diffusivity remains above the recently identified Planckian limit (, where is the sound velocity and ) and its anisotropy in is set by the anisotropy of . Comparison with cubic members of the IV-VI family leads to a consistent picture, where the diffusivity in all members of the family is set by the product of , and the ‘melting’ velocity derived from the melting temperature.