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 D . For both crystallographic orientations, thermal diffusivity remains above the recently identified Planckian limit ( D > vs2 τ P , where vs is the sound velocity and τ P = ℏ / k B T ) and its anisotropy in D is set by the anisotropy of vs. 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 vs, τ P and the 'melting' velocity derived from the melting temperature.