Thermal diffusivity and its lower bound in orthorhombic SnSe

Abstract

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 ($D>v_s^2{\tau }_P$, where $v_s$ is the sound velocity and ${\tau }_P=\hslash /k_{B}T$) and its anisotropy in $D$ is set by the anisotropy of $v_s$. 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 v${}_s$, ${\tau }_P$ and the 'melting' velocity derived from the melting temperature.