Thermal properties and ultra-low thermal conductivity of Zn₂GeSe₄

Material-related discovery continues to drive advancements in technologically significant fields of interest. Moreover, an understanding of the thermal properties of materials is essential for any application of interest. Here we report on the structural, optical and thermal properties of Zn₂GeSe₄, which forms in the sphalerite crystal structure with an indirect 1.85 eV band gap, as obtained from our optical spectroscopy measurements. Analyses and modeling of the temperature-dependent thermal properties reveal a low speed of sound and Debye temperature, with ultra-low thermal conductivity over a large temperature range due to low-frequency soft modes as well as strong lattice anharmonicity. Our findings are presented and discussed in the context of the current intense interest in metal chalcogenides and will facilitate the development of these and similar materials for applications where low thermal conductivity is of interest.