Low-temperature anharmonicity and the thermal conductivity of cesium iodide

Cesium halide has a simple cubic crystal structure and hosts low thermal conductivity, but its microscopic mechanism has not been fully understood. In the present work, we took cesium iodide (CsI) single crystal as an example, to investigate the lattice dynamics and thermal conductivity by performing inelastic neutron scattering (INS), heat transport measurements, and first-principles calculations. The temperature dependent phonon dispersions of CsI were obtained from INS and the low temperature anharmonicity of transverse optic (o) and transverse acoustic (a) phonon modes in CsI was observed. By performing the thermal conductivity measurement and first-principles calculations, it is shown that the low thermal conductivity of CsI originates from the combined effect of the small phonon group velocities and the large phonon scattering rates, which is dominated by the (a,a,a) and (a,a,o) phonon scattering processes. This work highlights the importance of phonon anharmonicity in lattice dynamics, which sheds light on the design of materials with low thermal conductivity.