Strong anharmonicity in tin monosulfide evidenced by local distortion, high-energy optical phonons, and anharmonic potential

Recently, SnSe has been found to possess a remarkable thermoelectric performance. As an isostructural compound to SnSe, SnS contains a more environmentally compatible and earth abundant element, which enables SnS a promising thermoelectric material for commercial application. In this work, we have performed systematic studies of the crystal structures and the lattice vibrations by means of neutron total scattering, Raman scattering measurements, and first-principle calculations. A structural transition has been observed by both temperature dependent neutron diffraction and pressure dependent Raman scattering. The lattice anharmonicity has been revealed by the local distortion induced by the Sn $5{s}^{2}$ lone pair and is also evidenced from the temperature dependent atomic displacement parameter of the Sn atom. By separating the intrinsic anharmonicity and lattice thermal expansion effects, we found that the former plays the primary role in the softening of the Raman active modes. Such anharmonicity has also been observed experimentally by the linewidth broadening of the high-energy optical modes and confirmed by frozen phonon calculations. Furthermore, our frozen phonon calculation reveals the presence of the quartic anharmonicity potential of those high-energy optical modes vibrating along the $b$ axis. Our data will contribute to a better understanding of the thermal conduction in SnS, which will be beneficial for the enhancement of the thermoelectric performance of this material.