Phonon anharmonicity in single-crystalline SnSe

Understanding the temperature-dependent behavior of phonons that affects the thermal transport properties is crucial for developing efficient thermoelectric materials. Previously, SnSe was reported to exhibit a record high thermoelectric performance above 900 K, which was debated in subsequent reports. Nonetheless, there is a growing consensus that anharmonicity is one of the contributing factors which yields ultralow thermal conductivity in SnSe. While there is experimental and theoretical evidence for anharmonicity in SnSe, its effect on the entire phonon band structure, which is responsible for the thermal transport, is investigated here. We performed a combined temperature-dependent polarized Raman spectroscopic and heat-capacity study on fully dense single-crystalline SnSe, which revealed that the anharmonicity is driven by soft optical modes in the b-c plane. These modes exhibited strong high-temperature broadening, indicating ultrashort phonon lifetimes and high scattering rates. Analysis of the Raman peak frequencies and linewidths revealed phonon decay to be dominated by a three-phonon scattering process. Consistent with this observation, the analysis of our temperature-dependent heat-capacity data also revealed the presence of strong anharmonicity in SnSe. The anharmonic coefficients alpha(R) and alpha(C) calculated from the Raman and heat-capacity measurements, respectively, are in excellent agreement with each other. This study provides a deeper understanding of the role of phonon-phonon scattering and anharmonicity leading to outstanding thermal transport properties of SnSe.