Phonon dynamics and anharmonicity in ScF3 and SnTe

Chen Li (Oak Ridge National Laboratory)

Knowledge on phonon anharmonicity is critical to understand thermal expansion and lattice thermal transport properties in many materials. Inelastic neutron scattering experiments were performed with time-of-flight and triple-axis spectrometers to study the temperature dependence of the lattice dynamics in the two systems. ScF3 has a large negative thermal expansion over a wide range of temperature. The measured phonon densities of states of ScF3 show a large anharmonic contribution. Phonon calculations with first-principles methods show that some of the modes with motions of F atoms transverse to their bond direction behave as quantum quartic oscillators, which account for phonon stiffening with temperature and a significant part of the negative thermal expansion. In SnTe, giant phonon anharmonicity is believed to contribute to the low thermal conductivity. Phonon dispersions and linewidths in single-crystalline SnTe measured at a series of temperatures were compared with phonon lifetime and molecular dynamics simulations with first-principles methods to identify the anomalies in phonon modes. The anharmonic coupling is likely to provide a key insight on understanding the surprisingly low thermal conductivity of the rocksalt tellurides in general.

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