Phonon anharmonicity and negative thermal expansion in SnSe

TL;DR

This study combines experiments and simulations to explore phonon anharmonicity in SnSe, revealing its role in negative thermal expansion and providing detailed insights into phonon thermodynamics and electronic structure effects.

Contribution

It provides a comprehensive analysis of phonon anharmonicity in SnSe, linking experimental measurements with first-principles simulations to explain negative thermal expansion.

Findings

Strong temperature-dependent phonon spectrum shifts and broadening

Reproduction of negative thermal expansion through anisotropic strain modeling

Quantification of harmonic, dilational, and anharmonic phonon contributions

Abstract

The anharmonic phonon properties of SnSe in the Pnma phase were investigated with a combination of experiments and first-principles simulations. Using inelastic neutron scattering (INS) and nuclear resonant inelastic X-ray scattering (NRIXS), we have measured the phonon dispersions and density of states (DOS) and their temperature dependence, which revealed a strong, inhomogeneous shift and broadening of the spectrum on warming. First-principles simulations were performed to rationalize these measurements, and to explain the previously reported anisotropic thermal expansion, in particular the negative thermal expansion within the Sn-Se bilayers. Including the anisotropic strain dependence of the phonon free energy, in addition to the electronic ground state energy, is essential to reproduce the negative thermal expansion. From the phonon DOS obtained with INS and additional calorimetry measurements, we quantify the harmonic, dilational, and anharmonic components of the phonon entropy, heat capacity, and free energy. The origin of the anharmonic phonon thermodynamics is linked to the electronic structure.