The Anharmonic Origin of the Giant Thermal Expansion of NaBr

TL;DR

This study reveals that anharmonic effects, especially cubic anharmonicity, are the primary cause of the giant thermal expansion in NaBr, and simple quasiharmonic models fail to predict its behavior accurately.

Contribution

The paper demonstrates that including anharmonicity in ab initio calculations is essential to accurately predict phonon behavior and thermal expansion in NaBr, challenging the adequacy of the quasiharmonic approximation.

Findings

Phonons in NaBr show large shifts and broadenings due to anharmonicity.

Quasiharmonic approximation fails to predict temperature dependence of phonons and thermal expansion.

Anharmonicity from cubic interactions dominates the thermal behavior of NaBr.

Abstract

All phonons in a single crystal of NaBr were measured by inelastic neutron scattering at temperatures of 10, 300 and 700 K. Even at 300 K the phonons, especially the longitudinal-optical (LO) phonons, showed large shifts in frequencies, and showed large broadenings in energy owing to anharmonicity. Ab initio computations were first performed with the quasiharmonic approximation (QHA), in which the phonon frequencies depend only on $V$, and on $T$ only insofar as it alters $V$ by thermal expansion. This QHA was an unqualified failure for predicting the temperature dependence of phonon frequencies, even 300 K, and the thermal expansion was in error by a factor of four. Ab initio computations that included both anharmonicity and quasiharmonicity successfully predicted both the temperature dependence of phonons and the large thermal expansion of NaBr. The frequencies of LO phonon modes decrease significantly with temperature owing to the real part of the phonon self-energy from explicit anhamonicity, originating from the cubic anharmonicity of nearest-neighbor Na-Br bonds. Anharmonicity is not a correction to the QHA predictions of thermal expansion and thermal phonon shifts, but dominates the behavior.