Phonon Softening and Elastic Instabilities in the Cubic-to-Orthorhombic Structural Transition of CsH

TL;DR

This study uses ab initio calculations to analyze the pressure-induced cubic-to-orthorhombic transition in CsH, revealing phonon softening and elastic instabilities linked to electronic structure changes.

Contribution

It predicts the transition pressure and elucidates the connection between phonon softening, elastic instabilities, and electronic structure modifications in CsH.

Findings

Transition occurs at about 17 GPa

Transition is related to phonon softening and elastic instabilities

Electronic structure changes drive the transition

Abstract

The cubic-to-orthorhombic structural transition occurring in CsH at a pressure of about 17 GPa is studied by ab initio calculations. The relative stability of the competing structures and the transition pressure are correctly predicted. We show that this pressure-induced first-order transition is intimately related to a displacive second-order transition which would occur upon application of a shear strain to the (110) planes. The resulting instability is rationalized in terms of the pressure-induced modifications of the electronic structure.