Investigating Anomalous Thermal Expansion of Copper Halides by Inelastic Neutron Scattering and Ab-inito Phonon Calculations

TL;DR

This study combines neutron scattering and ab-initio calculations to understand the lattice dynamics and negative thermal expansion in copper halides, revealing phonon softening as a key factor.

Contribution

It provides a detailed comparative analysis of phonon behavior and thermal expansion in CuX compounds, linking soft phonon modes to negative thermal expansion.

Findings

Identification of low energy phonons responsible for NTE

Good agreement between calculations and experimental data

High-pressure calculations relate NTE differences to unit cell volume

Abstract

We investigate detailed lattice dynamics of copper halides CuX (X=Cl, Br, I) using neutron inelastic scattering measurements and ab-initio calculations aimed at a comparative study of their thermal expansion behavior. We identify the low energy phonons which soften with pressure and are responsible for negative thermal expansion. The eigenvector analysis of these modes suggests that softening of the transverse-acoustic modes would lead to NTE in these compounds. The calculations are in very good agreement with our measurements of phonon spectra and thermal expansion behavior as reported in the literature. Our calculations at high pressure further reveal that large difference in negative thermal expansion behavior in these compounds is associated with the difference of the unit cell volume.