Phonon Dispersion Relation, High-Pressure Phase Stability and Thermal Expansion in YVO4

TL;DR

This study investigates the phonon dispersion, phase stability under high pressure, and thermal expansion properties of YVO4 using neutron scattering experiments and theoretical calculations, revealing instability at high pressures and anisotropic thermal behavior.

Contribution

It provides the first detailed phonon dispersion relation for YVO4 and combines experimental and theoretical approaches to analyze its high-pressure phase stability and thermal expansion.

Findings

Phonon dispersion relation measured up to 65 meV.

Unstable phonon modes at high pressure indicating potential phase instability.

Large anisotropy in thermal expansion due to elastic and mode Grüneisen parameters.

Abstract

The orthovanadates are useful as host matrices for immobilization of radioactive wastes. The thermodynamic stability of these materials is crucial for their applications in high pressure and temperatures environment. It is necessary to investigate the phonons in the entire Brillouin zone, beyond the zone-centre phonons accessible in previous Raman and infrared experiments. We have carried out extensive neutron inelastic scattering experiments to derive the phonon dispersion relation of YVO4 up to high energy transfer of 65 meV using a single crystal, which are perhaps reported for the first time in any orthovanadate compound. The measured phonon dispersion relation is in good agreement with our first principles density functional theory as well as shell model calculations. The calculated pressure dependence of phonon modes in the zircon and scheelite phases shows unstable modes and violation of the Born stability criteria at high pressure, which may be lead to instability in YVO4 at high pressures. We also calculate large anisotropy in the thermal expansion behavior which arises from difference in anisotropic elasticity and mode Gr\"uneisen parameters.