Lattice anharmonicity effects in fluorite oxide single crystals and anomalous increase in phonon lifetime in ceria at elevated temperature

TL;DR

This study explores how lattice anharmonicity affects phonon lifetimes in fluorite oxide crystals, revealing an unexpected increase in phonon lifetime in ceria at high temperatures due to phonon dispersion changes.

Contribution

It demonstrates that anharmonic phonon interactions can cause anomalous phonon lifetime behavior in ceria, highlighting the importance of phonon renormalization effects at elevated temperatures.

Findings

Ceria shows an anomalous linewidth reduction between 1023-1123 K.

First-principles calculations do not fully explain the linewidth anomaly.

Phonon dispersion changes reduce scattering phase space, increasing phonon lifetime.

Abstract

We investigate the temperature dependence of the frequency and linewidth of the triply-degenerate T$_{2g}$ zone-centered optical phonon in flux-grown ceria and hydrothermally-synthesized thoria single crystals from room temperature to 1273 K using Raman spectroscopy. Both crystals exhibit an expected increase in the phonon linewidth with temperature due to enhanced phonon-phonon scattering. However, ceria displays an anomalous linewidth reduction in the temperature range of 1023-1123 K. First-principles phonon linewidth calculations considering cubic and quartic phonon interactions within temperature-independent phonon dispersion fail to describe this anomaly. A parameterization of the temperature-dependent second order interatomic force constants based on previously reported phonon dispersion measured at room and high temperatures, predicts a deviation from the monotonic linewidth increase, albeit at temperatures lower than those observed experimentally for ceria. The qualitative agreement in the trend of temperature-dependent linewidth suggests that lattice anharmonicity-induced phonon renormalization plays a role in phonon lifetime. Specifically, a change in the overlap between softened acoustic and optical branches in the dispersion curve reduces the available phonon scattering phase space of the Raman active mode at the zone center, leading to an increased phonon lifetime within a narrow temperature interval. These findings provide new insights into higher-order anharmonic interactions in ceria and thoria, motivating further investigations into the role of anharmonicity-induced phonon renormalization on phonon lifetimes at high temperatures.