Phonon dispersion curves and thermodynamic properties of \alpha-Pu2O3

TL;DR

This study uses computational methods to analyze phonon dispersion and thermodynamic properties of lpha-Pu2O3, revealing weaker Pu-O bonds, a vibrational frequency gap, and temperature-dependent phase stability.

Contribution

It provides the first detailed phonon dispersion curves and thermodynamic analysis of lpha-Pu2O3 using density functional +U methods, extending previous work on PuO2.

Findings

Weaker Pu-O bonding in lpha-Pu2O3 compared to PuO2.

Presence of a vibrational frequency gap between oxygen and plutonium.

Thermodynamic stability of lpha-Pu2O3 depends on temperature and oxygen availability.

Abstract

A recent inelastic x-ray scattering study [Manley et al., Phys. Rev. B 85, 132301 (2012)] reveals that the phonon dispersion curves of PuO2 is considerably consistent with our previous density functional +$U$ results [Zhang et al., Phys. Rev. B 82, 144110 (2010)]. Here in the present work, using the same computational methods, we further obtain the phonon dispersion curves for \alpha-Pu2O3. We find that the Pu-O bonding is weaker in \alpha-Pu2O3 than in fluorite PuO2, and subsequently a frequency gap appears between the vibrations of oxygen and plutonium atoms. Based on the phonon dispersion curves and Helmholtz free energies of PuO2 and \alpha-Pu2O3, we systematically calculate the reaction energies for the transformations between Pu, PuO2, and \alpha-Pu2O3. It is revealed that the thermodynamic equilibrium of the system is dependent on temperature as well as on the chemical environment. High temperature and insufficient oxygen environment are in favor of the formation of \alpha-Pu2O3.