Phonon spectrum, thermodynamic properties, and pressure-temperature phase diagram of uranium dioxide

Abstract

We present a study of the structural phase transition, mechanical and thermodynamic properties of UO${}_2$ by means of the local density approximation (LDA)$+U$ approach. A phase transition pressure of 40 GPa, which agrees well with the experimental value of 42 GPa, is obtained from theory at 0 K. Pressure-induced enhancements of elastic constants, elastic moduli, elastic wave velosities, and Debye temperature of the fluorite phase are observed. Phonon spectrums of both the ground state fluorite structure and high pressure cotunnite structure calculated by the supercell approach show that the cotunnite structure is dynamically unstable under ambient pressure. Based on the imaginary mode along the $\Gamma$$-$$X$ direction and soft phonon mode along the $\Gamma$$-$$Z$ direction, a transition path from cotunnite to fluorite has been identified. We calculate the lattice vibrational energy in the quasiharmonic approximation using both first-principles phonon density of state and the Debye model. Calculated temperature dependence of lattice parameter, entropy, and specific heat agree well with experimental observations in the low temperature domain. The difference of Gibbs free energy between the two phases of UO${}_2$ has predicted a boundary in the pressure-temperature phase diagram. The solid-liquid boundary is approximated by an empirical equation using our calculated elastic constants.

11 pages, 10 figures

References in corpus (6)

• Ground state properties and high pressure behavior of plutonium dioxide: Systematic density functional calculations
• Electronic Correlation and Transport Properties of Nuclear Fuel Materials
• Phonon spectrum, thermal expansion and heat capacity of UO${}_2$ from first-principles
• Structural behavior of uranium dioxide under pressure by LSDA+U calculations
• Crystal field and magnetic structure of UO${}_2$
• First-principles study on oxidation effects in uranium oxides and high-pressure high-temperature behavior of point defects in uranium dioxide

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• Avoiding critical-point phonon instabilities in two-dimensional materials: The origin of the stripe formation in epitaxial silicene
• On the temperature and density dependence of dislocation drag from phonon wind
• Doubly screened Coulomb correction approach for strongly correlated systems
• Phonon thermal transport in UO${}_2$ via self-consistent perturbation theory
• Ideal strengths and bonding properties of PuO2 under tension