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

Abstract

We present a study of the structural phase transition and the mechanical and thermodynamic properties of UO2 by means of the local density approximation (LDA)+U approach. A phase transition pressure of 40 GPa is obtained from theory at 0 K, and agrees well with the experimental value of 42 GPa. The pressure-induced enhancements of elastic constants, elastic moduli, elastic wave velocities, and Debye temperature of the ground-state fluorite phase are predicted. The phonon spectra 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 Γ−X direction and soft phonon mode along the Γ−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. The calculated temperature dependence of lattice parameter, entropy, and specific heat agrees well with experimental observations in the low temperature domain. The difference of the Gibbs free energy between the two phases of UO2 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.

Phonon density of states and anharmonicity ofUO2

“…While recent first-principles simulations for UO 2 [3][4][5] have reported good agreement with experimental elastic constants, bulk moduli, and lattice heat capacity measurements, a reduced level of agreement was found for phonon dispersion and phonon density of states (PDOS) simulations compared to the experimental inelastic neutron scattering (INS) results of Dolling et al [6]. Moreover, Pang et al [2] reported results similar to other ab initio phonon dispersion simulations [2][3][4][5], but differences of more than a factor of two between ab initio simulations and INS measurements of phonon linewidths (i.e., inverse lifetimes) for UO 2 [2], especially at high temperature (1200 K).…”

“…Counterintuitively, however, the major source of the discrepancy in the phonon linewidth simulations was identified [2] to lie within the phonon energies and dispersion. This suggests that the presently available ab initio simulations [3][4][5] of the second-order interatomic forces are not sufficiently accurate for handling anharmonicity and phonon linewidths, and hence, for simulations of thermal transport in UO 2 . Considering the large anharmonicity of UO 2 and the limitations of the quasiharmonic approximation to account for temperature effects as used in Ref.…”

Pressure-driven insulator-metal transition in cubic phase UO 2

“…We chose U = 6.0 eV and J = 0.6 eV in our calculations, in accord with the previous DFT + DMFT calculations [16]. For the double counting term, we used the fully localized limit scheme [30], which is commonly used in DFT + U or DFT + DMFT calculations for correlated insulators [9][10][11][12][13]. All of the calculations were carried out at the inverse temperature β = 100 eV −1 (T ≈ 116 K), which is much higher than the experimental Néel temperature (T N = 30.8 K) [2].…”

“…The ground state of UO 2 is an antiferromagnetic Mott insulator with a sizeable band gap of ∼2.1 eV [8]. To study the electronic structure of UO 2 , many traditional first-principles approaches have been employed, such as the local density approximation (LDA) (or generalized gradient approximation (GGA)) plus Hubbard U approach [9][10][11][12][13], the hybrid functional method [14], and the self-interaction corrected local spin-density approximation (SIC-LSDA) [15]. However, none of the above methods can provide a satisfactory description of UO 2 over a wide range of conditions.…”

Slave Boson Theory of Orbital Differentiation with Crystal Field Effects: Application toUO2

“…At ambient pressure it is a Mott insulator and crystallizes in a cubic fluorite structure. Given the importance of this material, its electronic structure and energetics have been extensively investigated both experimentally and theoretically, e.g., with DFT+U [30][31][32] and other single-particle approaches [33,34]. However, within these techniques it is not possible to address the properties of the paramagnetic state of this material, which is stable above the Néel temperature T N 30.8 K [35].…”