Atomistic modeling of the vibrational and thermodynamic properties of uranium dioxide, UO2

Modeling the thermal properties of uranium oxide is of immense interest to the nuclear industry. UO2 belongs to the family of superionic conductors whose solid-state diffusion coefficients at high temperatures are comparable to that of liquids. We report lattice dynamics and molecular dynamics studies carried out on oxide UO2 in its normal as well as superionic phase. Lattice dynamics calculations have been carried out using shell model in the quasiharmonic approximation. The calculated equilibrium structure, elastic constants, bulk modulus, phonon frequencies and specific heat are in excellent agreement with the reported experimental data. Pressure variation of the phonon dispersion and equation of state have also been predicted. Molecular dynamics simulations have been carried out to study the diffusion behavior and the thermodynamic properties in UO2. The diffusion constant of O in UO2 has been determined. The pair correlation functions, O–U–O bond angle and thermal amplitude of vibration for the oxygen atom provide a microscopic picture of the local structure thereby throwing light on the gradual increase in the disorder of the oxygen sub-lattice which is a signature of superionic transition. The calculated transition temperature of UO2 is 2300 K, which compares well with experimental value of about 2600 K.