First-principles investigation of phonon spectrum, elastic, mechanical and thermophysical characteristics of an actinide-oxide ceramic

Abstract

In this paper, first-principles calculations and density functional perturbation theory (DFPT) based on the ab-initio approach, have been utilized to investigate mechanical, elastic anisotropy, dynamical, and thermal properties of nuclear fuel UO. The results reveal that the obtained equilibrium lattice parameter at zero temperature and pressure has a good consistency with the experimental lattice constant. The elastic constants are evaluated using the strain-stress method. These coefficients are estimated to be positive and satisfied the Born-Huang relations, which is indicative of the mechanical stability of this compound. The mechanical properties such as shear modulus, bulk modulus, and Young's modulus of UO are calculated, and the results show the compound's stiffness. Pugh's criterion (B/G) is obtained to be approximately 3.5, which confirms that UO is a ductile substance. Elasticity analysis including universal anisotropy and Zener's anisotropy factor reveals that UO falls into the category of anisotropic materials. The phonon spectrum curve is calculated using DFPT along the high symmetric paths and no negative frequencies are observed in the dispersion relations, implying the dynamical stability of the crystalline structure. The quasi-harmonic Debye (QHD) model is employed to study the thermophysical properties such as Debye temperature, isochoric heat capacity, coefficient of thermal expansion, entropy, and Grüneisen parameter at high pressures and temperatures. We found that molar specific heat was more affected by the temperature than pressure.