Study on Structural, Dynamical, and Thermal Properties of Nuclear Fuel Thorium Mononitride Using First-Principles Calculations

This paper explores the vibrational, thermophysical, and structural properties of thorium nitride using density functional theory. An agreement is observed between the calculated and experimental properties of the lattice constant and bulk modulus. In the density functional perturbation theory, diagrams of phonon spectrums and vibrational densities of states along high symmetry paths are calculated. Based on the phonon dispersion diagram, no imaginary frequencies are found, indicating that the crystalline structure is dynamically stable. The compound also exhibits a phonon gap in the range 154-300 cm-1. Under high temperature and pressure, quasi-harmonic Debye models are used to evaluate thermodynamic properties such as Debye temperature, thermal expansion coefficient, entropy isothermal bulk modulus, and vibrational specific heat capacity. As the temperature increases, the volume of the system at a constant pressure decreases, while it increases for all pressures at a constant temperature. As temperature increases at a constant pressure, the coefficient of thermal expansion increases, indicating that the crystalline lattice is transferring more heat.