Computational modelling studies for high temperature monazite systems

Abstract

Monazite is widely considered as a potential source of fissile feedstock for nuclear power generation, as it generally contains significant quantity of thorium (Th), uranium (U) and rare earth elements (REEs) within the mineral ore structure. Currently, a new process of high-temperature cracking of monazite is under development to improve the liberation of these cations from the robust monazite lattice. The current process for extraction of the REEs from the mineral involves complicated processes which makes use of harsh chemicals. These process therefore has significant scope for optimisation. With this in mind, the stability of monazite systems, t-CeSiO4 and m-LaSiO4 was investigated using first-principle calculation employing density functional theory (DFT) to gain a better understanding of the inherent molecular structures and the influence of temperature on conformation. The calculated lattice parameters for the model of t-CeSiO4 were found to be in good agreement with the experimental values (within 3%) and deemed a sufficient m-LaSiO4 of the system to be exposed to simulated reaction conditions. The elastic properties suggested that t-CeSiO4 is the more stable structure compared to m-LaSiO4 structure at 0 K. Semi-empirical embedded atom method interatomic potentials incorporated in the LAMMPS code were also employed to investigate the lattice expansion of t-CeSiO4 at high temperatures. It was found that calculated a and b lattice parameters expand with a linear ratio to a temperature of 2200 K whereas the c parameter was found to contract in the same temperature range. The findings provided an in depth understanding of the monazite molecular structure change at higher temperatures that may be helpful in plasma cracking optimisation experimental methodologies.