Derivation of Transferable Pair Potentials and the Calculation of Intrinsic Defect Properties for Xenotime

A new force field has been empirically derived that is transferable across the YPO4, Y2O3 and P2O5 phases, utilising a reverse Monte Carlo (RMC) method. This method employs a simulated annealing technique with a logarithmic quench to fit potential parameters to observed crystallographic and mechanical properties, producing a force field suitable for simulating radiation damage events in an atomistic molecular dynamics regime. These potentials are used to investigate the defect properties of xenotime, where a wide range of intrinsic defects including Schottky, Schottky-like, Frenkel pairs and anti-site defects have been investigated, both at infinite dilution and as defect clusters. A common feature in the lowest energy defect configurations was the presence of polymerised phosphate tetrahedra, forming P2O7 units. The trend in the formation energies for the Frenkel pair defects at infinite dilution was in good agreement with previously published simulations. However, the binding energy associated with the aggregation of point defects was found to have a profound impact on the defect formation energies, significantly lowering the formation energy of the phosphorous Frenkel pair in particular. The intrinsic defect calculations presented here have been compared with previous work in zircon, to gain insight into differences that may contribute to the disparity in the radiation resistance of the two minerals.