Computational study of plutonium–neodymium fluorobritholite Ca9Nd0.5Pu0.5(SiO4)(PO4)5F2 thermodynamic properties and threshold displacement energies

A force field for the modelling of neodymium fluorobritholites, having the general composition Ca10−yNdy(SiO4)y(PO4)6−yF2, is derived. The validity of the established analytical potentials is tested by calculating the crystallographic parameters of the compositions for 0⩽y⩽6 and comparing to the experimental values. The validity is further tested by calculating the variation of the phonon spectra when transitioning from the fluoroapatite Ca10(PO4)6F2 to the full silicate neodymium fluorobritholite Ca4Nd6(SiO4)6F2. Trivalent plutonium ions are introduced into the mono-silicate neodymium fluorobritholite using transferable potentials in order to obtain the composition Ca9Pu0.5Nd0.5(SiO4)(PO4)5F2, which corresponds to a Pu concentration of about 10 wt%. The thermodynamic properties of the resulting structure are calculated, and the threshold displacement energies of various sublattices are estimated by applying the sudden approximation (SA) method. The activation energies for Frenkel defect annealing are also calculated using the rational function optimisation (RFO) procedure for the saddle point research.