Inelastic neutron scattering, lattice dynamics and high-pressure phase stability inLuPO4 and YbPO4

Abstract

The combination of physical and chemical properties of orthophosphates,such as high melting point, structural and chemical stability and long-termcorrosion resistance, makes them an ideal medium for the storage and permanentdisposal of nuclear waste materials. We have carried out lattice dynamicalstudies using a shell model based on transferable interatomic potential forLuPO4 and YbPO4 in their zircon and scheelite phases. We also report inelasticneutron scattering measurements of the phonon density of states forYbPO4. The calculations in the zircon phase are in good agreement with the reportedexperimental data on phonon density of states and phonon dispersion relation forLuPO4 and ourdata for YbPO4. The lattice dynamical model is found to be useful for the calculation of variousthermodynamic properties such as the specific heat, thermal expansion and equation ofstate in both zircon and scheelite phases of these compounds. The calculated free energy inzircon and scheelite phases shows their relative stability below and above 22 GPa,respectively.