Inelastic neutron scattering, lattice dynamics and high pressure, phase stability in LuPO4 and YbPO4

The combination of physical and chemical properties of orthophosphates such as high melting point, structural and chemical stability and long term corrosion resistance makes them an ideal medium for the storage and permanent disposal of nuclear waste materials. We have carried out lattice dynamical studies for LuPO4 and YbPO4 in order to analyze and understand the phonon spectra as measured by inelastic neutron scattering. The calculated phonon density of states and phonon dispersion relation for LuPO4 are in agreement with the available phonon data in the literature and our inelastic neutron scattering measurements from the polycrystalline sample of YbPO4. The lattice dynamical model is used for the calculation of the pressure variation of phonon frequencies and various thermodynamic properties such as the specific heat, thermal expansion and equation of state etc. The calculated free energies in zircon and scheelite phase shows that zircon and scheelite phases are stable below and above 22 GPa, respectively. (author)