Inelastic neutron scattering, lattice dynamics, and synchrotron x-ray diffraction study of FePO₄

The quartz structure compounds MPO₄ (M=Al, Fe, and Ga) are of interest to understand the nature of bonding in the tetrahedral coordinated phosphates. We report detailed lattice dynamical studies involving experimental inelastic neutron scattering measurements and theoretical shell model calculations of vibrational and thermodynamic properties of FePO₄. X-ray diffraction measurements are also carried out for FePO₄ from 100 to 300 K using an Elettra synchrotron radiation source. The measured phonon density of states and thermal expansion for FePO₄ have been interpreted based on the shell model calculations. The lattice dynamical model is used for calculating various thermodynamic properties like specific heat, thermal expansion, bulk modulus, and thermal amplitudes. The calculations are in good agreement with numerous experimental data including our phonon density of states and thermal expansion measurements. Our calculated pressure variation of phonon frequencies shows that a zone boundary mode at (1/3,1/3,0) becomes soft at 4 GPa, which may be associated with the mechanical instability of the quartz structure of FePO₄ at high pressure.