Inelastic neutron scattering and lattice dynamics studies of AlPO₄ and GaPO₄

The compounds AlPO₄ and GaPO₄ show phase transitions at high pressure depending on the compressibility of the constituent tetrahedra. Semi-empirical interatomic potentials are available for AlPO₄ and GaPO₄. Molecular dynamics simulations have been reported using these potentials to understand the nature of phase transitions in different polymorphs of these compounds. In order to check these potentials we have carried out lattice dynamical studies for AlPO₄ and GaPO₄. The phonon density of states measurements from the polycrystalline samples of low-cristobalite phase of AlPO₄ and GaPO₄ are carried out using High-Resolution Medium-Energy Chopper Spectrometer at ANL in the energy transfer range 0-160 meV. The calculated phonon spectra for both the compounds using the available potentials show fair agreement with the experimental data. However, the agreement between the two is improved by including the polarizibility of the oxygen atoms in the framework of the shell model. The lattice dynamical model is used for the calculation of specific heat and thermal expansion.