Dynamic disorder and the α-β Phase transition in quartz-type FePO ₄ at high temperature investigated by total neutron scattering, Raman spectroscopy, and density functional theory

Quartz-type iron phosphate (FePO ₄) was studied by total neutron scattering and Raman spectroscopy up to 1150K in order to investigate disorder and the mechanism of the α-β transition. The increasingly large underestimation of P-O and Fe-O distances in Rietveld refinements of the average long-range structure as compared to the bond lengths obtained from the pair distribution function as a function of temperature is a clear indication of the presence of significant dynamic disorder, particularly from 850 K up to and above the α-β transition near 980K. A significant broadening of the Fe-O distance distribution is also observed. Reverse Monte Carlo modeling confirms the presence of such disorder with broadened angular distributions in this temperature range, in particular for the Fe-O-P and Fe-P-Fe distributions. The Raman spectrum, calculated using density functional theory, is in very good agreement with experiment. These calculations indicate that there is an inversion of two low-energy A ₁ vibrational modes with respect to AlPO ₄. The principle mode, which exhibits strong damping in the Raman spectrum above 850 K, is thus not a tetrahedral libration mode, but a mode that principally involves large amplitude translations of the Fe atoms along with a degree of oxygen displacement. The transition mechanism from a dynamic point of view is thus different from the transitions in SiO ₂ and AlPO ₄. The strong damping of this mode is also further evidence of a high degree of dynamic disorder, which is different from the disorder observed in SiO ₂ and AlPO ₄. This mode does not exhibit any significant softening with temperature near the phase transition, which is further evidence that the α-β transition is not of the simple displacive type. The difference in behavior between FePO ₄ and the other quartz homeotypes arises from the weaker bonding between the 3d5 transition metal cation and oxygen.