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摘要
Quartz-type iron phosphate (FePO4) was studied by total neutron scattering and Raman spectroscopy up to 1150 K in order to investigate disorder and the mechanism of the alpha-beta 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 alpha-beta transition near 980 K. 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 AlPO4. 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 SiO2 and AlPO4. 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 SiO2 and AlPO4. This mode does not exhibit any significant softening with temperature near the phase transition, which is further evidence that the alpha-beta transition is not of the simple displacive type. The difference in behavior between FePO4 and the other quartz homeotypes arises from the weaker bonding between the 3d(5) transition metal cation and oxygen.
- 出版日期2012-10-4
