A neutron diffraction study of quartz-type FePO4: high-temperature behavior and α–β phase transition

The structural evolution of quartz-type iron phosphate, FePO4, was studied by time-of-flight neutron powder diffraction from 294 K up to 1073 K. The refined structural parameters of the low-temperature α phase tend towards the values obtained for high-temperature β-quartz-type FePO4. In particular, the Fe—O—P bridging angles increase and the tetrahedral tilt angles decrease strongly as the α–β phase transition at 980 K is approached. Important discontinuities in these structural parameters are observed at this first-order transition. Thermal expansion in the α phase is strongly non-linear and dominated by these angular variations. There is essentially no thermal expansion in the β phase due to the absence of these mechanisms. The behavior in the α phase is distinct from those of other α-quartz homeotypes in that the angular variations are greater than for the other materials, including SiO2 and AlPO4, and do not scale with the initial structural distortion. This may be an indication of a dynamic instability in α-quartz-type FePO4.