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

Abstract

Abstract The structural evolution of quartz-type iron phosphate, FePO 4 , 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 FePO 4 . 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 SiO 2 and AlPO 4 , and do not scale with the initial structural distortion. This may be an indication of a dynamic instability in α -quartz-type FePO 4 .