Haines et al., A neutron diffraction study of quartz-type FePO4: high-temperature behavior and α-β phase transition Z. Kristallogr. 218, 193-200 (2003).
FePO4 containing ferric iron and dihydrate in majority is commonly used in batteries. The colour of hydrated FePO4 is nearly white or slightly yellowish, it turns more yellow when water is lost and become white-yellowish powder when dehydrated. It is a quartz-type iron structured as α-quartz, a tetrahedral structure, in relatively low temperature. This research paper studied about FePO4’s change in structure using neutron powder diffraction with the temperature range of 294K to 1073K. As it’s A cation is a transition metal, FePO4 is unique in the quartz. The first-order transition occurs at 980K that it becomes more concentrated and turns to an octahedral structure, known as the β-phase. As temperature increases, the cell parameters also increase in a non-linear way. Angular variations of the Fe-O-P bridging and tilt angles are main factors contributed to the thermal expansion in α-phase. The rate is calculated as α (K-1)= 2.924 x 10-5 + 2.920 x 10-10 ( T-300) 2. It is shown that the single order parameter tilt angle δ, imitates transition of α-β phase. As mean tilt angle for both FeO4 and PO4 tetrahedral uses δ, it is also used for FePO4 due to their correlation. As δ is fixed to zero, there is no thermal expansion happen in β-phase resulting this mechanisms to be absent. The formular δ2 =2/3δ02