The research done by Haines on how the atomic structure of FePO4 varies when temperature increases from 294K to 1073 shows few crystal chemical relationships between
SiO2 and FePO4. There is a first order variation of cell parameters and atomic orientations with temperatures lower than 980K and α-phase plays a dominant role over β-phase at high temperature. For the α-phase FePO4, the cell parameters increase with a non-linear trend when temperature increases linearly. The volume of the cell expands with a thermal expansion coefficient of α (K-1
. The reasons for the cell expansion are the cell angular variations and tilt angles, which were caused by the variations in Fe-O-P bridging angles as they are not symmetrical. The δ …show more content…
The variation of the average of δ and θ versus temperature was studied in the paper. In terms of the crystal chemical relationship between quartz SiO2 and FePO4, the Fe-O distance in the tetrahedral phase decreases with increasing temperatures. The very alike situation happens in Si-O distance in α-quartz with increasing temperatures. As the fact there are many variations caused by temperature increases, energy increases and vibrations, the non-physical behavior could be used for analysis of all the disorders. From the research, the equation of δ2
^1/2] shows the changing correlation of tilt angle δ with temperature from α phase to β phase. The δ0 is the decreased tilt angle at 980K and Tc is the temperature for the second-order transition.
FePO4 structural behavior is different from other α-quartz homeotypes as the tilt angle δ decreases faster than quartz.
Figure 1: Crystal structure of quartz
Figure 2: Crystal structure α-phase FePO4
Figure 3: The variation graphs of cell parameters and cell volumes with temperatures
The α-β transition happens when the temperature increases from 294K to 980K. The α-
FePO4 cell has a trigonal structure with lattice parameters a=b=c and …show more content…
With increasing temperature, the α-phase at low temperature evolves to β-phase at high temperature to β quartz type FePO4. With increasing disorders because of increasing energy and high amplitude vibration in α-quartz, this non-physical behaviour promotes the α-β transition. The angle and the length of bonds change when there is a tetrahedral distortion. And tetrahedral tilt happens when only the angle changes and bond length doesn’t. When temperature reaches 980K, the α-β transition happens with Fe-O-P bridging angles increase and tetrahedral tilt angels decrease.
Research also showed that there is a correlation between the distortion of β-quartz structure and tetrahedral tilt angle δ and the intertetrahedral bridging angle θ. The α-β transition doesn’t happen very often the δ is larger than 220 and θ is smaller than 1360
FePO4 where the structure is δ=21.50 and θ=137.80
. The bond length and tetrahedral