What Is Fepo4 Is Distinct From Other Α-Quartz Isotypes?

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FePO4 is distinct from other α-quartz isotypes because its A cation is a transition metal. It is studied at different temperature ranged from 294K to 1073K by neutron powder diffraction. At relatively low temperature, it adopts the structure of α-quartz, which is tetrahedral and shown below. At high pressures, a phase change occurs to a more dense octahedral structure, which is known as β-phase. The transition temperature is 980K.
During the first-order transition, discontinuities can be observed. For α-phase, the cell parameters and volume increase significantly as temperature increase, but the dependency is not linearly. Its thermal expansion coefficient a is defined as α (K-1)= 2.924 x 10-5 + 2.920 x 10-10 ( T-300)2 . Angular
(Figure 1)(Figure 2).Both of them contains 1 formula unit in its unit cell, whileα- FePO4 has a trigonal unit cell andβ- FePO4 has a hexagonal unit cell. Table below shows the lattice parameters ‘s correspondence with temperature.
Group B: Model Answer
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About symmetrical difference, the symmetry of α- FePO4 is trigonal while the symmetry ofβ- FePO4 is hexagonal as shown above. From the above table, we can see that as the temperature increases (below 980K), the changing in the crystals is shown below. The cell parameter increases markedly as the temperature increased, which will lead to a rise in volume. There is preferential expansion along a direction, so the c/a ratio will decrease as temperature increases. And the bent Fe-O-P bond will expand and increase significantly. (shown in figure below)
  T=465K T=659k T=865K
As temperature reach the phase transition value(980K), The refined structural parameters of the low-temperature α phase tend towards the values obtained for high-temperature β-quartztype FePO4. It change fromαphase to β phase. (Shown
It basically contains PO4 tetrahedrons which is of great importance in determining the structural integrity and properties. The tetrahedral tilt angle δ, in addition with intertetrahedral bridging angle θ, is believed to constitute tetrahedral distortion. The bond length change as well as the the O-PO angle may also contribute to the tetrahedral distortion upon elevated temperature. But up to present, it is proper to treat the tetrahedrons as a rigid body because the tetrahedral tilt plays a much more important role in determining the tetrahedral distortion. In short, tetrahedral distortion is a result of tetrahedral tilt, which is quantified by tilt angle δ and is significantly temperature dependent. For quartz-type FePO4, the cell parameters and volum of the α phase increase markedly and non-linearly as a function of temperature. The principal contribution to the thermal expansion arises from angular variations manifested by changes in the two symmetrically-independent intertetrahedral Fe-O-P bridging angles and the correlated tilt angles, thus the temperature dependence of thermal expansion is actually the temperature dependence of the angular variations of intertetrahedral bridging angles and tetrahedral tilt angles. The temperature dependence of this angle can be reflected using Landau-type model expressed as