Paragraph 1 – Introduction and comparison of Crystal Structures and Chemistry of Quartz, α-FePO4 and β-FePO4
This essay is based on the Haines et al. research paper, which examines how FePO4’s crystal structure changes over temperatures ranging from 294K to 1073K. Quartz has the chemical composition SiO2, and comprises SiO4 tetrahedra where each oxygen atom is shared between two tetrahedra. It has a trigonal crystal shape. α-FePO4 has a similar crystal structure, with Fe3+ (brown) and P5+ (Purple) ions replacing the Si atoms within the tetrahedra, as shown below:
With increasing temperature, the cell parameters of the α-phase increase non-linearly, described by the equation: α(K-1
)=2.924 * 10-5
+2.920 *10-10 (T-300)2
This thermal expansion is smaller than that of …show more content…
The crystal structure of FePO4 consists of rings of tetrahedra, with FeO4 and PO4 present in equal proportions. The smaller PO4 tetrahedra have a larger tilt angle than the FeO4 tetrahedra, which is consistent with other ABO4 compounds. As temperature increases, the tilt angle δ in FePO4
(circled in blue) decreases with increasing temperature, dropping sharply to zero right before the transition at 980K of α-FePO4 to β-
FePO4. The two tilt angles converge to zero together, similar to the behaviour of AlPO4. The rate of decrease of PO4 tilt angle is higher than that of FeO4 tilt angle.
The Fe-O1-P angle increases with temperature, from 137.9° at 294K to 147.3° at 969K, before jumping to 154.9° at 1005K. This increase in Fe-O-P angle reflects the decreasing tilt angle with increasing temperature.
Tetrahedral distortion has several forms. One is inversion of the carbon, creating a square pyramidal shape, and usually resulting in bond strain. Another is the flattening of the tetrahedron through the increase in bond angle between two of the bonds. An extreme case of this would be