Fepo4 Lab Report

Writing Exercise – FePO4 Polymorphs
1. Compare and contrast the crystal structures and crystal chemistry of quartz, α-FePO4 and β-FePO4. FePO4 is a quartz-type of iron phosphate. It is different from other α-quartz isotypes due to the fact that the A-cation is a transition metal. The low-temperature α-phase can be from 294k neutron powder diffraction to high-temperature β-phase 1073K. As the quartz start to transit at
980K, from α-β, the Fe-O-P bridging angles start to increase value and the tetrahedral tilt angle decline drastically. Due to the high intensity of thermal heat, it will turn to a more complex and heavy octahedral structure, which is the β-phase.
In the α-phase, thermal expansion is strongly non-linear and overcome by angular
This volume data is controlled by a thermal expansion coefficient α (K-1)= 2.924 x 10-5 + 2.920 x 10-10 (T-300)2. This also meant the α-quartz type
FePO4 is very instable as the angular variations are much bigger than other materials like SiO2
(quartz).

Fig 1. Fig 2.

Structure of FePO4 (tetrahedral)
According to the polyhedral representation, it is also known that the crystal structure of α-quartz type
FePO4 at 294K. The angles correspond to the tilt of the bond vectors to 45 degrees, which is their orientation in the β-quartz type structure. For the α -> β transition  δ2= 2/3 δ02 [1 + (1 – ¾ (T – Tc/T0 – Tc))^1/2], the temperature dependence of tilt angle δ. δ0 is the reduction in tilt angle at the phase of change (980K) and Tc refers to the temperature for the second order change. The characteristic of FePO4 is different and thus does not relate to other kinds of α-quartz homeotypes. Angle of δ starts to decline faster than quartz as a result.
Therefore, FePO4 produces two very distinct characteristics in terms of form, reaction and structure in their α-phase and β-phase respectively.
2. Illustrate and describe the symmetrical differences between α-FePO4 and β-FePO4.

FePO4 294K with unit cell FePO4 1073K with unit
The crystal formation of FePO4 is consisting of FeO4 and PO4 tetrahedral rings. The α–β structural phase transition (SPT) in FePO4 is important for its relatively high Tc value and for the highest volume increase. At the high temperature of 980K, this will be the transitional phase that changes α-β FePO4. Now, these PO4 tetrahedrons are crucial to identifying the properties that the crystal possesses. The tilt angle δ depends on the initial structural distortion present at the ambient temperature. This angle also starts to decrease rapidly than the least distorted materials like SiO2 and AlPO4. When the value of δ is larger than 22 degrees and θ (intertetrahedral bridging angle) is smaller than 136 degrees, there are no observations of any materials going through the α-β transition.
However, the exact structural parameters of FePO4 stay very close to the limiting values, which are (δ=21.5o and θ= 137.8o). The bond distance modifies as well as the O-P-O angle commit to the changes of the tetrahedral with rising temperature. Across the observation of heating this structure, the Fe-O2 bonding scales down in length and results in the angle of Fe-O2