Crystal Chemistry Lab Report

Compare and contrast the crystal structures and crystal chemistry of quartz, α-FePO4 and β-FePO4.
The structural evolution of Iron phosphate FePO4 from a temperature of 294K to 1073K was discussed in this article. ABO4 compounds ( B = Si, Ge, P, As; A = Si, Ge, Al, Fe, Ga) family consists of FePO4 which crystallized in a quartz-like structure. Quartz, berlinite (AIPO4) and iron phosphate undergo high temperature α – β phase transition. The structural analysis of FePO4 at high temperature is very tough due to measurements of cell parameters. At a low temperature, FePO4 exhibits the structure of α – quartz which forms the shape of a tetrahedral. While the temperature is extremely high at 980K, it transform to β phase and the structure change

The bigger number of diffraction lines intensities decreases during the transition phase. Bonding distances and angles changes drastically at the transition of β phase. A larger bond distance gives β phase a higher degree of dynamic disorder. No thermal expansion was seen in the β phase as well. This resulted the tilt angle to reduce as the temperature rises. Moreover, temperature also leads to the increase in cell volume of quartz-type FePO4 which is shown in the table below, taken from the research paper. Base on the table, we can determine that the volume of FePO4 was not affected much by the increase of temperature even it reaches 980K. This shows that FePO4 has changes from α – FePO4 to β –

During the transition phase, the average tilt angle δ is used for FePO4 as both individual average tilt angle while PO4 tetrahedral is based on fractional atomic coordinates of O1 and O2. However, they are still related. Quartz-type FePO4 consists of PO4 tetrahedrons which is a key factor in determining the structural integrity and properties. Tilt angle δ together with intertetrahedral bridging angle θ is believed to cause tetrahedral distortion. Bond length O-P-O angle also causes tetrahedral distortion at elevated temperature although not as significant as compared to tetrahedral tilt. Therefore, we can conclude that tetrahedral distortion is caused by tetrahedral tilt and also very sensitive to temperature. Temperature dependence of this angle δ can be expressed by the