Crystal Chemistry Lab
The structural evolution of FePO4 arises due to the deviation in temperatures from 294K to 1073K via a process known as the neutron powder diffraction. This happens when the temperature of 980K is reached to result in the α to β transition. There are however certain observations of discontinuities in structural parameters during this transition. The stark contrast is that during the α phase, there are huge thermal expansions or contractions that present a non-linear angular variation. This is the same concept as expansion occurs when temperature increases and thus there will be an increase in volume which is related to the tilting of the tetrahedra. …show more content…
Whereas in high temperatures above 980K, the beta phase has a octahydro structure where the lattice symmetry is hexagonal as the dispose group is p6422. These are the lattice symmetry of the tree structures.
Illustrate and describe the symmetrical differences between α-FePO4 and β-FePO4. For this part, firstly, for temperature of 294K up to 980K, the alpha to beta phase transition occurs and unit cell structure is trigonal for the alpha FePO4. The lattice parameters is a=b=c and the angles α = β = γ ≠90°. As for the range of temperatures from 980K to 1073K, the unit cell is hexagonal for the beta FePO4.
The lattice parameter is a=b=/c. Furthermore, the angles α = β = 90° and γ =
120°. By comparing the different ATOMS drawings at the different temperatures, it can be observed that the tilt angles actually decrease as temperature increases. As such, we can conclude that both phases have the same extent of symmetry from figure 1. This can also be proven by the ATOMS drawing by comparison along the same axis, namely by looking down the z-axis. However, one significant point to take note of is that even though there is a change …show more content…
Describe changes in the FeO4 and PO4 tetrahedra with temperature and explain tetrahedral tilting quantitatively.
Firstly, the FePO4 crystal structure comprises of both FeO4 and PO4-tetrahedral links. A tetrahedral distortion occurs when there is both a change in angle and lengths of the bonds that are present there. However, in contrast, in the case of a tetrahedral tilting, there is only a change in angle and no presence of change in bond lengths. A tetragonal distortion can be said to be a result of a tetrahedral tilt, which is due to the tilt angle delta. These are the major differences between the two forms. Second point to bring about is that when there is an increase in temperature, we are basically sizing up the volume of the “cages”. Changing the angles brings this about, which basically means there is tetrahedral tilting, and also the relationship between the FeO4 and PO4 tetrahedral. However, there is also a limit to this vast increase in volume. Once it has been converted to the high temperature beta phase, since the change in volume is due to the change in tilting angle or the angles between the tetrahedral, the expansion is being constricted and limited to a certain point. As such, ultimately, there will reach