Transition Of Feo4: Lab Analysis Of Quartz-Type Fepo4

Quartz-type FePO4 was investigated from 294 up to 1073K by neutron powder diffraction. The cell parameters and fractional atomic coordinates in the α phase tend towards the values found in the β phase. The first order transition of FePO4 is at 980 K, during which several observations are made. Firstly, the cell parameters and fractional atomic coordinates show discontinuous shifts. Secondly, the intensities of a large number of diffraction lines decrease, as shown in Fig. 1.
Lastly, bond distances and angles change dramatically at the transition to β phase. In particular, the Fe—O distances drop significantly. α-β transition in quartz can be modelled using a single order parameter – the tilt angle δ. The temperature dependence of this angle
As compared to that of FePO4 (980 K), Tc for FePO4 is independent of the initial structural distortion, whereas for other α-quartz homeotypes, the temperature stability is dependent on the initial structural distortion present at ambient temperature.
The transition temperature of FePO4 is 980 K. At temperatures below 980 K, FePO4 is in the α phase, also described as α-FePO4. Similarly, at temperatures above 980 K, β-FePO4 is formed. Symmetrically, α-FePO4 adopts a trigonal structure whereas β-FePO4 adopts a hexagonal structure. To illustrate this, the structures of FePO4 at two extreme temperatures 294 K and 1073 K are shown below (from the [001] direction).
To look at how the structure of FePO4 changes with temperature, we assess the structure at various temperatures both below and above the transition temperature 980 K. As the temperature increases, the cell parameters and volume increase (Fig. 3 and 4), the tilt angles δ decrease (Fig. 6), and the Fe—O distances decrease (Table 4). The decrease in Fe—O distances is caused by the increasing disorder at high temperature arising from excited low-energy, high-amplitude vibrations. The changes stated can also be observed from the ATOMS drawings below. Thermal expansion in the α phase is in a strongly non-linear manner and dominated by angular
The bonds within the tetrahedral are much stronger than the bond between the tetrahedral. The temperature dependence of the volume strongly follows the behaviour of the average tilt angles and inter-tetrahedral angles as a function of temperature. With increasing temperature, the tetrahedral Fe—O distances decrease, and this is mainly caused by increasing disorder. The structural behaviour of α-quartz in the same temperature range is disrupted by changes in the tilting of the tetrahedral. It has been proposed that the transition temperature in α-quartz homeotypes scales with a term defining the depth of the potential well related to tetrahedral tilting, where m is the mass per tetrahedral unit, is a function of the frequency of the lowest A1 phonon made and is a function of sin δ.
Fig. 1. Experimental data (+), calculated and difference profiles (solid lines) from the Rietveld refinements of α-quartz-type FePO4 at 294 K, at 659 K and β-quartz-type FePO4 at 1005 K using neutron diffraction data. Intensity is in arbitrary units and the difference profile is on the same scale. Vertical bars indicate the positions of all calculated reflections.
Fig. 9. Evolution of the tilt angle d as a function of temperature for a series of α-quartz hemeotypes. Data for SiO2