Structural change on reduction. Ordering of defects

Although they differ in detail, it may be accepted that the basic unit of the cluster is a tetrahedron with one interstitial iron (most likely Fe3+ [52, 53] surrounded by four vacancies on the nearest octahedral site, which is found locally in the magnetite structure. The wiistite structure is then understood to have these unit tetrahedra arranged in some ordered manner. From this point of view, the measurements suggesting three phases separated by second- or higher-order transitions within the wiistite phase [22, 22a, 78] can be interpreted as successive loss of different types of order as the temperature is raised or the number of the unit tetrahedra decreases (the reduction proceeds). However, no definite conclusions have yet been drawn and indeed, the existence of these three subphases is still disputed [19, 20, 23, 24, 28]. 

Namely, the reaction proceeds topotactically and 7-Fe2 03 results when the particle size is small, but in large particles accumulated strain causes the nucleation of a-Fe2 03 before the completion of the reaction. 

The mechanism for the transformation from 7-Fe203 to a-Fe203 has been proposed [29] to involve associated movements of the oxygen close packed planes in the (112) direction of the f.c.c. lattice and of Fe3+ ions so that the tetrahedral ions in 7-Fe203 move into vacant octahedral sites. 

For example, W03 takes the Re03 structure, in which the unit octahedron shares six vertices with six nearest neighbour octahedra. When vacancies are introduced, they tend to arrange as shown in Fig. 7(a), for example, and they are annihilated by crystallographic shear movement indicated by arrows resulting in Fig. 7(b). 

This view of the process of reduction of the oxides and the oxysalts of W and Mo has been supported by an electron microscopy study [34] and by electron spectroscopy [32, 35]. However, the actual mechanism by which vacancies arrange cooperatively in ordered manners to give rise to crystal shear is yet to be elucidated. 

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