Phase transitions, order and disorder

We will concentrate our attention on a small number of special cases of solid-solid transitions, without presuming to infer too much in generality. The approach taken is different to the usual view, and the examples which yield to this attack are inaccessible to standard ways of thinking. The utility of our approach will be apparent from the examples chosen. 

When the solid is compressed (e.g. upon firing), the enhanced repulsion distorts the linear arrangement of positive cations. They move sideways, and the structure becomes more elastic. The only coordination feature that changes is the cation-cation packing, which is eidianced. The new structure is relaxed and this is reflected in the cation - cation distance, which is larger in the more compressed form. 

This phenomenon, which is widespread in metals, is triggered by a cooperative phase transformation associated with the ensuing change in size of the atoms (or equivalently the position of the potential minimum in their 

The standard (postulated) mechanism for the transition is the Bain deformation, which involves a 20% contraction in one direction and a 12% expansion in two perpendicular directions. This was described by Bowles and 

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