Pauling third rule

The hematite structure appears to violate Pauling s third rule. Explain how this is done without excessively destabilizing the structure. 

Pauling s third rule states that the presence of shared edges, and especially of shared faces, in a coordinated structure decreases its stability this ect is large for cations with large valence and small ligancy. 

For octahedral structures, the Ti02 polymorphs usually serve as illustrative examples of Pauling s third rule. In the stable rutile structure, a TiO octahedron shares two of its edges with neighboring octahedra in the less stable brookite and anatase polymorphs, however, the number of shared edges increases to three and four [5]. Also, the rutile type is much more common than the other two. 

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