Octahedra sharing faces, edges, and vertices

Structures of this type include the corundum (a-Al203) and 7-Cd(OH)2 structures. The construction of models of both of these structures is described in MSIC. In the corundum structure there are pairs of octahedra with a common face, and in addition there is sharing of edges and vertices of coordination groups around the Al ions. The complexity of this structure for a compound A2X3 appears to arise from the need to achieve a close packing of the anions and at the same time a reasonably symmetrical environment for these ions-see also the remarks on p. 216. In the structure of 7-Cd(OH)2 (Fig. 5.42) pairs of rutile chains are joined through common faces and the double chains then share vertices. 

Other structures in this class include the high-pressure form of Rh203 (p. 451) and K2Zr20j (JSSC 1970 1 478). 

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