The pseudobrookite structure, A2BO

In this structure there is octahedral coordination of the B ions, but the coordination of A is unusual. It can be described either as a highly deformed octahedron or as a distorted tetrahedron, for in Fe2Ti05 Fe has four neighbours at a mean Fe-0 distance of 1-92 A and two more at 2-30 A. Ti has six octahedral neighbours as in the forms of Ti02, FeTi03, Al2TiOs also has the pseudobrookite structure for TisOs see p. 466. 

We noted in Chapter 4 a number of structures in which metal ions occupy one-half of the octahedral interstices in c.p. structures. Three h.c.p. structures of this kind are  

Since both A and B ions occupy octahedral interstices in all of these structures they are suitable only for the smaller metal ions, as may be seen from the following examples ZnSb20g (trirutile), (Fe, Mn)Nb20g (niobite), and NiU206 (Na2SiFg structure). 

The CaTa206 structure has been illustrated with other octahedral structures in Chapters. As in the CaFe204 structure Ca is 8-coordinated the ninth, more distant, 0 would complete a tricapped trigonal prismatic coordination group. 

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