Structure and principles of oxidic hexaphases : A review

Abstract This paper is about the general structure and the formation of oxidic hexaphase types according to their crystal chemistry. Oxidic hexaphases exhibit hexagonal symmetry and consist of mixed layer structures stacked along the crystallographic c-axis. There are two main types of hexaphases: the magnetoplumbite and β-alumina type. Every hexaphase has a spinel structure that integrates blocks with either a rock salt or β-alumina structure. Both block types contain a characteristic cation. The formation of oxidic hexaphases depends on various factors: In addition to the size and charge of the characteristic cation the size of the spinel block in the crystallographic a-b direction is a decisive factor. Furthermore, the spinel block can be modified by replacing the present trivalent scaffolding cations with bivalent cations, resulting in more complex hexaphases such as the X, W, Z and Y types or β‴ types.