Cristobalite-related oxide structures

Cristobalite-related oxide structures are reviewed and described in terms o f concerted rotations of [110](p) (p = C9 parent structure) tetrahedral str ings about two mutually orthogonal (110), axes, All known structures are cl assified according to their patterns of rotations, This simple mechanism of distortion from C9 is the same as that proposed for the dynamically disord ered model of beta-cristobalite, Only fully Na stuffed alpha-cristobalite-t ype and beta-NaFeO2-type structures behave as if they comprise regular, rig id tetrahedra. The collapse of the various structures from their C9 parent and the regularity of the resultant tetrahedra are investigated. In fully o r partly stuffed structures it is the size of the interstitial cations whic h determines the degree of collapse of the C9 framework. (C) 1998 Academic Press.