Structures related to those of spinel and the phase, and a possible mechanism for the transformation olivine spinel

The structures of intermediate phases, such as the phase, are shown to be readily derived from that of spinel (y) by the regular insertion of anti-phase boundaries (apbs) or mirror planes parallel to (010) of the body-centered tetragonal unit cell of the latter [(TlO) planes of the face-centred cubic cell]. The process leads eventually to a hypothetical structure, co, from which y, etc. can be derived by similar apbs. Unit layers parallel to (0T2) in the co structure are identical to those parallel to (001) in olivine (a, setting Pnma). A simple mechanism is proposed for the transformation a co. Further transformation to p etc. and y requires only slip, to generate apbs.