‘Off-centre’ ions in compounds with spinel structure

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Abstract
Experimental evidence from several sources suggests that, contrary to general belief, the crystal structure of cubic spinels is not satisfactorily referred to the space group Fd3m. The discrepancy is most likely to be associated with a small displacement of the octahedrally coordinated metal ion along a [111] direction. It is shown that the wide variation among Debye-Waller factors reported for spinel compounds is consistent with a mechanism analogous to that responsible for producing ‘off-centre’ displacements in the alkali halides. Careful refinement of the crystal structure parameters of spinels at different temperatures could be a means of investigating the repulsive interactions between ion cores and could also give useful information concerning magnetic exchange interactions.