The role of the coordination defect (CD) in the structures of anion-deficient, fluorite-related compounds

The various superstructure phases that occur with the anion-deficient compositions of binary oxides MO2-x with the fluorite structure as parent are explored here in terms of the original 'coordination defect' (or CD) concept in which each vacant oxygen site, □, is 'coordinated' by six O atoms thereby creating the octahedral 'structure-determining' entity [M 3.5□O6]. It emerges that the structure and composition of each anion-deficient (polymorph) phase can be described in terms of crystallographic 'motifs' which comprise sets of parallel coplanar polygons based on 210F and 111F CD linkages.