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 MO_2 − 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<sub>3.5</sub>□O<sub>6</sub>]. 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 ½〈210〉_F and ½〈111〉_F CD linkages.