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(3.5)□O(6)]. 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.

Compounds with a 'stuffed' anti-bixbyite-type structure, analysed in terms of the Zintl-Klemm and coordination-defect concepts.

The bixbyite structure (Mn(2)O(3)) (Ia3) is often described as a distorted face-centered cubic (f.c.c.) array of Mn atoms, with O atoms occupying 3/4 of the tetrahedral holes. The empty M(4) tetrahedra are centred at 16c. In anti-bixbyite structures (Mg(3)N(2)), cation vacancies are centred in empty N(4) tetrahedra. If 16 hypothetical atoms were located at this site they would form the structure of gamma-Si. This means that anti-bixbyite structures are ideally prepared to accommodate Si(Ge) atoms at these holes. Several compounds (Li(3)AlN(2) and Li(3)ScN(2)) fully satisfy this expectation. They are really anti-bixbyites 'stuffed' with Al(Sc). The presence of these atoms in 16c is illuminated in the light of the extended Zintl-Klemm concept (EZKC) [Vegas & García-Baonza (2007). Acta Cryst. B63, 339-345], from which a compound would be the result of 'multiple resonance' pseudo-structures, emerging from electron transfers between any species pair (like or unlike atoms, cations or anions). The coordination-defect (CD) concept [Bevan & Martin (2008). J. Solid State Chem. 181, 2250-2259] is also consistent with the EZKC description of the pseudo-structures. A more profound insight into crystal structures is gained if one is not restricted to the contemplation of classical anions and cations in their conventional oxidation states.