Structural theory for non-stoicheiometry. Part I. Defect fluorite-type structures: lanthanoid oxides MOx with 1·7 ⩽x⩽ 2·0

Abstract

The concept of an octahedrally co-ordinated anion vacancy □O₆ is introduced to develop a rational structural theory for the known series of ordered non-stoicheiometric lanthanoid oxides of formula M_nO_2n–2(n= 7–∞). The resulting co-ordination defect (c.d.) has the composition M7//₂□O, and it is suggested that this retains its structural integrity in all the homologues. Topological analysis has been employed to establish the fundamental superstructures of the fluorite lattice which characterize these oxide phases. It emerges that the anion deficiency is accommodated by point defects gathered in oblique {213} planes each being separated by (n– 1) oxide-intact {213} planes. Effectively, all these phases are based on regularly spaced intergrowths of the progenitors ι-Pr₇O₁₂ and PrO₂. It is shown that the homologues are structurally coherent and that irregularity in spacing of intergrown phases provides a logical description for the grossly non-stoicheiometric α-phase which overlays the region MO_1·7–MO_2·0 at higher temperatures. The extended {213} sheets of defects can be circumscribed coherently by PrO₂ to generate finite discs of Pr7//₂□O₆. Order–disorder transformations are considered in terms of the interchangeability of Pr7//₂□O₆ and Pr7//₂□O₇ structural sub-units which are topologically identical. The non-existence of the homologue Pr₈O₁₄ and a possible structure for the β′-Pr₁₂O₂₂ phase are examined in terms of the cubic pyrochlore (E8₁) structure.