A structural theory for non-stoicheiometry. Part II. Defect fluorite-type structures: lanthanoid oxides MOx with 1.50 ⩽x⩽ 1.72

Abstract

The hyperstoicheiometric composition region MO_1.500+δ of binary lanthanoid oxides is discussed in terms of the concept of octahedral oxygen co-ordination of anion vacant sites (c.d.s) in a fluorite-type (C1) lattice. The structural model proposed for the hypostoicheiometric composition range MO_2-δ, based on isolated c.d.s. has been extended to the hyperstoicheiometric region by allowing for the formation of clustered defects, through their corner- and edge-sharing, achieved formally by the deletion of (213) oxygen-intact planes coupled with a crystallographic shear mechanism. The terminal member of the series C-M₂O₃ is shown to possess an anion lattice based on the tris-chelation of c.d.s with both chiral types being involved. Although ordered binary structures have not yet been found for the σ-region (0 < δ < 0.214), this extension of the model seems to provide a useful insight into the phase characteristics of this grossly non-stoicheiometric region and, as well, affords a convenient description of the polymorphic transformation of the cubic type C to the hexagonal type A sesquioxides. The structure derived by this extension of the model for the M₆O₁₀ phase is identical with that observed for the ternary oxides Sr₂UO₅ and Cd₂UO₅. The known structure of β-Bi₂O₃ is interpreted in terms of the corner-sharing of c.d.s in three dimensions. A structure for β-Pr₁₂O₂₂ is suggested on the basis of recently published unit-cell data.