A structrual theory for non-stoicheiometry. Part III. Defect fluorite-type structures: vacancy superstructures in ordered praseodymium oxides

Abstract

The hypostoicheiometric composition region MO_{2 –δ}(0.286 δ 0) if binary lanthanoid oxides is re-examined in the light of new electron difraction data on ordered praseodymium oxides Pr_nO_{2n– 2} with n= 7,9,10,11, and 12. Model structures have been devised which are consistent with the topological requirements of the co-ordination defect (c.d.) and the implications of the new unit-cell data. The metal-centered pairing of c.d.s along [111]_F, to yield the structural unit of composition Pr₇O₁₂□₂, is a fundamental determinant in all phases. The vacancy sets for the triclinic homologues have been defined uniquely and those possible for the monoclinic phase β-Pr₁₂O₂₂ have been reduced to two alternatives. Planar ordering of anion vacancies characterizes both the triclinic (odd-n) and monoclinic (even-n) hormologues. Possible inter-relatios between their structures are analysed in terms of an extension of the crystallographic shear formalism.