Characterization and Defect Structure of Anion-Excess Fluorite-Related Phases in LnOF-LnF_3 Systems 9Ln=Nd,Sm,Eu,Gd): From Isolated Defect Clustering to 'Vernier'Modulated phases

摘要

The phases LnO1..~F1+~, derived from the fluorite-type struc-ture by anionic excess, form, for medium size Ln cations(Ln = Nd, Sm, Eu, Gd), a homogeneous series of orthorhombicsymmetry (space group Pmmn, a b aF J2/2;c = aF). Struc-tural comparison of these phases by X-ray powder diffractionallows us to make a better determination of their defect struc-ture: 0/F order in sheets perpendicular to the [001] axis and formation of 1:0:2 clusters which can be associated in dimer units. For Nd, an orthorhombic—tetragonal transition is cx-plained by a change from 1:0:2 to denser 1:0:3 clusters. Coin-parison of the structural features of these phases with “vernier”modulated phases, isolated for Sm—Lu cations, allows a general classification of Ln01~F1+~ phases at low temperature to be proposed. All are characterized by the preservation of an anionic O/F order in sheets perpendicular to the [OOl]F axis and by a progressive loss of freedom in the accommodation of the anionic excess (nature and orientation of the clusters, tendency to long-range ordering) from La to Lu, caused by the decrease in Ln size and therefore in Liz coordination.