Characterization and Defect Structure of Anion-Excess Fluorite-Related Phases in LnOF–LnF3 Systems (Ln=Nd, Sm, Eu, Gd): From Isolated Defect Clustering to "Vernier" Modulated Phases

The phases LnO1-xF1+2x, derived from the fluorite-type structure by anionic excess, form, for medium size Ln cations (Ln=Nd, Sm, Eu, Gd), a homogeneous series of orthorhombic symmetry (space group Pmmn, abaF2/2;c=aF). Structural comparison of these phases by X-ray powder diffraction allows us to make a better determination of their defect structure: O/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 explained by a change from 1:0:2 to denser 1:0:3 clusters. Comparison of the structural features of these phases with "vernier" modulated phases, isolated for Sm–Lu cations, allows a general classification of LnO1xF1+2x phases at low temperature to be proposed. All are characterized by the preservation of an anionic O/F order in sheets perpendicular to the [001]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 Ln coordination.