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 LnO(1-x)F(1+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 Pmmm, a approximate to b approximate to aF root2/2;c = a(F)). 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 LnO(1-x)F(1+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 Lrt size and therefore in Ln coordination. (C) 2001 Academic Press.