THE DEFECT STRUCTURE OF ANION-EXCESS (CA1-XYX)F2+X WITH X = 0.06

The defect structure of the anion-excess fluorite (Ca1-xYx)F2+x with x = 0.06 has been investigated using single-crystal neutron diffraction. Least-squares refinement of the measured Bragg intensities has been performed considering four positions for the disordered anions within the Fm3m unit cell: in 48i sites at ( 1 2, u, u) with u = 0.39; in 32f sites at (w, w, w) with w = 0.38; in 32f sites at (v, v, v) with v = 0.27, and in the empty cube center 4b sites at ( 1 2, 1 2, 1 2). The results indicate that only the ( 1 2, u, u) and (v, v, v) sites are significantly occupied. Information concerning the local defect configuration has been provided by least-squares refinement of the measured distribution of coherent elastic diffuse scattering within the (110) plane of reciprocal space. The best agreement with both Bragg diffraction and diffuse scattering data is obtained by adopting a model of randomly distributed cuboctahedral anion polyhedra, of the type observed in ordered anion-excess fluorite superlattices. © 1992.