STRUCTURAL RELATIONS BETWEEN MONOCLINIC AND ORTHORHOMBIC FLUORITE-RELATED SUPERSTRUCTURES M0.5-XLN0.5+XF2+2X SINGLE AND TWINNED PHASES

The coefficients of a general similarity matrix which allows one to relate the fluorite structure to orthorhombic, monoclinic, and triclinic superstructures are determined for the phases K0.333Ln0.667F2.34 (Ln = Y, Er, Ho, Dy, and Tb), K0.333Gd0.667 F2.34, K0.265Gd0.735 F2.47, K0.25Ln0.75F2.50 (= KY3F10), and M0.31Ln0.69F2.38 (M = K; Ln = Gd, Eu, Sm, Nd, Pr and M = Rb; Ln = La, Pr, Nd). The first four phases are either orthorhombic or monoclinic. They are characterized by the presence of chains of clusters Ln6F36 (or Ln6F37) parallel to the direction bF + cF, which alternate with chains of cubes LnF8, according to a sequence accounting for the ratio Ln M in the phase. The juxtaposition of these chains forms layers perpendicular to the direction aF. Likewise, these cubes and clusters form chains parallel to the direction [-bF + cF], but running alternatively at the levels 0, 1 2(bF + cF), 0, ...; they are connected by the polyhedra surrounding the alcaline cations, some of them forming chains parallel to the direction [bF + cF]. The fifth phase M0.31Ln0.69F2.38 is triclinic. The crystals are twinned and the model of the structure which is proposed contains only bits of chains, [cluster-cube-cluster], parallel to the direction [-bF + cF] and running alternatively at the levels 0, 1 4(bF + cF), 1 2(bF + cF), 3 4(bF + cF), 0, ..., with substitution of the chains MMM ... by chains MLnM ... A precise determination of the structure of the fifth phase is in progress. © 1991.