Cluster Self-Organization of Li- and TR-Containing Silicate Systems: Suprapolyhedral Precursors and Self-Assembly of LiYSiO_4and LiTRSiO_4 (TR = Y, Sc, Tm) Dimorphic Crystal Structures

摘要

The combinatorial and topologic analysis (the TOPOS 4.0 program package, the coordination sequences method) is carried out for silicates LiYSiO_4 (a synthetic phase (SIN), space group P2_1/b) and LiTRSiO_4 (TR = Y, Sc, Tm; the olivine structure type (OLI), space group Pnma). The framework of these silicates is built of polyhedral structural units YO_7 + SiO_4 and YO_6(ScO_6, TmO_6) + SiO_4, respectively. The framework structures TRTO_4, built of Y,O- and Si,O-polyhedra in SIN and OLI, respectively, are represented as three-dimensional (3D) Y,Si-nets with distal oxygen atoms. The crystal-forming 2DY,Si-net is the four-nodal net Y(l) (43333) + Y(2) (4334334) + Si(l) (434) + Si(2) (3343) for SIN and the two-nodal net Y 443333 + Si 4433 for OLI. In the 2D Y,Si, nets, equivalent chains of four-nodal precursor clusters Y-Si-Y-Si with three shared bonds are identified. In the dimorphic crystals structures of LiTRTO_4, polyhedral precursor clusters Li_2TR_2T_2 are linked into a ring with two Li atoms lying one above the other below the center of the ring, thus retaining the center of symmetry 1. Precursor clusters Li_2TR_2T_2 through the matrix self-assembly mechanism control the evolution of high-level crystal-forming clusters. Cluster coordination numbers in layers are equal to six.