Cluster self-organization of Li- and TR-Containing silicate systems: Suprapolyhedral precursors and self-assembly of LiY[7]SiO4 and LiTR[6]SiO4 (TR = Y,Sc, Tm) dimorphic crystal structures

The combinatorial and topologic analysis (the TOPOS 4.0 program package, the coordination sequences method) is carried out for silicates LiY^[7]SiO₄ (a synthetic phase (SIN), space group P2₁/b) and LiTR^[6]SiO₄ (TR = Y, Sc, Tm; the olivine structure type (OLI), space group Pnma). The framework of these silicates is built of polyhedral structural units YO₇ + SiO₄ and YO₆(ScO₆, TmO₆) + SiO₄, respectively. The framework structures TRTO₄, 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 2D Y,Si-net is the four-nodal net Y(1) (43333) + Y(2) (4334334) + Si(1) (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 LiTRTO₄, polyhedral precursor clusters Li₂TR₂T₂ 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(bar 1). Precursor clusters Li₂TR₂T₂ through the matrix self-assembly mechanism control the evolution of high-level crystal-forming clusters. Cluster coordination numbers in layers are equal to six.