Cluster self-organization of Li-and TR-containing germanate systems: Suprapolyhedral precursors and self-assembly of LiScGeO4 and LiHo3(GeO4)2F2 crystal structures

The combinatorial and topologic analysis (the TOPOS 4.0 program package, the coordination sequences method) is carried out for the crystal structures of the orthogermanates Li^[6]Sc^[6]Ge^[4]O₄ (the olivine type, space group Pnuma) and Li^[6]Ho^[8]Ho ₂ ^[7] (GeO₄)₂F₂ (space group I2/c). The same type of 2D TR,Ge-net with the Schläfli index of 3⁴4² + 3²4² and the site ratio of TR: Ge = 1: 1 is discovered for both structures. Four-polyhedral ring precursor clusters (TR)₂T₂ are identified using the two-color decomposition of structural graphs. All the clusters have a symmetry center; they differ in the types of TR polyhedra (ScO₆ and HoO₆F), which are linked through GeO₄ orthotetrahedra into a ring. The Li atoms reside above and under the centers of the clusters. The Li₂(TR)₂T₂ clusters determine the formation of crystal-forming clusters of a higher level by means of matrix self-assembly. The coordination number of the precursor clusters in the 2D net is six, which is the highest possible value.