Verbindungsbildung in den quasi-bin�ren Systemen AcX4?MX2 (Ac = Th, U; M = Ca, Sr, Ba, Eu, Ge, Sn, Pb; X = Br, I)

Coloring variants of the Re3B type

The Re3B type, space group Cmcm, has boron-centered trigonal prisms as central building units and is one of the basic structure types with numerous binary and ternary representatives. The coloring of different atoms on the two crystallographically independent rhenium sites leads to a manifold of compounds with different bonding peculiarities that are rather isopointal than isotypic with the prototype. Typical compounds are the S-phase precipitate MgCuAl2, the silicide ScPt2Si or the iodide Th0.5Pb0.5I3 (PuBr3 type). Differences in size or composition might force symmetry reductions. This is discussed for YZn3 (space group Pnma) and the different coloring variants ScRhSi2 and TaNi2P which show different twists of the trigonal prisms. Striking singular representatives with lower symmetry structures are BaThBr6 (Pmma) and NbCo2B (P21/c) which allow different ordering/distortion patterns for the prisms. All these crystal chemical details are discussed on the basis of group subgroup schemes (Bärnighausen trees).

High-temperature monoclinic α-SrHfF6, and isostructural α-SrZrF6: associating Hf2F12 bipolyhedra and SrF8 snub disphenoids

The structure of the high-temperature monoclinic variety α-SrHfF₆ (strontium hafnium hexafluoride) [and of isostructural α-SrZrF₆ (strontium zirconium hexafluoride)] associates Hf₂F₁₂ bipolyhedra and SrF₈ snub disphenoids, forming zigzag twisted [SrF₆]_n layers. The distribution of the Hf and Sr polyhedra forms a three-dimensional framework which can be related to the family of anion-excess ReO₃-related superstructures. α-SrHfF₆ corresponds to a new ABX₆ type and is compared to the other main families already described. A partial amorphization of this structure is observed in samples quenched from the melt.