No abstract

No abstract

Ternary BaAl4-type derivative stluctures are frequently found with rare-earth-transition-metal silicides and germanides with the rare-earth atoms occupying the Ba sites and the two other kinds of atoms distributed in ordered fashion on the A1 sites. The possible ternary BaA14-type derivative structures with the same unit cell as BaA14 are investigated. If it is assumed that transition-metal-transition-metal contacts are to be excluded only seven hypothetical BaAl4-type derivative structures are possible. Three of these are already known as ThCr2Si2, CaBe2Ge 2 and BaNiSn 3 types. Segments of these three structures and of other BaAl4-type derivative structures are found in various intergrowth structures. The BaA14-type derivative structures can be classified according to the type of coordination polyhedron around the transition metal, which can be a tetrahedron or a square pyramid. The tendency of the transition metal to have one or the other coordination polyhedron determines which BaAl4-type derivative structure is formed and explains also the occurrence of the HfFe2Si 2 and U2C03Si 5 types -BaAla-type derivative structures with larger unit cells.

No abstract

No abstract

Abstract. ScRh3SiT: a = 7.5056 (6), c = 19.691 (4)/k, D x = 5.706 Mg m -3,/l(Mo Ka) = 9.6 mm -1, F(000) = 1524; Sclr3SiT: a = 7.5010 (2), c = 19.909 (1)/k, Dx = 8.403 Mg m -3, g(Mo Ka) = 67 mm -1, F(000) = 2100. ScRhaSi 7 and SclraSi7 crystallize with a new rhombohedral structure type in space group R3c, Z = 6. For ScRh3Si 7 (Sclr3SiT) the final R = 0.037 (0.072) for 272 (250) reflections. The Rh and Sc arrangement found in ScRh3Si 7 is similar to that found in the PdFa-type structure. Sc (Pd) are at the centres of octahedra formed by a hexagonal close-packed array of Rh (F) atoms. The crystal structures of ScRh3Si 7 and Sclr3Si7 are stabilized by strong Rh-Si and Ir-Si interactions with each Si surrounded by three Rh or Ir atoms.

No abstract

Partheite is a zeolite-like mineral of composition Ca 2 Al4Si 4 Oi5(OH) 2 ·4Η 2 0. It crystallizes with monoclinic symmetry, space group C2/c, α = 21.555(3), b = 8.761(1), c = 9.304(2) Ä, 0 = 91.55(2), Z=4, £>", = 2.39 g/cm 3 , D x = 2.41 g/cm 3 . Its structure consists of Γ-centered T0 4 tetrahedra (Γ= Al,Si) which are connected via corners to a 3-dimensional framework. Due to the presence of hydroxyl groups the framework is interrupted at every second A10 4 tetrahedron. Water molecules and Ca atoms are situated in large channels which cross the structure parallel to c. The channels are delimited by zig-zag chains of nearly flat 10-membered rings. The framework also contains 8-membered rings and two types of 4-and 6-membered rings of oxygen tetrahedra.