Volume 175, Issue 3-4

K 2 HfF 6 is monoclinic: a = 6.592(3), b = 11.485(5), c = 6.910(3) Å, β = 90.56(5)°, space group C 2/ c , Z = 4. The structure was refined to R = 0.0214 and R w = 0.0270 by least-squares methods, using 780 unique observed reflections. K 2 HfF 6 is isostructural with K 2 ZrF 6 . The structure consists of chains of [HfF 8 ]-polyhedra. The coordination geometry around the Hf atom is a distorted bisdisphenoid (or triangulated dodecahedron) of symmetry 2. The [KF 8 ]-polyhedra are intermediate between a square antiprism and a bicapped triangular prism.

The reconstructive high-temperature transformation of tridymite to cristobalite has been studied with X-ray precession photographs on single crystals. For this purpose inclusion-free and optically untwinned tridymite crystals, separated from a used refractory-grade silica brick were heat-treated between 1500 and 1620°C. The X-ray patterns of the heat-treated tridymite crystals show additional though weak reflections corresponding to those of the pseudo-orthorhombic tridymite T PsOrh-5 . Diffuse intensity parallel [001]* (pseudo-orthohexagonal setting) indicates reconstructive stacking disorder of the tetrahedra layers of tridymite. According to the X-ray pictures the newly formed cristobalite crystallites are in a definite orientation to tridymite, with [112] c //[100] T , [[unk]l0] c //[010] T , and [111] C //[001] T [orthohexagonal setting of tridymite (T), and cubic setting of cristobalite (C)]. At the beginning of the transformation a one-sided structural orientation of cristobalite with respect to tridymite is observed. With the progress of transformation a second generation of cristobalite with alternate orientation occurs. Both orientations are convertible by rotation around [111]* of cristobalite (i.e. [001]* of tridymite) by 180°. The X-ray patterns of tridymite, being completely transformed to cristobalite exhibit alternate orientations of cristobalite again. The close structural relationship between tridymite and cristobalite probably causes a transformation with preservation of structural units. The reconstructive rearrangement of the “antiparallel” tetrahedral layer sequence in tridymite to the “parallel” layer sequence in cristobalite requires rotation and shifting of the tetrahedral layers which occurs by cooperative movements of layer fragments. Complete decomposition of the tridymite tetrahedral framework, and subsequent nucleation and growth of cristobalite is less probable.

The crystal structure of KZrF 5 · H 2 O has been determined by single crystal X-ray analysis. The crystals are monoclinic, a = 7.628(1), b = 12.081(1), c = 11.100(1) Å, β = 99.62(2)°, space group P 2 1 / c , Z = 8. The structure was refined by full-matrix least-squares methods to an R value of 0.0236 for 1674 independent observed reflections. The structure can be described as built up by K + cations and [Zr 2 F 10 (H 2 O) 2 ] 2− double-polyhedron units which are linked into infinite chains parallel [100] by single F bridges. The double-polyhedron unit [Zr 2 F 10 (H 2 O) 2 ] 2− results from the association of two independent nonregular [ZrF 7 (H 2 O)] polyhedra sharing a triangular face. The chains are additionally strengthened by hydrogen bonds and are connected to layers parallel (010) by further hydrogen bonds. These hydrogen-bonded layers are held together by potassium cations located between the layers in irregular coordinations.

Similarities of the populous Cu 2 Sb and TlAsPd 5 structures (both tetragonal) are noted, and questions concerning the formation of the TlAsPd 5 variant and its building principles and MXN 5 formula are discussed. These are resolved in terms of the axial ratios ( c / a ) and interatomic distances found in phases with the TlAsPd 5 structure, and the fact that so far it has been found to be formed only with N = Pd, Pt and Ga. The structure is seen to be a Parthé-type segmental intergrowth structure, the building principle of which is a geometrical packing of the atoms.

Mr = 460.36, monoclinic. P 2 1 / c , a = 13.718(1), b = 11.090(1), c = 14.985(1) Å, β = 98.162(2)°, V = 2256.67(9) Å 3 , Z = 4, D x = 1.355 Mgm −3 , μ = 0.933 mm −1 , F (000) = 968, room temperature, final R = 0.047 ( R w = 0.048) for 3184 [ I ⩾ 3 σ ( I )] observed reflections. The title compound has the “three-legged piano stool” geometry, i.e. the coordination geometry of the iron can be regarded as a distorted octahedron with a η 5 – C 5 Me 5 group occupying three coordination sites. Like its nonmethylated analogue it contains an unusually monodentate dithiophosphate ester ligand.

The crystal structure of U 2 NiC 3 was determined from single-crystal X-ray data and refined to a residual of R = 0.029 for 530 structure factors and 34 variable parameters. It is of a new type with orthorhombic symmetry, space group Pnma , a = 6.4145(3) Å, b = 3.5457(2) Å, c = 13.2315(7) Å, V = 300.94 Å 3 , and Z = 4 formula units per cell. The two crystallographically different uranium atoms have large coordination polyhedra formed by all three constituating components. The nickel atoms have no near nickel neighbour. One third of the carbon atoms occupies octahedral voids formed by four uranium and two nickel atoms. The other two thirds form pairs (C – C-distance 1.43 Å). One of the carbon atoms of each pair has approximately octahedral coordination with five uranium and one carbon neighbour, the other carbon atom has trigonal prismatic uranium coordination with two nickel and one carbon atom as additional neighbours outside the rectangular faces of the prism. The hydrolysis of U 2 NiC 3 in hydrochloric acid yields mainly methane and ethane as well as some ethylene, propene and propane.

Using single crystal X-ray diffraction data the ternary phase Zr 3 AlN was found to have an orthorhombic unit cell a = 0.33690(5) nm, b = 1.1498(2) nm, c = 0.89825(15) nm, V = 0.34795(10) nm 3 , D x = 6.01 g cm −3 , Z = 4, F(000) = 560, space group Cmcm . Zr 3 AlN is isotypic with filled Re 3 B. The final reliability value obtained for the refinement was R = 0.042 for 281 contributing reflections.

Tris (glycine) calcium(II) dichloride crystallizes in the orthorhombic system, space group Pb2 1 a with a = 14.774(4), b = 19.573(6), c = 9.067(1) Å and Z = 8. The structure of this complex which is isomorphous with tris (glycine) calcium(II) dibromide was determined using the intensity data collected in a CAD-4 diffractometer ( R = 0.051 for 2685 Mo Kα reflections). Each of the two Ca atoms is coordinated to six O atoms and one Cl atom. The two Ca coordination polyhedra in the structure are linked distorted pentagonal bipyramids. All the hydrogen atoms have been located from a difference Fourier map. The glycine molecules exist in the crystal as Zwitterions, NH 3 + CH 2 COO − . The glycine molecules are enclosed between sheets of hydrogen bonds which stabilize the structure.

A systematic description and classification of inorganic structure types is proposed on the basis of homogeneous or heterogeneous point configurations (Bauverbände) described by invariant lattice complexes and coordination polyhedra; subscripts or matrices explain the transformation of the complexes in respect (M) to their standard setting; the value of the determinant of the transformation matrix defines the order of the complex. The Bauverbände (frameworks) may be described by three-dimensional networks or two-dimensional nets explicitely shown with structures types of the I - and F -family. Examples of structure types of different orders are listed for the homogeneous or pseudohomogeneous I -, P -, F -, Y **-, S -, + Y -, Q - and C -families and for the heterogeneous [ I + W ]-, [ D + T ]- and [ Y (31) + Dx ]-families. Homeotypic are those structure types, which replace their points of the configurations by polyhedra like dumb-bell (21), triangle (31), tetrahedron (4t), octahedron (6o), cuboctahedron (12co), icosahedron (12i) etc. forming the I [polyhedra]- P [polyhedra]- etc. families. Furthermore, layer descriptions are applied with two-dimensional nets of H , G , N , N [43 2 43] and N [6434] with different stacking sequences and in combination with other nets. The system is proposed after the study of the cubic inorganic structure types and the structures of intermetallic compounds with tetragonal symmetry.

The crystal structure of natural mordenite from Elba (Italy) with chemical formula (Na 1.49 K 2.80 Mg 0.04 Ca 2.05 Sr 0.05 )(Al 8.98 Si 39.13 )O 96 · 29.07 H 2 O was refined in its hydrated form up to a wR value of 5.2%. Systematic absences were consistent with the Cmcm space group but the asymmetry of the extraframework ions indicated as more probable the acentric subgroup Cmc2 1 . Consequently the atom O8 is no longer set on an inversion center and the T – O8 – T angle is not constrained to 180°. The Si/Al distribution is partially ordered, with an enrichment of Al in the tetrahedra of the four-membered ring. All Ca ions are localized in a eight-coordinated site; K ions alternate with water molecules in a six-coordinated site.

Silicon crystals grown in a hydrogen atmosphere were heattreated at different temperatures. With the aid of a double-crystal diffractometer rocking curves of the specimen were recorded. Only the sample heat-treated at 1000°C showed significant diffuse scattering due to clustering of hydrogen. The mean cluster radius was determined.

Both, calcium azide and strontium azide, can incorporate dimethyl sulfoxide to form crystals of the composition given in the title. The lattice constants of the isotypic calcium-(strontium-)-compounds are: a = 8.801 (8.978), b = 13.362 (13.511), c = 10.862 (11.347) Å, space group Cmca , Z = 4. The crystal structures were investigated by X-ray single crystal techniques, R = 0.055 (0.060). Calcium (strontium) is octahedrally surrounded by four azide ions and two oxygen atoms of the solvated DMSO. The octahedra are connected through four azide groups to form layers. The DMSO molecules are located between these layers, and they are highly disordered.

The structures of Na 2 TiF 6 and Na 2 SiF 6 were redetermined ( R = 0.036, 0.029). The space group C 2 was found instead of trigonal symmetry. Measurements on twinned crystals and powder showed the coexistence of different pseudotrigonal phases.

The crystal structure of ethyl 5-methyl-l,2,3-triazino [5,4-b]-indole 4-carboxylate N(3)-oxide (II), (C 13 H 12 N 4 O 3 ), has been detnrmined by X-ray diffraction techniques. The crystals are monoclinic, space-group P 2 1 / n , with a = 13.137 (1), b = 8.48 8(1), c = 12.70 9(1) Å and β = 116.63(2)°, Z = 4, D c = 1.4278 g · cm −3 . The crystal structure was solved by direct methods and refined by least-squares to R = 0.059 for 838 independent reflexions. The molecule is planar and the side-chain backbone is all-trans and presents a staggered conformation.

The crystal structure of the orientationally disordered cubic high-temperature phase of AgClO 4 has been investigated using single crystal X-ray diffraction data. The lattice constant is a = 703(1) pm (170°C), the space group is Fm 3 m , Z = 4. The ClO 4 − -anions are orientationally disordered and occupy statistically 6 equilibrium orientations, characterized by the fact that in each orientation only one of the three [unk]-axes of a ClO 4 -tetrahedron lies parallel to a cubic crystal axis. The structure can be described by choosing Cl in 4a (000), Ag in 4b (1/21/21/2) and oxygen in 96j (0 yz etc., occupation factor 6 1 ). The conventional least-squares refinement yields y = 0.120, z = 0.160. The Cl – O bond length amounts to 148(4) pm (corrected for thermal motion), the angles in the ClO 4 2212 -anion are 109(2)° and 110(2)° (corrected for thermal motion). A rigid-body-motion analysis using the isotropic temperature parameter of chlorine and the anisotropic temperature parameters of oxygen yields an angle of 17° for the libration of the C1O 4 − -anion around its [unk]-axis and of 10° around the axes perpendicular to the [unk]-axis. In order to account for the large librations of the ClO4 4 − -anions an additional structure refinement has been carried out, using a method worked out by Hohlwein (1984) for structures with orientationally disordered groups. The refinement confirms the existence of 6 equilibrium orientations of the kind described above. The oxygen parameters ( y = 0.122, z = 0.170) correspond to a Cl – O bond length of 147(3) pm. The thermal motions of the ClO 4 -group and of the Ag- and O-atoms agree well with those derived by the conventional refinement.

Interpolyhedral O – O distances in inorganic crystal structures have as a rule O – O > 2.90 Å, mostly O – O > 3.00 Å, but exceptions are occasionally found. Several such distances with O – O ≤ 2.90 Å are presented, among them three from silicates. The shortest interpolyhedral O – O distance found in a silicate occurs in phenakite, Be 2 SiO 4 , and measures 2.75 Å. It is discussed from the point of view of stereochemistry.

The study of infrared reflection spectra of SbSBr crystals necessitated redetermination of its structure and revision of its description, along the lines of the isostructural compounds BiSCl, BiSBr and SbSeI. The cell constants are: a = 8.2424(6), b = 9.7556(4), c = 3.9680(2) Å, Z = 4; space group Pnam . The Sb atom is seven-coordinated by four Br and three S atoms in the form of a composite polyhedron, linked to a symmetry equivalent one. This characteristic double polyhedron is repeated along the c axis, thus forming an infinite double chain, which is further linked to each of four similar neighboring chains. Analysis of the infrared spectra for the polarization E ⊥ c , taking into account the composite polyhedra and the chains, showed for the first time in a member of the group of compounds A V B VI C VII , where A = Sb, Bi; B = S, Se and C = Cl, Br, I, the presence of 10 lattice vibration bands along directions parallel to the plane (001) with symmetry B 2u + B 3u , exactly as predicted by group theory for the structural model in the space group Pnam , with four formula units in the cell. This indicates the existence of relatively strong forces between the double chains in the structure.