Band 214, Heft 6

Two high-temperature phases were found in Al-Rh at compositions close to Al 5 Rh 2 . One can be indexed assuming a primitive cubic unit cell of a = 0.7680(2) nm and another, at a slightly lower Al content, by assuming a primitive monoclinic unit cell of a = 1.0309(5) nm, b = 0.3808(2) nm, c = 0.6595(3) nm and β = 102.42(3)°. The formation of the low-temperature hexagonal Al 5 Rh 2 phase reported earlier was confirmed.

In deriving all sphere configurations which may occur with symmetry R [unk] m – . m 36 topological different types were found. The sphere configurations of 19 types disintegrate into disconnected partial configurations: subunits with point-group symmetry occur four times, such with rod-group symmetry or layer-group symmetry eight times or five times, respectively. Seventeen types correspond to homogeneous sphere packings, and two types to interpenetrating sphere packings. In addition, the possible deformations of all homogeneous sphere packings in the subgroups R [unk], R 32, and R [unk] c of R [unk] m were investigated. Structural examples that can be described with the aid of sphere packings are given.

Single crystal X-ray diffraction experiments on α -quartz (trigonal, space group P 3 1 21) were performed to pressures up to 19.3 GPa using synthetic samples. Volumina of the unit cell were determined to 13.1 GPa, by combining the data with data taken from the literature; the bulk modulus and pressure derivative calculate to 38.7(3) GPa and 5.2(1) respectively according to a Birch-Murnaghan equation of state. Intensity data were collected at 10.9(1), 12.0(1), 12.1(1), 12.6(1) and 13.1(1) GPa. From the five intensity data sets, one was collected using synchrotron radiation at HASYLAB/DESY and the other four using a conventional X-ray tube. Results show that the SiO 4 tetrahedra, the building blocks of the structure, become increasingly distorted as pressure is increased but no significant change in the polyhedral volume is observed. The volume reduction is mainly compensated by the decrease in the inter-tetrahedral Si–O–Si-angle, which is correlated to the increase in the tilt angle φ . In addition to the five intensity data sets collected, X-ray diffraction experiments were carried out up to 19.3 GPa in order to address the question of when the structure undergoes amorphization. It was observed that up to 19.3 GPa, no pressure-induced amorphization takes place. This result is in agreement with studies carried out on powder quartz and theoretical studies.

The high pressure behavior of ruby ( α -Al 2 O 3 doped with Cr 3+ ) was investigated to pressures up to 31 GPa. Whereas data on compressibility has been obtained at pressures up to 175 GPa, no structure determination exists above 9 GPa. Results show that the highly incompressible material has a bulk modulus of K = 242(8) GPa and its pressure derivative K ′ is calculated to be 6(1). In order to compensate for the applied forces, the MO 6 octahedra (M = Al, Cr) volume decreases with increasing pressure. At pressures above 20 GPa, a distinct change in the oxygen positional parameter is observed. The reason might be a short M–O distance of 1.814(2) Å which stays constant throughout the pressure range investigated. A comparable effect of a considerable change of the internal structure under high pressure despite a very low compressibility was recently observed for the mineral spinel [1].

New ternary phosphide Ni 7 Si 2 P 5 has been synthesized by direct interaction between powders of pure components at 1070 K and its crystal structure has been determined using X-ray single crystal method. The structure is of a new type with space group Pbcn (no. 60) and lattice constants as follows, a = 23.84(2) Å, b = 5.925(2) Å, c = 4.922(1) Å, Z = 4. The final residuals are R F = 0.048, R w = 0.044 for 362 symmetry independent reflections (diffractometer KM-4, Mo K α -radiation). Crystal structure of Ni 7 Si 2 P 5 is closely related to Ni 2 SiP and Ni 5 Si 2 P 3 structures which belong to the series of linear inhomogeneous structures M 2 m +2 n X 2 m +2 n (M and X correspond to metal and non-metal atoms, respectively) and are derivatives from the MnP-type structure.

We report on the synthesis and thermal decomposition of Th 2 X 5 (X = S, Se) compounds and the structure refinement of Th 2 Se 5 by Rietveld procedures. The latter compound crystallizes in the space group Pcnb with the lattice parameters: a = 792.23(4) pm, b = 793.75(4) pm, c = 1071.55(3) pm ( Z = 4). There is no homogeneity range in such compounds. Under high vacuum conditions Th 2 S 5 and Th 2 Se 5 decompose between 873 K and 1073 K into ThS 2 and ThSe 2 , respectively, which crystallize in a PbCl 2 type structure. The crystal chemistry of An 2 X 5 compounds (An = Th, U, Np; X = S, Se) is discussed and an explanation is given for contradictory descriptions of their structures in the literature. The common orthorhombic structure type may be explained as a distortion of a higher symmetric tetragonal arrangement by the formation of X 2 2− dumb-bells, i.e. alternating short and long X–X distances in a linear chalcogenide chain. It may be shown by an analysis of Cheshire groups that seemingly different sets of positional parameters are equivalent descriptions of the same structure.

Neutron, synchrotron and X-ray high pressure powder diffraction experiments on synthetic leucite, Rb 2 [ZnSi 5 O 12 ], were performed up to the maximum pressure of 22 GPa. The linear volume compressibility b = 38(2) GPa is similar to the value of analcime. A pressure-induced phase transition was found to appear at abut 0.45 GPa. The transition is reversible, proper ferro-elastic and discontinuous. The spontaneous strain scales to an order parameter having E g symmetry. Above 4.5 GPa the structure transforms into an amorphous phase.

Sixtyseven structure factors for a spheroidal, single crystal of diamond (max diam = 0.052 cm, min diam = 0.040 cm) were measured by XRD with synchrotron radiation and λ = 0.4 Å. The data were corrected for isotropic extinction from measurements taken with λ = 0.7 Å. The whole sphere of reflections, up-to a resolution of 0.27 Å, was observed. The esd's of the structure factors were of the order of 0.006 e/atom and 0.002 e/atom for the low (except 111) and high order reflections, respectively. Despite of the large size of the specimen, the observed data agreed better with published theoretical structure factors than previous experimental structure factors. It is the first complete XRD single crystal data-set since 1939.

The crystal structure of bis(phenylthio)acetylene, C 14 H 10 S 2 , was studied by X-ray diffraction at 87 K. The space group is P 2 1 / c , a = 14.6790(8) Å, b = 5.6481(3) Å, c = 14.7115(11) Å, β = 105.60(1)° from repeated centrings on resolved Mo K α 1 peaks, Z = 4. Intensity data were collected with Mo K α radiation on three different crystals, LO data with (sin θ )/ λ < 0.714 Å −1 on one crystal, and HO data in the (sin θ )/ λ range 0.682–0.857 Å −1 on two different crystals. The standard corrections were applied to the data, including corrections for absorption, before merging. Structure solution was by direct methods, refinement based on the full data set, 5757 F o converged at R w = 0.036, a separate refinement including only non-hydrogen atoms and based on 3206 F o with (sin θ / λ > 0.65 Å −1 gave a final R w = 0.031. The atomic sequence –S–C≡C–S– is distinctly nonlinear, the S–C (acetylene) bonds, average 1.6647(9) Å, have significant double bond character, and the central C≡C bond, 1.2224(12) Å, is about 0.04 Å longer than expected for a vicinally substituted carbon-carbon triple bond. The pattern of angular deviations from 120° in the phenyl rings shows that the S atoms are electronegative substituents combining σ -electron withdrawal with some π -electron donation. Both phenyl groups are nearly eclipsed with the acetylenic moiety which gives rise to two short intramolecular H(phenyl)…C(acetylene) contacts, 2.49 Å and 2.59 Å.

Single crystals of H 4 [Mo(CN) 8 ] · 2 O(C 2 H 5 ) 2 · CH 3 OH · 2 H 2 O have been obtained by adding diethylether to a solution of H 4 [Mo(CN) 8 ] in methanol. The needle shaped crystals are triclinic with space group P [unk] and lattice constants a = 1073.2(1) pm, b = 1119.1(1) pm, c = 1199.1(1) pm, α = 84.41(1)°, β = 78.28(2)°, γ = 76.38(2)°. The molybdenum atom has a distorted square antiprismatic environment of eight cyanide ligands. The substance itself can be considered as a coordination polymer with a double N–H–N linkage incorporating hydrogen bonded oxonium ions as well as methanol and diethylether molecules.

The three-dimensional crystal structure of a commercial paraffin wax has been determined from electron diffraction intensity data collected from epitaxially oriented samples. The orthorhombic space group is A 2 1 am , where a = 7.46(5) Å, b = 4.97(1) Å, c /2 = 41.2(1) Å. On average, the lamellar packing resembles the B-form of n -C 31 H 64 . Although disorder occurs near the lamellar surfaces, as indicated by fractional atomic occupancies, the final structure ( R = 0.19, based on 38 unique hkl intensities) is still very well-ordered.

The bicyclic sulfamidite (C 17 H 17 NO 2 S) ( P 2 1 2 1 2 1 , a = 9.735(3) Å, b = 16.27(1)Å, c = 9.467(6) Å, Z = 4, R = 0.037), shows a R configuration of the S atom, deduced from the known absolute S configuration of the asymmetric carbon in the starting prolinol derivative. This is also deduced from proton 1 H NMR. The molecules form intramolecular C–H … O bonds.