Band 167, Heft 1-2

X-ray diffraction (293 K) and neutron diffraction (60, 120, 293 K) data have been measured from SiP 2 single crystal. The lattice constants, interatomic distances and temperature parameters of Si and P atoms vary linearly in the range 60 K to 293 K. The change of the P – P bond length is very small (2.156 Å – 2.159 Å). The vibrational amplitude of the Si atom becomes 10% larger than that of the P atom at 293 K. The deformation densities ϱ X – N and ϱ X – X ) shows a strong bonding electron density (0.65e Å −3 ) at the centre of the P 2 pairs. There is a small electron density maximum (0.2 e Å −3 ) in the Si – P bond region but not located on the Si – P straight line. The results have been compared with that obtained in FeS 2 and MnS 2 .

Electron double diffraction patterns obtained from decomposing cadmium iodide and cadmium bromide crystals into cadmium in the electron beam have been reanalysed. The possible diffraction patterns considering the diffracted beams of strong {110}, {220}, and {300} reflections of CdI 2 /CdBr 2 as primary beams, besides the incident beam, for diffraction from cadmium have been drawn and compared with the experimentally observed ones. An excellent agreement, geometrically as well as from an intensity point of view, was found between them. A new simple method of indexing electron double diffraction patterns has been proposed.

Measurements of the crystal rotation angle ( ω ) for centred reflections provide the most reliable data for the refinement of unit cell parameters in two-circle single-crystal diffractometry. Zero-layer reflections are least affected by crystal misalignment, and can be used in a least-squares refinement of the corresponding reciprocal-cell parameters. The introduction of equations for a general crystal orientation allows the precise refinement of all six cell parameters from measurements of reflections on a range of layers for a misaligned crystal; only one crystal mounting is required. The possibilities of applying the general orientation algebra to intensity data collection are discussed.

The crystal structure of the title compound (1) has been determined [space group P [unk], a = 9.229(3), b = 10.568(3), c = 11.672(4) Å, α = 116.69(2), β = 70.32(4), γ = 100.68(3)°, z = 2] and refined to R = 0.069 for 2939 unique observed reflections. The planar part of the molecules from stacks with an interplanar separation of 3.47 Å.

A probabilistic procedure is described for the estimation of the onephase seminvariants of first rank in accordance with the theory of representations (Giacovazzo, 1977, Acta Cryst. A33, 933 – 944).

The electrooptic Kerr coefficient R c = | R 1122 – R 1111 | of ADP and DKDP crystals has been investigated. The results show that R c is less than 1 · 10 −19 m 2 /V 2 for ADP and 5 · 10 −20 m 2 /V 2 for DKDP crystals. The electronic contribution to R c has been theoretically estimated to be 3 · 10 −22 m 2 /V 2 for ADP and 1 · 10 −22 m 2 /V 2 for DKDP in agreement with the experimental results.

The absorption factor of a powder usually assumes a homogeneous material distribution with a locally independent absorption coefficient αμ , α : relative mass density. To correct for the influence of inhomogenity, a semiempirical correction factor κ has been developed in several steps. First a general approximation formula for κ has been derived. On the basis of the κ -formula found, a computer program has been established for a 2-dimensional powder model. It represents the grains by circles of diameters equal to the mean grain diameter [unk] and is useful to an upper limit ϑ G of the Bragg-angle, where the correction is due to the absorption in exactly the volume of a single grain. Comparison of experimental P ( ϑ )-curves of Suortti (1972) – P ( ϑ ) = 1 − κ ( ϑ ) – with corresponding model curves in the range ϑ [unk] ϑ G showed qualitative, and in some respects quantitative agreement. Therefore P ( ϑ )-formulas were sought, on one side giving good agreement with Suortti's experimental P ( ϑ )-curves, on the other reflecting the theoretical outline made before. The result is: For ϑ G , given by tg ϑ G = [unk], we have P ( ϑ G ) = μ g[unk] − αμ [unk]/sin ϑ G , g = 0.83 ± 0.01. Theoretically g [unk] means the mean absorption path in a single grain, which in our model we have g = 8/(3 π ) = 0.85. For ϑ < ϑ G we obtain P ( ϑ ) = P ( ϑ G ){1 + 0.5( μ [unk] + α )[( ϑ G − ϑ )/ ϑ G − 10°)] 3 }. The accuracy of these formulas has been estimated simply by spot checks. For ϑ G and ϑ = 20° the mean absolute deviation between experimental and theoretical P -values resulted in 3 · 10 −3 . The range ϑ > ϑ G was treated similar to the range ϑ < ϑ G . As theoretical input we used the equation P (90°) = 0. deduced by Harrison and Paskin (1964). We obtain P ( ϑ ) = P ( ϑ G ){1 − [ ϑ − ϑ G )/(90° − ϑ G )] 3 }. For completeness the equation P (90°) = 0 is discussed.

Hansagelb G has the structure formula N-Phenyl-2-(4-methyl-2-nitro-phenylhydrazono)-3-oxo-butyramide. Like other “azo”-pigments the molecule prefers – at least in solid state – the hydrazono – to the azo-tautomeric form. It crystallizes in the monoclinic space group P 2 1 / n with 4 molecules in the unit cell. It could be shown that the cell constants given in the literature are wrong. The molecules are crystallizing in layers built by endless tapes joined together like staircases.

Synthetic dodecasil 3C, 136 SiO 2 · 16 M 12 · 8 M 16 , is cubic with space group Fd 3 and a 0 = 19.402(1) Å. Structure refinement of a crystal containing M 12 = N 2 and small amounts of CH 4 and Ar, M 16 = N(CH 3 ) 3 and CO 2 led to a weighted R w = 0.064 for 883 independent reflexions. The clathrate compound is isotypic with the cubic clathrate hydrate type II. The clathrate compound has a silica host framework with guest molecules M in two different types of cage, and it is the cubic three-layer member of the polytypic series of dodecasils. Cornersharing [SiO 4 ] tetrahedra form a 3-dimensional 4-connected net which is built up from pseudohexagonal layers of face-sharing pentagondodecahedra ([5 12 ] cages), the fundamental cage of the dodecasil series. On connecting such layers in the sequence [unk]ABC two types of cagelike void arise, the [5 12 ] cage again and the larger [5 12 6 4 ] cage. The guest molecules M 12 and M 16 which must be present during synthesis show up in difference Fourier synthesis. The entrapped molecules are oriented within the cages such that van der Waals contacts between guest molecules and host framework are optimized. The mean value of the Si – O – Si angles (174.5°) is considerably higher and the mean value of the Si – O distance (1.566 Å) considerably lower than corresponding angles and distances in silica polymorphs. If aspheric guest molecules are used during synthesis, the room temperature phase is of lower symmetry than cubic. Calorimetric and hot-stage microscope experiments with dodecasil 3C containing M 16 = tetrahydrofurane show that a displacive phase transformation to the cubic high symmetry form takes place at about 110°C.

The crystal structure of the title compound was determined by single crystal x-ray diffraction. The crystals are triclinic, a = 9.338(1), b = 10.786(3), c = 11.684(2) Å, α = 92.59(1)°, β = 107.86(1)°, γ = 113.69(1)°, space group P [unk], Z = 1, d x = 2.84 Mgm −3 . The structure was solved by Patterson and Fourier methods and refined to a conventional R = 0.041 for 3101 independent Mo Kα counter reflections. Calcium is coordinated to seven azide groups, the coordination polyhedra are distorted pentagonal bipyramids. Pairs of them are related by a center of symmetry and share a common edge. The cesium ions are irregulary surrounded by azide-groups.

The program SYM (1982) for the derivation of the symmetry operations from the space-group symbols is improved. In the new program the symbol is checked for validity. In addition to the symmetry operations a group multiplication table is computed and seminvariance conditions are evaluated. Different settings of the 230 space-group types described by 541 different symbols can be generated using the origins of the International Tables or origins fixed by the user. Computer language is FORTRAN IV.

The crystal structure of Sr-gehlenite Sr 2 Al 2 SiO 7 is tetragonal with space group P [unk]2 1 m , a = 7.820(1), c = 5.264(1) Å, Z = 2 and D x = 3.821 g · cm −1 , and was refined to R = 3.6% using 345 non-equivalent reflections. This compound is confirmed to be isotypic with gehlenite Ca 2 Al 2 SiO 7 . In this Sr-gehlenite there is the same partial ordering with Al in T(1) tetrahedra and Al 1/2 Si 1/2 in T(2) ones as in natural gehlenite. This Al/Si distribution is in good agreement with those predicted through the universal bond-strength method and the one corrected for oxygen coordination. Cross-hatched and lamellar structures observed in the latter, however, were neither recognized in the former nor in synthetic gehlenite with the same Al/Si ordering, and so it is not conceivable that these lamellar structures are due to only Al/Si ordering. Judging from the crystallographic geometry of melilite isomorphs, the chemical expansion of their lattice parameters is interpreted as mainly due to the effect of rotation of the T(1) tetrahedra and enlargement of the T(2) – O(1) – T(2′) angles.

The molybdenum antimony oxide Sb 2 MoO 6 crystallizes in the triclinic space group P [unk]. Cell parameters measured on an X-ray diffractometer CAD4 were a = 7.481(2) Å; b = 7.504(1) Å; c = 10.120(2) Å; α = 70.43(2)°; β = 70.91(2)°; γ = 83.35(2)°. From neutron diffraction the following parameters were obtained a = 7.4774(2) Å; b = 7.5017(2)*; c = 10.1259(2)*; α = 70.374(1)°; β = 70.889(1)°; γ = 83.246(1)°. The structure determination is based on a single crystal X-ray diffraction study and a powder neutron diffraction profile refinement. 4124 independent reflections collected on an X-ray diffractometer lead to an R factor of 7.4%, while neutron diffraction profile refinement gave an R factor of 4.8%. MoO 6 octahedra are distorted and the neighbourhoods of the Sb atoms are described up to the 5 closest oxygen atoms. MoO 6 octahedra share corners and build sheets with the MoO 4 composition. MoO 4 sheets are separated by Sb 2 O 2 layers. This arrangement is very similar to that of keochlinite Bi 2 MoO 6 except that, due to centrosymmetry, the former compound is not ferroelectric.

EuSb 2 , which crystallizes in the monoclinic CaSb 2 -type structure of space group P 2 1 / m , orders with a collinear antiferromagnetic structure below T N = 26.2 K with the magnetic moments pointing along [010]. The magnetic space group is P a 2 1 / m (Sh 11 55 ), k = ½00. The approximate moment value is μ y = 5.7(5) μ B .

C 2 H 2 N 2 O, M = 70.051; Pnma ; a = 8.519(6), b = 5.427(4), c = 7.235(5) Å; Z = 4; D x = 1.391 g·cm −3 ; V = 334.49 Å 3 ; (Mo Kα ) = 0.71069 Å; F(000) = 144. The structure was solved with direct methods and refined by least-squares analysis to an R = 0.059 including 363 observed reflections. The molecule lies on the mirror plane and must therefore be completely planar. By two intermolecular hydrogen bridges towards the oxygen and the nitrogen atom of the carbonyl and the nitrile group respectively, neighbouring molecules of the same layer are bound together.

Crystals of the new compound LaCo 8 P 5 were prepared by reaction of the elemental components in a tin flux. They have orthorhombic symmetry: a = 10.501(3), b = 3.596(1), c = 9.342(2), V = 352.8 Å 3 , space group Pmmn , Z = 2. Their structure was determined by direct methods and refined to R = 0.040 for 49 variable parameters and 927 unique F values. It belongs to a large family of structures of which the Fe 2 P and TiNiSi type structures may be considered as the most simple representatives. Some similarities and differences of the coordination polyhedra in these structures are discussed.