Volume 209, Issue 8

Using Pólya theory of colorings and the cycle indicators of the corresponding color-permutation groups, all the proportional colored symmetry structures are derived from the colored symmetry groups of friezes.

The strong focusing of neutron beams by reflection from two bent crystals in antiparallel (+, +) setting is discussed. A previous analysis made for thin perfect crystals is extended, by numerical computations, to the case of a thick and mosaic first crystal. It is concluded that microfocusing is also achievable with flat mosaic first crystals. The combination of strong beam focusing onto sample with narrow diffraction lines in a given limited range, which is a requirement in stress scanning, is further examined. Results of test measurements, with a flat mosaic thick silicon as a first crystal and pneumatically bent thin silicon as a second crystal, are presented. The strong focusing of the beam onto sample is confirmed to be compatible with the focusing in scattering. The feasibility of microfocus diffraction on existing three-axis instruments with flat mosaic monochromators is also confirmed. Techniques for plastic bending of high-reflectivity crystals will have to be developed to take full advantage of microfocusing.

The Na 4.5 FeP 2 O 8.5 F 0.5 (NFP) and Na 4 TiP 2 O 9 (NTP) crystals are superionic conductors with Na + ion conductivity. These compounds are isostructural, but the symmetry changes taking place during the phase transition in the same temperature range (520K– 540K), are essentially different. For NTP crystals, the phase transition in the range 520K-540K is reduced to the disordering of the Na sublattice and gives rise to a symmetry from monoclinic (sp. gr. P [unk], a = 8.631(1) Å, b = 7.541 (1) Å, c = 7.049(1) Å, = 116.10(2)°, with wave vector [unk] = 0.2[unk]+0.2[unk]*) to orthorhombic (sp. gr. Ibam , a = 15.647(8) Å, b = 14.989(8) Å, c = 7.081(5) Å). Moreover, the geometrical configuration of the conductivity channels is considerably changed. Also, the site occupancy coefficients of Na atoms are changed at the phase transition. For NFP crystals, the phase transition in the same temperature range leads to a symmetry change from Ibam ( a = 15.520(1) Å, b = 14.820(1) Å, c = 7.119(1) Å) to sp. gr. I [unk] ( a = 15.645(1) Å, b = 14.866(1) Å, c = 7.144(1) Å, [unk] = 4/15[unk]*). Above and below this phase transition, the Na sites are only statistically occupied. The site occupancy coefficients of Na atoms and the geometrical configurations of the main conductivity channels are not essentially changed at the phase transition.

The crystal structures of two phases in the neodymium-gold system were determined by single crystal X-ray diffractometer methods, λ (Mo K α = 0.7107 Å. Nd 3 Au 4 ( hR 42) is trigonal, R [unk] (n. 148), a = 13.739(3) Å, c = 6.088(2) Å, Z = 6, R = 0.083 with 405 reflections, isotypic with Pu 3 Pd 4 . Nd 17 Au 36 ( tP 106) is tetragonal, P 4/ nmm (n.129), a = 15.835(4) Å, c = 9.278(2) Å, Z = 2, R = 0.082 with 540 reflections. The structure of Nd 17 Au 36 , which represents a new type, can be described as packing of bicapped square antiprisms with vertices occupied by five Nd and five Au atoms and centered by part of the gold atoms. This kind of polyhedron is identical to that of aluminium in the pyramidal site of the BaAl 4 structure. In the Nd–Au system the phase Nd 17 Au 36 replaces the phase given as “NdAu 2 ”.

Magnesium aluminium phosphate, MgAlPO 5 , is monoclinic with space group P 2 1 / c , a = 7.111(2) Å, b = 10.362(3) Å, c = 5.455(3) Å, β = 98.38(5)°, Z = 4 and D calc = 2.71 g/cm 3 . Single crystals were prepared synthetically. The structure was refined to R (unweighted) = 0.067 and P (weighted) = 0.040 using 1132 nonequivalent observed reflections. The structure contains Mg 2+ in fivefold coordination by oxygen in the form of a distorted trigonal bipyramid.

A high-pressure modification of tungsten dioxide (hp-WO 2 ) was synthesized at 80 kbar and 1120 K. The structure was determined from X-ray powder diffraction data: hp-WO2, M = 215.85, space group Pnma, a = 9.7164(3) Å, b = 8.4380(3) Å, c = 4.7564(2) Å, Z = 12, D x = 11.03 g cm −3 , R wp = 0.12 and R B = 0.078 from Rietveld refinement. Although the heavy atom structure may be reasonably well described in space group Cmcm, the total structure can only be described in Pnma. The important information contained in non observable and very weak reflections is discussed. In the hp-WO 2 , modification the W atoms occupy half of the octahedral interstices of an hcp arrangement of oxygen atoms. It can be described as a distorted rutile structure which is twinned at the unit cell level. An essential feature is that groups of three edge-sharing WO 6 octahedra are formed at twin planes. The relation between the ambient-pressure (distorted rutile type) and the high pressure modifications is discussed.

The title compound 3 is the product of an attempted twofold Bischler-Napieralski cyclization of the bis-amide 2 of malonic acid and homoveratrylamine. It crystallizes from methanol monoclinically in the space group C/c with a = 901.1(1) pm, b = 2749.1(5) pm, c = 907.2(1) pm and β = 104.68(1)°.

The chalcones derived from acetonaphthones are potential germicides, fungicides and bactericides. The title compound (C 19 H 13 NO 3 ) is synthesized and the crystal structure has been solved. The absorption peaks corresponding to carbonyl, CH, trans-H and −NO 2 groups have been identified from infrared spectrum. Crystals belong to monoclinic space group P 2 1 / n ; the unit cell parameters are a = 16.157(2) Å, b = 3.882(1) Å, c = 24.224(2) Å and β = 106.36(1)°, V = 1457.8(4) Å3, μ = 0.73 mm −1 and D cal = 1.38 Mgm −3 . The structure is solved by direct methods and refined by full-matrix least-squares procedures. The C = O group is in s-cis conformation. The nitro group is coplanar with the phenyl ring to which it is attached. The molecules are stabilized by van der Waal's forces.

The structures of the [2.3]paracyclophane 1 , the [3.3]paracyclophane 2 and the [4.4]paracyclophanes 3 and 4 show a gradation of structural distortions that are typical of paracyclophane systems, the steric effects of the forced bridgehead proximity becoming less as the bridges become longer. For bridges of equal length, the rings are parallel but slightly rotated with respect to each other. Unequal bridges cause the rings to tilt. The central bond of the four-atom bridges is significantly shorter (ca. 1.52 Å) than a standard C–C single bond.

The crystal structure of C.I. Solvent Brown 1, C 16 H 14 N 4 , has been determined by single crystal X-ray diffraction techniques. It crystallises in the orthorhombic system with a = 12.771(1) Å, b = 13.021(1) Å, c = 16.244(3) Å, space group Pcab , (No. 61), Z = 8. The structure has been solved by direct methods and least-squares refinement has been completed on three dimensional data (2561 reflections, MoK α , radiation). The hydrogen atoms have been found but only their positional parameters refined. Final residual 0.096 (all intensity data). The molecule exists as an azo-compound in the trans configuration. The molecules are packed in columns parallel to the a axis, but, unusually, the molecular plane is almost parallel to the column axis. The molecules are linked by one hydrogen bond per molecule almost parallel to the b axis.

In [(CH 3 )(C 6 H 5 )SnCl 2 · H 2 O] 2 ·18-crown-6 C 26 H 44 Cl 4 O 8 Sn 2 (triclinic, P [unk], a = 9.727(1) Å, b = 10.236(2) Å, c = 11.077(2) Å, α = 64.22(2)°, β = 87.42(2)°, γ = 62.65(2)°) a chlorine bridge (Sn–Cl = 3.403(3) Å) links two adjacent (CH 3 )(C 6 H 5 )SnCl 2 · H 2 O units together to form a ccntrosymmetric dimer having an Sn–Cl … Sn–Cl ring; the tin atom occupies a distorted trans -octahedral (C–Sn–C = 147.8(1)°) site. The coordinated (Sn–O = 2.363(2) Å) water molecule forms twin, bifurcated hydrogen bonds with the crown ether molecule.