Band 145, Heft 1-2

The crystal structure of N,N′-disalicylidene-meso-1,2-diphenylethylenediamine (C 28 H 24 N 2 O 2 ) was determined directly by means of the symbolic addition method. The crystals are orthorhombic: Pbca – D 2 h 15 with the lattice constants a = 9.578, b = 14.311, c = 16.949 Å, Z = 4, d c = 1.202 g cm −3 . The least-squares refinement by block-diagonal matrices and anisotropic temperature factors for nonbydrogen atoms yielded an R value of 5.2% for 1470 reflections with F ≥ 3. The moelcule shows an inner molecular hydrogen bond O–H…N.

All twelve components of the quadratic electrooptic tensor of calcium formate, point symmetry mmm , have been determined. The effect exhibits the same order of magnitude as in other crystals which had been investigated earlier. The total electrooptic effect is such that an electric field applied to the crystal causes a reduction of the indices of refraction. It is shown that the three constituents of the total effect, namely true electrooptic effect, electrostrictive-elastooptic effect, and effect resulting from the attraction of the electrodes possess the same order of magnitude. True effect and electrostrictive-elastooptic effect have opposite signs. This difference is qualitatively interpreted by the reduction of total polarizability and the electrostrictive volume contraction, which both are caused by an electric field.

The structure of Cu 2 As 3 O 6 CH 3 COO ( a = 9.93 Å, b = 9.40 Å, c = 5.48 Å; β = 102.0°; space group P 2 1 / m – C 2 h 2 ; Z = 2) has been solved from three-dimensional intensity data. The basic structural information was derived from a three-dimensional Patterson synthesis followed by a three-dimensional Fourier synthesis. Cu atoms coordinate with five oxygen atoms to form tetragonal pyramids which are joined by common edges and corners to produce chains parallel to [010]. These chains, together with the As 3 O 6 rings and the acetate groups, bound only to Cu atoms, form sheets parallel to (100).

The crystal structure of the mineral trimerite was determined and refined to R = 0.033. The crystals are monoclinic, space group P 2 1 / n , with four formula units per unit cell of the dimensions: a = 8.09 8 Å, b = 7.61 3 Å, c = 14.06 5 Å, β = 90°. The crystals are triple twins with composition planes (110) and (1[unk]0). An interpretation is given by aid of the crystal structure. Trimerite is composed of an ordered Be,Si tetrahedral framework with two different sorts of channels in the direction of the b axis containing the Ca and Mn cations. The structure is derived from the beryllonite structure.

Powder data are given for the chlorine derivatives of two azo pigments, namely 1-(2,5-dichlorophenylazo)-2-hydroxy-3-naphthoic acid 4-chloro-2-methoxyanilide (chlorine derivative of C.I. Pigment Red 9) and 1-(2,5-dichlorophenylazo)-2-hydroxy-3-naphthoic acid 4-chloro-2,5-dimethoxy-anilide (chlorine derivative of C.I. Pigment Brown 1). The patterns have been indexed using single-crystal cell dimensions; the problem of multiple indexing has been reduced by comparison with observed structure factors from single-crystal data.

Nyerereite has the idealized composition (Na 0.82 K 0.18 ) 2 Ca(CO 3 ) 2 with significant minor substitution of SO 4 2− , PO 4 3− , F − for CO 3 2− , optical properties α = 1.5112, β = 1.5333, γ = 1.5345, 2 V α = 29°, and density 2.541 g cm −3 . The average unit cell has space group Cmc 2 1 , dimensions a = 5.044, b = 8.809, c = 12.743 Å and contains four formula units. Non-Bragg reflexions have h = n + mδ where n and m are integers and δ = 0.383 and indicate a structural modulation. The natural mineral is an inversion product from a high-temperature hexagonal polymorph, high nyerereite with a = 5.05, c = l2.85 Å and space group P 6 3 mc . The average structure of the natural mineral (low nyerereite) is discussed and the structure of high nyerereite is inferred. The high-nyerereite structure is the aristotype for all phases except buetschliite in the system Na 2 Ca(CO 3 ) 2 –K 2 Ca(CO 3 ) 2 . Mineralogical nomenclature in this system is discussed.

The magnetic structure of NiWO 4 has been determined from neutron-diffraction measurements on single-crystal specimens. It is a collinear anti-ferromagnet in which the nickel ions have moments of 2.25 ± 0.04 μ B at 19.4 ± 2.6° to the c axis in obtuse β . The form factor of the Ni ++ ions follows closely the free-ion form factor. CoWO 4 is shown to have an isomorphous magnetic structure.

Because of their high symmetry, most of the silica polymorphs contain only a few nonequivalent Si–O distances. However, the structure of coesite contains two nonequivalent silicate tetrahedra, eight Si–O bonds, five Si–O–Si and twelve O–Si–O angles, thus making it an excellent candidate for testing Si–O bonding theory. In order to obtain the best possible set of Si–O distances, we refined the structure using single-crystal diffraction data from a synthetic crystal (unweighted R = 0.024). Cell parameters for this crystal are a = 7.135(1) Å, b = 12.372(1), c = 7.173(1), and β = 120.36(1). A Mulliken population analysis was calculated with all Si–O bond lengths clamped at 1.614 Å and using the valence angles determined by A raki and Z oltai (1969) for which R = 0.076, and those obtained from the present experiment. The Si–O bond overlap populations calculated using the valence angles obtained for the new coordinates were found to be more highly correlated ( r 2 = 0.91 vs . r 2 = 0.60 for the older data) with the observed Si–O bond lengths, suggesting that precise structural data are necessary to obtain a near-linear correlation between bond-overlap population and bond length. The individual Si–O bond lengths to the oxygen in the Si–O–Si linkages in coesite vary (1) non-lineary when plotted against Si–O–Si angle and (2) linearly when plotted against −1/cos [unk] (Si–O–Si), shorter bonds being associated with wider angles. Shorter bonds also tend to be involved in the wider O–Si–O angles. The results of this experiment show that there is no evidence for departure from stoichiometry, that there is no unusual thermal vibration of the oxygen atom in the linear Si–O–Si linkage, and that variations in individual Si–O distances are closely related to calculated bond-overlap populations, to the geminal nonbonded repulsions and to the charge of the oxygen atom.

The crystal structure of an homogeneous single crystal of composition K 0.58 Na 0.42 AlSi 3 O 8 (Or58), C 2/ m , a = 8.434(1); b = 13.015(2); c = 7.1717(9) Å; β = 116.09(1)°; Δa = −0.11 Å (S tewart and W right , 1974), grown from a melt in the system Ab–Or–H 2 O at 700°C and 2.5 kbar, has been refined by least-squares and Fourier methods to a weighted R of 0.029. Structural and chemical homogeneity were verified, within the limits of resolution of the instruments, by transmission electron microscopy and electron-microprobe analysis. The best fit between experimental data and model was obtained using a structural model which split the alkali site into two half-sites, M(1) and M(2), with an ordered arrangement of K and Na, M(1) = 0.5 K, M(2) = 0.08 K + 0.42 Na, and assumed anisotropic thermal motion. Although we are not able to distinguish between the effects of thermal motion and positional disorder because of the small separation (0.08 Å) between the two half-sites, we favor the interpretation that K occupies a single site near the center of the alkali cavity whereas Na occupies two or more sites in the b , c plane displaced from the K position, as was found to be the case for Na in high albite (R ibbe et al ., 1969). The orientation of the vector [14 · 0 · 1], separating M(1) and M(2), is approximately parallel to the normals of ([unk]01) and ([unk]01), the planes along which K and Na are commonly intergrown. We therefore suggest that K, Na ordering, as described above, is a possible precursor to the unmixing of intermediate composition alkali feldspars. A linear trend exists between mean M–O distance and composition for six Al, Si disordered alkali feldspars including Or58, suggesting that structural features can be extrapolated from existing knowledge of end-member alkali-feldspar structures. A simple configurational-entropy calculation shows that the potential contribution of an alkali-site order-disorder relationship to S configurational ] (~ 1.3 cal deg −1 mol −1 ) is roughly one third of that resulting from Al, Si order-disorder.

The crystal structure of zinc Perchlorate hexahydrate resembles that of lithium Perchlorate trihydrate (space group: P 6 3 mc ) as far as the perchlorate-water arrangement is concerned. The distribution of zinc atoms among twice their number of available sites gives rise to orthorhombic symmetry (space group: Pmn 2 1 ), and crystals of this salt grow as three-component twins, in which the perchlorate-water arrangement is continuous throughout a crystal, discontinuity at the twin boundaries being only in the distribution of zinc atoms. Moreover, in the odd-layer Weissenberg photographs from rotation about the c axis, certain spots containing contributions from zinc atoms only streak out to form a hexagonal network. This observation has been explained in terms of mistakes in repetition along the b axis of the orthorhombic cell of each of the three twin components; the probability of such mistake has been estimated quantitatively from the intensity distribution of a suitable diffuse streak.

Powder data are given for the polymethine dye, 1,3-bis(dimethylamino)-trimethinium Perchlorate. The pattern has been indexed from the single-crystal dimensions; the problems of multiple indexing have been reduced by comparison with observed structure factors from single-crystal data.

The unit-cell dimensions of the compounds with the formula M(C 5 H 5 N) 2 Ni(CN) 4 , where M = Mn, Fe, Co, Ni, Cu, Zn and Cd, were determined from x-ray powder-diffraction patterns. The already existing results of the single-crystal structure analysis of the cadmium compound were used to index the powder patterns. The similarity of the powder patterns and of the lattice constants suggests that these compounds are the members of an isomorphous series.