Zeitschrift fur Kristallographie - Crystalline Materials

Stacking disorder is a common phenomenon in phyllosilicates but its nature is difficult to be deduced using conventional diffraction techniques. In contrast, recent investigations using high-resolution transmission electron microscopy (HRTEM) have elucidated the structure of stacking disorder in various phyllosilicates, by directly observing individual layers and stacking sequences. Furthermore, simulations of X-ray or electron diffraction patterns using the information from the HRTEM results can complement the limited analysis area in TEM and quantify the density of the stacking disorder.

Although the bonding between adjacent layers is similar, there is a significant difference in the stacking disorder between two counterparts of dioctahedral and trioctahedral 2 : 1 phyllosilicates: pyrophyllite vs. talc and sudoite vs. trioctahedral chlorite. In pyrophyllite and sudoite, stacking disorder is caused mainly by two alternatives of the lateral displacement directions between the two tetrahedral sheets across the interlayer region. On the other hand, rotation of 2 : 1 layer is also an origin of the stacking disorder in talc and trioctahedral chlorite. This difference is explained by the corrugation of basal oxygen planes on the dioctahedral 2 : 1 layer formed by the tetrahedral tilting to enlarge trans-vacant octahedral sites.

Taeniolite, KLiMg₂Si₄O₁₀F₂, was synthesized from a mixture of KF, LiF, MgO and SiO₂, and the crystal structure was refined with three-dimensional x-ray diffraction data. The crystals are monoclinic (1 M type) with the space group C2/m, a = 5.231(1), b = 9.065(2), c = 10.140(1) Å, β = 99.86(2)°, and Z = 2. Refinements with the full-matrix least-squares procedure gave the final R value of 0.024 for 1303 observed reflections. All the bridging Si–O distances are 1.638(1) Å, whereas the nonbridging one is 1.586(1) Å. A comparison of several 1 M-type micas clearly indicates the effect of octahedral cations on the distortion of (Si,Al)O₄ tetrahedra. Li⁺ ions are more concentrated at the M(2) site than at M(1) in conformity with the size difference between the coordination octahedra. The ditrigonal distortion of the tetrahedral sheets is still smaller than that of KMg_2.5Si₄O₁₀F₂, having the tetrahedral rotation angle α of 1.08°. The octahedral flattening angles, ψ are 57.8° and 57.9° for M(1) and M(2) sites, respectively.

SHELXE was designed to provide a simple, fast and robust route from substructure sites found by the program SHELXD to an initial electron density map, if possible with an indication as to which heavy-atom enantiomorph is correct. This should be understood as a small contribution to high-throughput structural genomics. The new sphere of influence algorithm combined with a fuzzy solvent boundary enables some chemical knowledge to be incorporated into the density modification in a general and effective manner. In the special cases of high solvent content (greater than 0.6) or very high resolution data (better than 1.5 Å) high quality maps can be produced. This raises the possibility of a new paradigm for atomic resolution structure refinement: instead of alternating atom parameter refinement with weighted electron density maps calculated with the phases of the current model, which inevitably leads to some model bias, all model building should be based on the model free experimental density map!

The crystal and molecular structures of four colorless Cd(S₂CNR₂)₂ compounds have been determined. The crystals of Cd(S₂CNiPr₂)₂, C₁₄H₂₈CdN₂S₄, (determined at 293 K) are monoclinic, space group P2₁/n with unit cell dimensions a = 11.611(5) Å, b = 11.228(9) Å, c = 16.767(2) Å, β = 108.90(2)°, Z = 4 and D _x = 1.494 Mg m⁻³. Crystals of Cd(S₂CN(Et)Cy)₂, C₁₈H₃₂CdN₂S₄, (200 K) are monoclinic, space group P2₁/c with unit cell dimensions a = 8.541(4) Å, b = 19.688(8) Å, c = 13.847(3) Å, β = 94.29(3)°, Z = 4 and D _x = 1.479 Mg m⁻³. Crystals of Cd(S₂CNCy₂)₂ as a 1/1 CH₂Cl₂ solvate, C₂₇H₄₆CdCl₂N₂S₄, (293 K) are triclinic, space group P[unk] with unit cell dimensions a = 13.557(7) Å, b = 14.49(2) Å, c = 9.604(2) Å, α = 100.70(6)°, β = 91.55(3)°, γ = 115.58(6)°, Z = 2 and D _x = 1.421 Mg m⁻³. Crystals of Cd(S₂CNiBu₂)₂, C₁₈H₃₆CdN₂S₄, (293 K) are monoclinic, space group C2/c with unit cell dimensions a = 49.97(5) Å, b = 9.597(3) Å, c = 23.675(8) Å, β = 116.41(3)°, Z = 16 and D _x = 1.361 Mg m⁻³. The structures were solved by direct methods and each refined by a full-matrix least-squares procedure to final R = 0.059 using 3485 reflections for Cd(S₂CNiPr₂)₂; to R = 0.055 using 2849 reflections for Cd(S₂CN(Et)Cy)₂; to R = 0.079 using 3069 reflections for Cd(S₂CNCy₂)₂; and to final R = 0.060 for 4851 reflections for Cd(S₂CNiBu₂)₂. The Cd(S₂CNR₂)₂ structures adopt a common dimeric motif in the solid state consistent with that observed for related systems. In the centrosymmetric dimeric unit, each cadmium atom is chelated by two dithiocarbamate ligands, one of which forms quite disparate Cd–S distances owing to a bridge formed to the second cadmium atom. The coordination geometries are distorted trigonal bipyramidal.

At high temperatures above 200 K, significant increase in thermal conductivity (κ) was observed for the Al-based icosahedral quasicrystals and their corresponding approximants. By comparing the measured κ of the Al—Re—Si 1/1-cubic approximants with their electronic density of states at the Fermi level (ε _F) determined by the low temperature specific heat measurements, we found that the increase in κ at high temperature was most pronounced when ε _F is located at the bottom of the pseudogap. This fact indicates that the large increase in κ is not brought about by the lattice but by the conduction electrons in associated with the narrow pseudogap at ε _F. It is also found that the behavior of κ at high temperature is well accounted for in terms of the bipolar diffusion effect of conduction electrons that is generally employed to analyze κ of the narrow-gap semiconductors and semimetals.