Band 128, Heft 1-2

Magnetite and sulfur, when heated together for 1 to 30 days days in silica tubes, where vapor is always present, react to form pyrite and iron-deficient magnetite (Fe 3– v O 4 , 0 < v ≤ 1/3) when S/Fe 3 O 4 ≤ 2/3 (at. ratio). For 2/3 < S/Fe 3 O 4 ≤ 7/4, FeSO 4 occurs as an additional phase. In this magnetite a measurable fraction of the vacancies is found to occupy tetrahedral sites in the inverse-spinel structure, thus forming “kenotetrahedral magnetite”. Sharpness of powder lines, absence of superstructure reflections, and difference in cell dimension distinguish this magnetite from the previously known “keno-octahedral” type, in which vacancies occur exclusively on octahedral sites. Total number of vacancies and number of tetrahedral vacancies are given on a nomogram as functions of density and cell dimension. Kenotetrahedral magnetite does not take measurable amounts of sulfur in solid solution. It forms when stoichiometric Fe 3 O 4 is heated in a flow of pure oxygen. Silica-tube experiments conducted over extended periods of time and collapsible-gold-tube experiments at 2 kbar show that “kenotetrahedral magnetite” is metastable. The reaction 3Fe 3 O 4 + 2S → 4Fe 2 O 3 + FeS 2 proceeds through the formation of metastable Fe 3– v O 4 and apparently a stage involving a metastable type of hematite before stable hematite is produced.

A description is given of the three-dimensional structure analysis of phyllochlorine ester. The problems arising from the determination of the 15 side-chain atoms are pointed out. The molecular structure will be discussed in comparison with related molecules.

Artificially grounded spheres of KCl single crystals (15 mm diameter) were subjected to growth at 45°C and at sufficient controlled supersaturations in otherwise pure aqueous solutions. Even a very slight increase of the supersaturation about more than 0.07 percent shows a change of the habit from a cube to an octahedron. This change is interpreted by different mechanisms of growth of the fundamental different relief of the two types of the faces: {111} faces are always distinctly rough and consist of cubic “subindividuals” with edges of about 5 to 10 μ m. {100} faces are always smooth and transparent. The increase of the supersaturation thus found to be favouring different growth velocities in different directions. The growth processes of KCl show much more differences compared to NaCl than other physical properties do (distinct anisotropy of growth behaviour). The growth velocities of {100}, {111} and {110} – other faces do not occur – sometimes differ more than by the factor of 10 to 100.

The oxide structures related to the fluorite structure type are capable of sustaining a high degree of anion sub-stoichiometry. In some binary and ternary mixed oxide systems, ordered intermediate compounds are found which are related to non-stoichiometric oxide phases. The structurally resolved oxide compounds and phases related to fluorite have arrangements which are distorted variants of idealized structures obtained by removing anions from an oxide of the fluorite type. Using this information, a condensed two-dimensional representation is proposed which displays some essential crystal-chemical principles of such oxide compounds and phases. From a consideration of the effect of removing anions from an idealized fluorite structure it was found possible to obtain relationships between overall non-stoichiometry and cation coordination number in mixed oxide systems

The crystal structure of lithium orthogermanate has been determined by means of three-dimensional Fourier syntheses and the least-squares method. The lattice parameters of the orthorhombic unit cell are: a = 7.76, b = 6.05 and c = 7.36 Å: space group Bmmb – D 2 h 17 . Li 4 GeO 4 is built up by [GeO 4 ] tetrahedra, being linked together by [LiO 4 ] tetrahedra. The average interatomic distances are found to be: Ge–O = 1.77 and Li–O = 1.98 Å.

The system MgO–Sb 2 O 5 has been reinvestigated. Besides the known MgSb 2 O 6 (trirutile type) and Mg 4 Sb 2 O 9 (I) (ilmenite type) further phases with the general formula Mg 8+7 k Sb 2( k +2) O 6(2 k +3) have been found converging for k → ∞ to the compound Mg 7 Sb 2 O 12 (II). By x-ray investigation has been shown, that (II) represents a new type of lattice based on hexagonal close packing of oxygen. The light absorption of the colouring cations Co 2+ and Ni 2+ after isomorphous incorportion in (I) and (II) gives evidence, that the bivalent cations have sixfold coordination in these phases.

Assuming photographic recording and balance at λ Kβ , the error of monochromatism is investigated as a function R of filter thickness. R is the ratio of the difference of the density produced by the radiation outside the pass band to the difference of the density produced by the pass-band radiation. It is shown that thinner filters are better monochromators than thicker ones. A stationary filter which has an absorption edge at the short-wave side of the Kα line improves monochromatism, provided that the filters of the pair are not too thick. An additional material without an absorption edge in the range in question increases the error R in the case of a Ni-Co filter pair, but it reduces R in the case of a Sr-Zr filter pair.

The metastable orange modification of HgI 2 , described earlier by several authors, was obtained by recrystallization of HgI 2 from 2-chloroethanol in the form of square, platelike crystals ranging in color from red-orange to yellow-orange. The x-ray photographs of the yellow-orange crystals indicated one-dimensional disorder along c, whereas the reflections from the red-orange form were sharp. The Friedel symmetry is 4/ mmm and the lattice constants are twice those of the red form, i.e., a = 8.776, c = 24.723 Å, c / a ≃;2√2. Both the hk 0 and hhl reciprocal-net planes are identical with the hk 0 plane of the red form and there are many systematic extinctions. The structure was solved from these observations. The structural motif is a Hg 4 I 10 group, consisting of four corner-linked HgI 4 tetrahedra in a tetrahedral arrangement. These groups are connected into HgI 2 layers which are stacked with I–I contacts in the same way as the HgI 4 tetrahedra in the red form. In contrast to the red form, there exist two kinds of stacking, leading to a cubic close packing of the iodine atoms. One results in a four-layer structure (red-orange crystals), the other in a two-layer structure. The disorder of the yellow-orange crystals is caused by an irregular succession of the two kinds of stacking. The intensity of the diffuse scattering arising from the disorder was calculated, and is in qualitative agreement with the observations.

The structure of C 41 H 28 NOBr · C 2 H 5 OH was solved by the heavy atom method with three-dimensional estimated intensity data. 3514 reflections with θ ≤ 60° ( λ = 1.5418 Å) were used, 327 of which were unobserved. Space group P 2 1 / a , a = 18.83 ± 3, b = 17.12 ± 3, c = 11.62 ± 2 Å, β = 111.7 ± ± 0.2°, D m = 1.32, D x = 1.29 g · cm −3 . By least-squares methods with anisotropic temperature factors, the structure was refined to R = 11.6%. The N–C bonds in the pyridinium ring are of usual length (1.36 Å), as is also the N–C bond (1.48 Å) that links the pyridinium and the phenolate ring. The dihedral angle between these rings is 65°. The dihedral angle between an outer phenyl group and the corresponding inner ring ranges from 18° to 70°. The C–O bond of the phenolate oxygen is very short (1.29 Å). The two adjacent C–C bonds are stretched (1.45 Å). A hydrogen bond (OH···O = 2.71 Å) connects the ethanol to the negatively charged phenolate oxygen.

Accurate and extensive three-dimensional intensity data for a single crystal of uracil have been collected by diffractometry using Mo K [unk] x-radiation. The average precision in the Bragg intensities is less than 2%. Secondary extinction was eliminated by powder-diffraction measurements. Multiple diffraction could not be detected. The interaction of thermal parameters with scattering factors is analyzed with a simple model. A refinement of the anisotropic thermal parameters, based on reflections for which (sin θ )/ λ exceeds.648 Å −1 , led to values which are significantly different from refinement based on all data. The carbon-atom parameter differences are consistent with recent studies of combined x-ray and neutron-diffraction analysis.

A method is reported of photometric intensity measurements on single-crystal x-ray photographs. By introducing a correction factor into the method given earlier by K aan and C ole the covered range of intensities per photograph is enlarged by a factor 5 and the accuracy increased to 6.5%.

Upon heating AlB 10 single crystals in air, a white, needle-like phase begins to appear on the surface of the crystals at about 700°C; this has been identified as 2Al 2 O 3 · B 2 O 3 . When heated in a helium atmosphere at 950°C, it transforms to a phase having a rhombohedral cell with a = 6.97 Å, α = 68.4°.