Band 124, Heft 3

The rate of growth of all faces appearing on polished single-crystal spheres of alkali halides was measured under carefully controlled conditions, and the influcene of small additions of 25 inorganic cations was determined. A method for measuring growth rates of individual faces after a short period of growth was developed. Cd 2+ and Pb 2+ showed the most pronounced influence on different alkali halides: The growth velocities of all faces are diminished. Those growing fastest in pure solutions, however, are more sensitive to smaller additions; they then show a morphological importance which is lost again when the concentration is further increased. The effect can be explained quantitatively as adsorption of foreign ions at growth centers on all faces according to Langmuir isothermes. The preferred adsorption of Cd 2+ and Pb 2+ cannot be explained by geometrical similarity between crystal structure and adsorbed particle.

The structure of hemimorphite, Zn 4 Si 2 O 7 (OH) 2 · H 2 O has been redetermined with three-dimensional data obtained both photographically and with a counter-diffractometer. R values of 7.1% and 4.0% were obtained for the two sets of data. There are no significant differences between the Si–O bond lengths (mean 1.627 Å) and the Si–O–Si angle is 150°. The Zn–O bonds (mean length 1.954Å) show considerable covalent character. Following Z oltai , the structure is described as a three-dimensional framework of tetrahedra. The water molecule shows large thermal vibrations and may be partly disordered.

The structure parameters of ferberite FeWO 4 were determined from x-ray and neutron-diffraction measurements. The least-squares refinement method was used to refine the parameters obtained from a three-dimensional difference Fourier synthesis. The metal ions (Fe 2+ and W 6+ ) occupy one half of the octahedral holes in a slightly deformed hexagonal closest-packed lattice of oxygen atoms. This structure is idontical to the structure of NiWO 4 other than for small parameter variations. It was shown by the neutron-diffraction measurements at liquid helium temperature (4.2°K) that the ferromagnetically planes (100) were antiferromagnetically ordered parallel along the x axis. All of the moments lie parallel in the plane (010). They build an angle of 29° with the x axis and are not perpendicular to any of the octahedral faces. The magnitude of the moment (2.19 μ B ) for a Fe 2+ ion corresponds approximately to the four unpaired electrons of the Fe 2+ ions when one considers only the spin components. The antiferromagnetic transition temperature was found to be 66°K for ferberite.

Butadiene sulfone is orthorhombic with a = 11.484 Å, b = 7.262 Å, c = 6.316 Å, and four molecules in the unit cell of space group Pnma . The molecules are distorted tetrahedra with C 2 v symmetry. The average bond lengths are 1.440 Å for S–O, 1.794 Å for S–C, 1.478 – for C–C, and 1.299 Å for C=C. The bond angles are 97.0° for C–S–C, 117.0° for O–S–O, 110.2° for O–S–C, 103.5° for S–C–C, and 118.0° for C–C=C.

In the orthorhombic modification of TeO 2 (tellurite) each tellurium atom is surrounded by four oxygen atoms at an average distance of 2.0 Å, similar to the tetragonal TeO 2 (paratellurite). The coordination polyhedron is a slightly distorted trigonal dipyramid, with both axial positions and only two of the three equatorial ones being occupied by oxygen atoms. Pairs of these TeO 4 groups are connected by a common edge to form Te 2 O 6 units and these are furthermore linked by the four “free” oxygen corners to ∞ 2 Te [4O] O 2 [2Te] sheets parallel to the perfect cleavage.

VOCl 2 is orthorhombic and crystallizes with two VOCl 2 moieties per unit cell. The measured cell parameters are: a = 3.842 Å, b = 11.761 Å and c = 3.383 Å. The x-ray density amounts to 2.99 g/cm 3 . Possible structures are discussed.