Volume 195, Issue 1-2

Investigations with synchrotron radiation have shown that atoms in anisotropic chemical environment can exhibit a pronounced anisotropy of their anomalous dispersion, i.e. for absorption edge energies, the energy dependent correction terms f ′ and f ″ of the atomic form factor can depend also on the crystal orientation with respect to the polarization of the incident photons. Then, extinction rules due to screw axes and/or glide planes can be violated giving rise to the observation of ‘forbidden’ reflections. The effect is discussed for a special case, a structure crystallizing in the space group P 2 1 2 1 2 1 and containing one ‘edge’ atom per asymmetric unit. The intensities of the ‘forbidden’ reflections are given as functions of both the azimuthal angle ψ and the atom's position. The developped results are also valid for unpolarized radiation. A simple method is proposed to extract partial structure information exclusively from the relative ψ -dependencies of the intensities of only 6 axial reflections, two on each lattice row. These data are, in principle, sufficient to obtain at most 16 equally possible locations of the atom in the asymmetric unit. Feasibility calculations promise satisfactory experimental results.

Electron micrographs and selected area diffraction patterns of single In 1 – x Ga x As epitaxial layers (cubic symmetry, space group F [unk]3 m ) have been studied using transmission through the specimen edges formed by intersection of the (001) growth surface and the (0[unk]), (1[unk]0) or ([unk]0) cleavage faces. For (001)/(0[unk]) edge transmission lamellae (thickness between 7 and 21 nm) of deformation twins parallel to ([unk]1) were detected unambiguously. Micron-deep grooves are formed where twins intersect the (001) surface. Twin growth proceeds by propagation of Shockley partial dislocations from a surface-near region towards the layer/substrate interface, leaving a sessile dislocation behind. A possible mechanism of twin growth is discussed on the basis of a repeated dissociation of such sessile dislocations into glissile Shockley partials.

Calculated and observed X-ray powder patterns of three ambient temperature modifications of tridymite: L1 – T o (= ordered low-tridymite MC), L2 – T D (= disordered low-tridymite PO 5/10 ) and L3 – T o (= ordered low-tridymite MX-1) are reported. Deviations in the diffractograms due to texture effect, stacking disorder and intergrowth of different modifications which complicate identification and indexing are discussed. Changes of unit cell dimensions due to thermal expansion and phase transitions were measured from room temperature to about 700° C. Near 160°C all ordered and slightly disordered tridymite polymorphs transform to monoclinic high tridymite H3 – T (OS) with non-integral modulation, at about 210°C to orthorhombic high tridymite H2 – T (OC) and finally at about 400°C to hexagonal high-tridymite H1 – T (HP). Different from pseudo-orthorhombic L2 – T D (PO 5/10 ) the ordered tridymite modifications L1 – T o (MC) and L3 – T o (MX-1) form an additional metrically orthorhombic modification H4 – T o (OP) between 110 and 160°C. L3 – T o (MX-1) shows a further metrically orthorhombic phase H5 – T o between 70 and 105°C. Strongly disordered tridymite L2 – T Ds has only one phase transition at 170°C. All phase transitions are reversible. The transformations near 160, 110 and 70°C show a temperature hysteresis.

The structure of d , l -glyceraldehyde has been redetermined by means of synchrotron and conventional X-radiation. At 295 K lattice parameters in monoclinic space group P 2 1 / n are: a = 6.403(2) Å, b = 6.961(2) Å, c = 8.830(3) Å, β = 105.00(2)°, V = 380.12 Å 3 , Z = 2. Three independent data sets of crystals with crystal volumes of 0.00009 and 0.02 mm 3 are compared. As a main advantage of the synchrotron experiment it was found that its results obtained with the small crystal are of similar precision compared to those based on the larger crystal measured with a conventional tube.

With the flux-technique single crystals of (Al, Ge)-mullites have been synthesized in the range of 1.28 A1 2 0 3 · 1 GeO 2 to 1.93 Al 2 O 3 · 1 GeO 2 . Chemical and optical (microrefractometer spindel-stage) data of the solid solution series have been collected. The results of a structure refinement of the mullite 1.93:1 are compared with those of a refinement of the mullite 1.50:1 (Neumann, 1984).

The structural phase transitions of betaine calcium chloride dihydrate have been associated with one or two two-component order parameters. Considering relevant dielectric and Raman-spectroscopic results we conclude that the available experimental data favor a model with only one order parameter.