Band 108, Heft 5-6

The crystal structure of cyanic acid trichloride was determined. The diffuse scattering was used to find the orientation of the molecule. The advantage of the combined use of strong as well as weak reflections for the selection of the parameters of the molecular translations is emphasized.

The rod-like shape of the cation and its orientation in the lattice was determined using diffuse X-ray scattering.

Phyllochlorine ester, a derivative of chlorophyll, is a suitable example for the study of diffuse X-rays scattered by thermal waves in crystals. The investigation was concerned with whether this diffuse scattering can be used advantageously for the determination of characteristic groups in complicated molecule structures. It was found that certain atomic configurations in the molecule can be recognized clearly in the diagram of the diffused scattering. In the case of phyllochlorine ester, it is the porphyrine ring which is recognized due to the dynamical attachment of its atoms to a rigid atom group. The diffuse scattering reveals these groups easier than the crystal reflections. It is proved experimentally that the distribution of diffuse scattering has no relation to the F ourier transform of the unit cell. This method, therefore, cannot be used directly for the determination of the phases of the crystal reflections.

This paper shows how the screw dislocation theory of crystal growth supplies a satisfactory mechanism for generating the known structural polytypes of cadmium iodide. It was found that all the structures of the “22…11” series could be generated by spiral growth from single nonintegral screw dislocations in an “ideal” 4 H structure. All of the other known structures not belonging to this series could be accounted for in a similar manner by assuming the presence of two or more cooperating dislocations and/or an alternate arrangement of the initial ions as they take their places at the bottom of a growing step. Structural series like “22…1111”, “22…33” and “22…2123” are to be expected as a result of these considerations. The proposed causes of dislocations are outlined and then it is shown how they are related to the former explanations of polytypism. Other polytypic substances such as silicon carbide and graphite are considered in the light of these findings. It is pointed out that polytypism is not to be expected in certain substances because of the nature of their “ideal” structures.

In order to understand the nature of the two transitions in Rochelle Salt, at − 18°C and +23°C, between which points the crystal is ferroelectric, the structures of the two non-ferroelectric phases have been studied with X-rays, preparatory to neutron examination. Both non-ferroelectric phases are in space group P 2 1 2 1 2. Multifilm W eissenberg data with CuKα , and with specimens encased in pyrex, were obtained at temperatures – 64°C, 0°C and + 35°C. Orthorhombic symmetry was assumed for all three phases, and six (001) projections were computed on X-RAC, employing as coefficients: F −64°C obs − F ÷35°C obs , F −64°C obs − F 0°C obs and F 0°C obs − F +35°C obs ; and F −64°C obs , F 0°C obs and F +35°C obs . Starting points for all phase assignments were the coordinates determined by B eevers and H ughes . Repeated refinement cycles led to R -factors of 10.5% for the − 64°C structure, and 11.6% for the 35°C structure. Shifts in atomic coordinates, and changes in thermal oscillations, are reported for both transition points.

Pentaerythrite crystals are usually twinned after (001). They are divided mostly four times by intergrowth planes parallel to the two (110) planes, perpendicular to each other. Opposite quadrants have the same orientation. Some rare specimens show, in addition, twinning after (100) or (110). Here the intergrowth planes run irregularly all through the crystal. The intergrowth planes of both twin laws penetrate each other without interference. The twinning after (001) is detectable optically but not with X-rays, the twinning after (100) or (110) with X-rays, but not optically.

α-yohimbine ( C 21 H 26 O 3 N 2 ) has been studied by X-ray powder diffraction method. With the help of the analytical methods of S tosick and H esse , the unit cell has been found to be hexagonal with a = 23,75 Å ane c = 15,71 Å and with 12 molecules per unit cell.

Eucairite crystallizes as pseudo-tetragonal orthorhombic crystals having a large supercell of a = 4.105 Å, b = 20.35 Å; c = 6.31 Å based upon a tetragonal subcell whose a dimension lies somewhere between 4.07 Å and 4.105 Å and whose c dimension is equal to 6.31 Å and which has the space group symmetry of P 4/ nmm . There are 2 ( CuAgSe ) per subcell with the Ag atoms at 1/4, 1/4, .449; 3/4, 3/4, .551. The Se atoms are at 1/4, 1/4, .873; 3/4, 3/4, .127 and the Cu atoms are at 1/4, 3/4, .105; 3/4, 1/4, .895. The true cell or supercell is a stacked sequence of these subcells in two orientations. At 190–195°C there is an inversion to a high temperature modification.

The experimentally demonstrated twins from vapor deposits of the alkali-halides can be explained graphically: Part of the arriving vapor molecules do not go into normal positions building up the lattice correctly but go into twin positions. This can be understood with the help of the present model in which the ions are represented by spheres. To enquire into the conditions which favor twinning, the contribution of the Coulomb forces to the deposition energy is determined by an approximation method due by F rank . This shows that the formation of twins on the surface of an ideal crystal is very improbable. However, it is favored by steps and fractures in the substrate. A mechanism, leading to twinning is given and the growing of the twins is described.

The structure of diglycine hydrochloride has been determined by solving the three-dimensional P atterson function for the approximate three-dimensional distribution of electron density with the aid of an image-seeking polyhedron. Since this is the first structure to be solved in three dimensions by this means, the theory of the method is first discussed and then applied to the data for this particular crystal. The resulting structure was refined by projections, difference syntheses and least squares. On the basis of the interatomic distances found in the structure, the bonding in the glycine molecule and between glycine molecules is discussed in considerable detail.

A mechanical time switch is described which is constructed for measurements of a fixed counting period with a counting-goniometer.

The cell dimensions of 1,8-dihydroxyanthraquinone are determined by rotation photograph method. The crystal belongs to tetragonal system with axial lengths a = b = 5.86 Å and c = 31.33 Å. The density is determined by the flotation method and the calculated number of molecules per unit cell is 4. From morphological and X-ray studies, the space group C 4 2 − P 4 1 or the equivalent C 4 4 − P 4 3 is uniquely assigned.

Investignations of the crystal structure of harmine, C 12 H 13 ON 2 , are reported here. Both X-ray and optical studies show the orthorhombic nature of the crystal with the axial ratios a : b : c = 0,5968: 1: 1.985. The unit-cell parameters are a = 5.77 Å, b = 9.67 Å and c = 19.2 Å, having the principle refractive indices α = 1,667, β = 1,723 and γ much higher than 1.84. The space-group is P 2 1 2 1 2 1 , and in the unit-cell there are four molecules.