Band 123, Heft 3-4

The structure of the low-quartz modification of AlPO 4 was in detail determined and refined with 539 independent reflections. The final R value obtained is 0.046. The intensities were measured on several untwinned crystal spheres. The structure factors were determined, due consideration being given to anomalous scattering and extinction effects. The refinement by least squares showed that the method of weighting had but slight influence on the results. The absolute configuration was determined with copper radiation by means of the anomalous scattering. The analogy between AlPO 4 and low quartz proved to be very marked. The physical properties, the laws of twinning, the optical activity and its relation to the absolute configuration, the distortion of the oxygen tetrahedra and the thermal oscillations of the atoms are analogous to the respective data of quartz. The two crystallographically independent oxygen atoms are chemically equivalent. The mean distances between Al and O (1.739 Å) and P and O (1.516 Å) are in good agreement with values given in the literature. The Al–O distance can be used as a standard for bond lengths of aluminium in fourfold coordination in silicates as is done with the Si–O distance of 1.607 Å in quartz. It was proved by a difference Fourier synthesis that the P–O bond is more covalent than the Al–O bond. Hence the structure may be considered as built up of Al 3+ and (PO 4 ) 3− and can from a formal point of view be regarded as a ionic structure Al 3+ (PO 4 ) 3− . On the other hand the analogies to the quartz type suggest an interpretation of the structure as (AlP)O 4 , i.e. as a frame-work.

The crystal of l -arginine monohydrobromide monohydrate belong to space group P 2 1 , with four molecules of C 6 H 14 O 2 N 4 HBr · H 2 O in the unit cell of dimensions a = 11.26 Å, b = 8.65 Å, c = 11.25 Å, and β = 91.5°. The positions of the two bromine atoms in the asymmetric unit were determined from the Patterson projections. A three-dimensional β synthesis based upon these atoms was calculated and this yielded the rest of the structure. The structure was refined by the three-dimensional least-squares method using about 1956 reflections. The final R factor is 0.102. The estimated standard deviations are 0.032 to 0.035 Å in bond lengths and about 2° in bond angles. The two crystallographically independent molecules in the asymmetric unit have a pseudotranslation of about c /2 when viewed along the b axis. The bond lengths and bond angles for the two molecules are normal. The two molecules differ in their configurations in the orientation of the guanidyl groups with respect to the aliphatic chain. The arginine molecules occur as zwitterions, both the guanidyl and the amino groups accepting protons from the acid groupings present in the structure.

The crystal structure of the monobromoacetyl derivative of a compound with the molecular formula C 13 H 16 O 2 , first isolated by N azarov and G urvich , has been determined. The structure determined is found to agree with that proposed by S waminathan et al ., and to be in disagreement with the earlier one proposed by N azarov . Crystals of C 15 H 17 0 3 Br (monobromoacetyl derivative of Nazarov's compound) belong to the orthorhombic system with space group Pbca and the unit-cell dimensions are a = 20.33; b = 17.63; c = 7.67 Å; Z = 8. The structure was determined using the heavy-atom method in three dimensions and refined by three-dimensional least-squares method to an R value of 0.15. There are no intermolecular bond systems in the usual sense of the term, but the presence of CH to O short contacts indicate the possibility of a weak CH…O hydrogen bond holding the molecules together. Otherwise the structure is stabilized only by van der Waals forces.

The crystal structure of p -chloroacetanilide has been determined. The crystals belong to the orthorhombic system with space group Pna 2 1 and the unit-cell dimensions are a = 9.73; b = 12.88; c = 6.56 Å; Z = 4. The structure was determined using projection data and refined by three-dimensional least-squares method to an R value of 0.14 (for the observed reflections only). The molecules are stabilized by NH…O hydrogen bonds parallel to the a axis.

At elevated temperatures complete solid solution exists between Cu 5 FeS 4 and Cu 9 S 5 (K ullerud and R oseboom , 1958). Homogeneous solid solutions of nine compositions on this join were synthesized at temperatures above the solvus. The crystallographic properties of the solid solutions were studied by the Weissenberg and precession methods at various temperatures. At elevated temperatures each composition exists as one phase, commonly in well developed single crystals, and at subsolidus temperatures two phases occur. All the synthetic materials, when studied over a temperature range, display x-ray reflections analogous to those of the high-, transitional-, and low-temperature forms of bornite and digenite described by M orimoto and K ullerud (1961, 1963). The structure of the high-temperature form is described on the basis of the cubic close packing of sulfur atoms with statistical distribution of metal atoms. During the inversion from the high- to the low-temperature polymorph, the transitional metastable forms temporarily appear. These are superstructures of the high-temperature form, and their structure types differ owing to differences in composition. The various superstructures are derived, by ordered arrangements of metal-vacant layers, from the structure of the high-temperature form. The transitional forms gradually change to the low-temperature forms. Among the low-temperature forms only two types corresponding to the structures of natural bornite (Cu 5 FeS 4 ) and digenite (Cu 9−x S 5 ) are stable at room temperature. Experiments were also carried out with natural digenite and bornite, and the results are explained on the basis of the structures of the transitional forms occurring on the Cu 5 FeS 4 –Cu 9 S 5 join. The mechanisms of the polymorphic transformations are considered from a structural point of view.

The symbols for lattice complexes, as given by C. H ermann , are simplified for easier typing or printing; the “representation” of a lattice complex is introduced as a new concept.

Since disordering can have a marked influence on absorption spectra in the infrared range, it appeared worthwhile to examine and compare the spectra of the various modifications of SiO 2 and AlPO 4 . The modifications with cristobalite and trydimite structures for which the various types of stacking faults are revealed in x-ray diffraction effects show a decrease in the intensity of certain infrared bands with increasing disorder. The effect of increased temperature on these structural types is the decrease in intensity and relatively sudden disappearance of certain bands. Their disappearance is coupled with polytropie transformations. These processes are almost completely reversible and show hysteresis. By comparing the absorption spectra of SiO 2 and AlPO 4 samples an attempt was made to deduce relationships between frequency and structural units. The absorption spectra of SiO 2 –AlPO 4 mixtures suggest that isomorphous substitution can occur at high temperatures up to about 5% at each end of the series.

An experimental arrangement is described for monochromatic x-ray diffraction analysis of polycrystalline thin films which utilizes the Seemann-Bohlin focussing principle. This permits the selective use of either film or scintillation-counter measurements with adjustable small angles of incidence.