Band 118, Heft 3-4

A brief survey is given of the results of extended investigations into certain systems of anomalous mixed crystals between uranium compounds and fluorides of the alkalis and alkaline earths. The systems in question are as follows: NaF–Na 2 U 2 O 7 , LiF–Li 2 UO 4 , KF–K 2 UO 4 , NaF–UO 2 (OH) 2 , CaF 2 –CaUO 4 , CaF 2 –UO 2 . These mixed crystals were subjected to examination under the polarizing microscope, by x-rays and by spectral photometry. In numerous cases it was possible to deduce the mechanism of intergrowth and this can serve as a base on which a later discussion of the absorption and luminescence phenomena of these systems can be founded.

Precession x-ray photographs showed that single crystals of MgSiO 3 obtained from the Norton meteorite converted to proto-enstatite between 1050 ± 50°C and 1300°C (the upper limit of the apparatus). Diffractometer study of synthetic MgSiO 3 showed that proto-enstatite forms up to about 1440°C. Proto-enstatite, upon cooling from high temperature, converted into a variety of products ranging from twinned clino-enstatite through disordered structures to a product similar to ortho-enstatite, but with stacking faults. Rapid quenching produced clino-enstatite or disordered material closely related to it. Slow cooling through the range 800–1000°C sometimes produced partly disordered material similar to ortho-enstatite. From consideration of these and earlier results by many investigators on both natural and synthetic enstatites, it is concluded that ortho-enstatite is the form stable below 1000°C while proto-enstatite is stable above this temperature, and that clino-enstatite and the disordered enstatites are metastable forms, occurring because they grow faster than ortho-enstatite. Disordered enstatites similar to those produced by the cooling of protoenstatite have been found in the Norton County and Cumberland Falls meteorites. Twinned clino-enstatite was found in the Juvinas meteorite. The occurrence of such enstatites may give information on the temperature history of meteorites, though it is possible that shearing stress may cause similar structural disorder.

As Fe–Cr substitution improves the stability of the Fe 3 C phase an x-ray investigation of the metal-atom distribution in (Fe,Cr) 3 C was undertaken. For the experiments x-rays were chosen which provide an enlarged difference of the scattering factors of iron and chromium by anomalous dispersion. By measuring the intensities of the especially suitable reflections 120 and 200 it could be shown that a favoured substitution into the eightfold position takes place. – Precise measurements of the distance of the reflections confirm that with increasing chromium content the ratio of the lattice constants b : a at first decreases and then, passing a minimum, increases.

A rutile-type AlAsO 4 polymorph with a = 4.359 Å and c = 2.815 Å has been synthesized at 90 kbar and 900 C. The density is 5.15 g/cm 3 , an increase of 54 per cent over the quartz modification. The indices of refraction of the rutile form of AlAsO 4 are ε = 1.950, ω = 1.905, both ± 0.005. Molar refraction data for the quartz and rutile polymorphs of AlAsO 4 show a percentage decrease of 10.4 relative to the quartz form and fulfill the Dachille-Roy prediction relating molar refraction and cation coordination. The main absorption band of the infra-red spectra of the quartz and rutile forms of AlAsO 4 shows a shift of 23 per cent in the direction of increasing wavelength with increasing cation coordination. A corresponding shift of the same magnitude is shown by the quartz and rutile forms of SiO 2 and GeO 2 .

The theoretical basis and the experimental specifics are presented for a generally valid, systematic procedure which effectively achieves simultaneous balance of x-ray filter pairs over a wide range of wavelengths. K irkpatrick's technique of adding a third material to one of the filters is necessarily employed. Corollary topics discussed include the preparation of filters and holders, criteria for balance, methods of testing for balance, and the role of pulse-height analysis in the balanced-filter technique. It is shown how a number of points of possible experimental difficulty may be recognized and overcome. Results obtained by following the described procedure are presented.

X-ray crystal-structure analysis of basic mercury(II) cyanide, known under the formula HgO · Hg(CN) 2 , showed that the compound is a bis -cyanomercuric oxide. It is orthorhombic holosymmetric with the space group D 2h 16 – Pnam . The unit cell with a = 18.93, b = 7.09 and c = 3.90 Å contains four formula units. The parameters of all atoms were obtained by means of Fourier and difference Fourier projections on (001) and from symmetry conditions of the space group. The structure consists of planar (NCHg) 2 O molecules the shape of which is shown in Fig. 2. Intra- and intermolecular distances as well as bond angles are given in Table 3. The molecules, related symmetrically by [001] screw axes, are linked in vertical columns through the shortest intermolecular approaches of 2.53 Å between Hg I and O atoms. With the exceptions of the vertical separations of Hg II –N I 2.84 Å and Hg II –N II 2.96 Å which ensure the connection of columns, all the other intermolecular shortest distances correspond to the sum of van der Waals radii. The coordination polyhedra around Hg I and Hg II are deformed tetrahedra and octahedra respectively, with the bivalency of Hg II being more accentuated than in the case of Hg I .

Single crystals of the three polymorphs of CaGa 2 O 4 reported by Jeevaratnam and Glasser 1 have been studied. CaGa 2 O 4 -I is orthorhombic, with a = 7.73, b = 9.14, c = 10.36 Å; CaGa 2 O 4 -II is orthorhombic with a = 5.32, b = 14.38, c = 16.82 Å; m -CaGa 2 O 4 is monoclinic with a = 7.86, b = 8.90, c = 10.55 Å, β = 93°. These cell dimensions do not resemble any of those previously reported for other RO · R 2 O 3 compounds.

A calculation is made of the integral reflectivity of ideal mosaic crystals according to the theory of mosaic structures. Both the reflection and transmission patterns are derived for different values of the Bragg angles θ 0 used for mono chromatization, of the reduction in beam cross-section according to the Fankuchen cut, of the effective total cross-section of the monochromator material, and of the thickness of the crystal. The influence of the latter on the change in beam cross-section is estimated. An intensity formula is proposed for the problem of image formation that arises in connection with the examination of small test crystals. Some measurements on real crystals were carried out.

The concept of gnomonic projection has been used to derive equations that will enable the indexing of flat-film Laue photographs, whether transmission, back-reflection, side-reflection or upper-reflection Laue photographs. This is done for all crystal systems in arbitrary orientations, (i.e. for symmetrical or asymmetrical Laue photographs). These equations involve the directions of the incident x-ray beam and the crystal axis which is vertical, the crystal to photographic plate distance and the coordinates of a given Laue spot with respect to axes chosen on the photographic plate itself.

We investigated the adsorption of urea by the planes {100} and {111} and of aminoacetic acid by the planes {100} and {110} of sodium chloride in aqueous solution. The results show a progressive growth of the adsorption of aminoacetic acid by the planes {100} and {110} with increasing concentration of aminoacetic acid in the solution. So it is supposed that the changing habit is caused by the adsorption of aminoacetic acid as impurity. If urea is added to the solution the adsorption of this substance at the {100} planes grows as far as a concentration of 8–10%. At higher degrees of concentration the adsorption of urea decreases and finally reaches negative values. Only negative values are found for the adsorption of urea by the {111} planes and the adsorption decreases with rising concentration. It is supposed that urea causes the association of the water molecules to decline. A favoured adsorption of the resulting monomeric molecules by the {111} planes is the reason for the change in habit.

The crystal structure of nickel formate, Ni(HCOO) 2 · 2 H 2 O, has been determined using Weissenberg photographs. Patterson and Fourier syntheses yielded the atomic positions, which have been refined by the three-dimensional “least-squares”-method, to a final value of R = 8%. The crystals belong to the monoclinic space group P 2 1 / c = C 2 h 5 and include four formula units in the unit cell with a = 8.60 Å, b = 7.06 Å, c = 9.21 Å, β = 96° 50′. The structure shows two kinds of Ni ions: one with six oxygens of different formate ions as nearest neighbours, the other with four water molecules and two formate oxygens coordinated.

Manganite is monoclinic, pseudorhombic. Its structure is a superstructure based upon the marcasite type. The crystals show polysynthetic twinning. By x-ray measurements on a thin untwinned cleavage splint improved Mn parameters were obtained. For the neutron investigations twinned crystals had to be used. Because of the polysynthetic twinning normally non identical reflections were superimposed, excepting ( hk 0), (0 kl ) and certain ( h 0 l ) reflections. From these reflections the H positions were derived. The neutron-diffraction measurement proved that the hydrogen is bonded to certain oxygen ions. High-temperature measurements showed that the superstructure exists up to the decomposition temperature of manganite. No coherent magnetic scattering was observed.

Phosphoferrit cristallizes in the crystal class mm 2. The space group is Pcn 2, but the Fe and P atoms nearly have the symmetry Pcnb . The change in the space group from Pcnb to Pcn 2 is caused by the positions of the PO 4 tetrahedra and the H 2 O molecules. The Fe atoms are in 6-fold coordination. The relation between structure and morphology is discussed.