Volume 120, Issue 4-5

The structure of silica glass has been studied by comparing calculated x-ray scattering patterns of structural models with experimental scattering patterns. The structural models chosen are of random arrangements of silica tetrahedra containing preferred intertetrahedral configurations resulting in a degree of order on a local scale in the otherwise random network. Examination of the derivation of the radial distribution function from x-ray scattering data has shown that the occurrence of such local configuration is not excluded. In the structural model chosen the tetrahedra link together preferentially in ways similar to the tetrahedral arrangements in the crystal forms cristobalite and tridymite so that non-planar rings occur consisting of mainly 5, 6 and 7 tetrahedra. In calculating the scattering patterns, diffraction ripples arise from the shape of the model and consideration is given to their removal. The calculated x-ray scattering patterns are found to be in agreement with experiment; and best agreement is obtained when the Si–O–Si angle is about 150° and when the model contains a proportion of 7-membered non-planar rings of tetrahedra rather than non-planar rings of only 5 and 6 tetrahedra. Experimental and theoretical estimates of the sensitivity of the x-ray scattering patterns to structural detail are discussed.

Crystals of compounds [ Me (II)(N 2 H 4 ) 2 ]Cl 2 , with Me = Mn, Zn, Cd, are monoclinic (space group C 2/ m ) and isotypic; they form twins according to various laws defined as (100), (001) and (110) respectively. Twin laws have been interpreted on the basis of structural relations at the boundary layer, using results of the structure determinations of these compounds. The structure is an array of chains of complexes parallel to [001]. Chains are not bound to one another by hydrogen bonds. The observed twin laws have been shown to be due either to different packing of the chains [(100) and (110) laws] or to change of stacking within each chain [(001) law]. Twinning is possible when the intermolecular distances between chains of the two individuals at the boundary layer are very near to those observed in the regular structure. The fact that these distances are not very different shows that the arrangement of molecules at the boundary layer has an energy only slightly different from that of the regular sequence. This seems to be the main condition for twinning.

The crystal structure of tetragonal US 2 has been derived from powder and some single crystal data. The cell dimensions, measured with CuK α 1 , wavelength 1.540 Å, are a = b = 10.28 ± 0.01 Å, c = 6.327 ± 0.005 Å. On the assumption that there are 10 US 2 per unit cell, the calculated density is 7.50 g/cm 3 . The proposed structure has the symmetry of I 4/ mcm ( D 4 h 18 ), with all of the atoms in special positions. This may be an idealization, because only small displacements of the atoms in the c -axial direction are necessary to reduce the symmetry to I 4 cm ( C 4 v 10 ) or I [unk] c 2( D 2 d 10 ). The two geometrically different uranium atoms in the structure have quite different environments. Of the ten uranium atoms in the unit cell, eight have a coordination of six sulfur atoms, with uranium to sulfur distances of 2.77 Å. The remaining two, which are statistically distributed in fourfold positions, have a coordination of eight, with uranium to sulfur distances of 2.81 Å. The structure type is new, and has no obvious relation to orthorhombic US 2 , which, like ThS 2 , has the PbCl 2 structure.

At room temperature (NH 4 ) 3 GaF 6 is cubic with a = 9.0409 Å and Z = 4. It crystallizes in a T h 6 − Pa 3 structure submicroscopically twinned across {110} and {100}, or in an O h 5 − Fm 3 m structure in which the fluorine and eight nitrogen atoms occupy higher-ordered lattice complexes at random. The gallium atoms lie at 4 a [000], four nitrogen atoms at 4 b [½00], eight nitrogen atoms at 8 c [ www ] or 32 f [ www ] with w = 0.231, and the 24 fluorine atoms at 24 d [[unk] yz ] or 192 l [ xyz ] with x = 0.013, y = 0.052, z = 0.203.

The lattice constants and space groups of the selenates of Mg, Mn, Co, and Cu were determined. These selenates all crystallize with the ZnSO 4 structure. Above 40 kilobars and 400°C they all undergo a quenchable transition to the NiSO 4 structure.

We investigated the adsorption of impurities (CH 2 NH 2 CH 2 COOH, CdCl 2 , MnCl 2 , HgCl 2 ) by different planes of sodium chloride. These impurities influence the habit of sodium chloride in aqueous solution. The results show a progressive growth of the adsorption to negative values with increasing concentrations of the impurities. This can only be explained by a more than monomolecular adsorption-film of water on the planes.

The lattice parameters of BaTiO 3 have been measured between room temperature and 200°C with a commercial high-temperature powder camera, modified for better temperature homogeneity and control. The accuracy of the parameters was about ± 0.0001 kX in the major part of the temperature range. From these measurements the expansion coefficients were evaluated graphically with an accuracy of 3% in the range from 85°C to 120°C and analytically with 1% accuracy in the other range. At about 95°C, a slight change in the mean volume expansion could be detected.

Single-crystal investigations were carried out on a mixed crystal α -Al 2 O 3 + 9 Mol-% Cr 2 O 3 and on crystals of Cr 2 O 3 . In the case of the mixed crystal a distortion of the oxygen lattice is effected by building in Cr 3+ ions. The structure of Cr 2 O 3 is strictly ordered and free of strain. Atomic parameters are given. The reason why mixed crystals in the system α -Al 2 O 3 –Cr 2 O 3 show distortion is considered to be not only the discrepancy between the ionic radii of Al 3+ and Cr 3+ but also a difference in the nature of the bonding.

X-ray studies at room temperature on inorganic salts have been made by the use of a diamond bomb, up to 10 kbar. New powder x-ray diffraction patterns are given for TlI, NH 4 HSO 4 , NaN 3 , KPF 6 and NaHF 2 at high pressure, each showing a phase change; x-ray reflection from high-pressure forms of NH 4 HSO 4 and NaHF 2 have been indexed. With NaN 3 the phase change appears to be of a higher order.

The chromium position in highly doped ruby has been determined from 00· l intensity data. Cr occupies a site 0.06 Å from the normal Al position. Contrary to recent crystal-field calculations, the shift is towards the nearest Al neighbor, increasing the smaller metal–oxygen distances.

A symbolism of coordination polyhedra is proposed and listet together with the topologically and metrically distinct crystal forms, based on a paper by A. N iggli . Their occurrences with respect to the different point symmetries are given. The proposed symbol for the coordination polyhedron consists of the number of equipoints and small letters, which give the configuration of the points.

A number of programs for facilitating the indexing of powder patterns, based essentially on trial-and-error methods, have been constructed for the electronic computers BESK and Facit EDB. The methods are efficient also for the analysis of powder patterns in which lines are missing either by accident or because of translational symmetry. The programs presuppose monoclinic or higher symmetry. The principles used are illustrated with three powder patterns. It should be possible to adapt the methods to any digital computer.

Albite and pericline twin lamellae have been produced in anorthite at room temperature in a simple way. The pattern of twins changes with increasing pressure. Most twin lamellae disappear on removing the stress, but some remain; in addition naturel twin lamellae can be made to disappear by the same process.

The structures of apatite and of alloys of the Mn 5 Si 3 type are very similar; the same is valid for the eulytite-[Bi 4 (SiO 4 ) 3 ] type and the Th 3 P 4 type.

A close similarity between the crystallographic constants of freudenbergite and sodium titanium dioxide bronze Na x TiO 2 is pointed out.

The unit-cell dimensions of copper biguanide chloride have been determined from oscillation and Weissenberg photographs. The crystal belongs to the triclinic system. The parameters are a = 6.78 Å, b = 9.58 Å, c = 12.71 Å, α = 113°27′, β = 98°59′ and γ = 99°45′. The number of molecules per unit cell comes out to be 2. The space group could be either P 1 or P [unk].