Volume 123, Issue 5

Tarbuttite is triclinic, space group P [unk]. The cell parameters are: a = 5.499 Å, b = 5.654 Å, c = 6.465 Å, α = 102°51′, β = 102°46′, γ = 86°50′. There are two formula units of Zn 2 (OH)PO 4 per unit cell. The crystal structure has been solved from Patterson projections and statistical methods. Three-dimensional isotropic least-squares refinement yielded a final discrepancy index R = 0.12. The arrangement of oxygens about the Zn ions is a nearly regular trigonal bipyramid with average Zn–O distances of 2.04 Å. The PO 4 configuration is a regular tetrahedron with a mean P–O bond length of 1.54 Å. The framework consists of zig-zag chains of Zn 1 coordination polyhedra along b -axis direction, connected together by discrete PO 4 tetrahedra and pairs of Zn 2 coordination polyhedra.

The crystal structure of the π -complex [C 4 H 6 Co(CO) 2 ] 2 has been determined in order to check that the molecule has the structure predicted from infra-red evidence. The material, which consists of copper-coloured orthorhombic crystals, with unit-cell dimensions a = 10.51, b = 7.897 and c = 16.14 Å, is relatively unstable in air, but intensity data for three projections collected from shellac coated specimens enabled the structure to be determined. The analysis has shown that the crystal space group is Cmcm . The molecule is in the cis form, but has mm 2 ( C 2 v ) rather than the m ( C s ) symmetry deduced from the infra-red data.

All twelve components of the piezooptic tensor have been determined by static methods. The piezooptic behaviour of calcium formiate is typical for structures with strong diagonal bonds. The high piezooptic anisotropy is related with the optical and elastic anisotropy.

A graphical method for indexing x-ray powder photographs of complex superlattices is described, and the example of ordered Ni 4 Mo is worked out to demonstrate this method. The limitations and advantages of the method are briefly discussed.

It is shown that the conical extinction curves, introduced by J oel and T ocher , exhibit symmetry ranging from that of class [unk] to that of class ∞/ mm ; and that, when 2 V = 90°, certain conical extinction curves and the indicatrix itself exhibit elements of antisymmetry.

Sodium-A sorbs up to 6 molecules of bromine per unit cell. The occluded molecules occupy definite positions in the large cavity and form a cluster consisting of 6 molecules. The bromine atoms are lying near the vertices of a cuboctahedron. The maximum point symmetry of an individual bromine cluster is [unk]. The atomic coordinates of the framework atoms and cations in the hydrated and in the brominated zeolite differ somewhat.

Employing precision optical techniques, a survey was made of rhombohodral faces of natural quartz. The gradient near the summit of several growth pyramids was found to be less than that away from it. The gradients in all the three directions are illustrated and interpreted. From the study of the slopes of the planes of the pyramids the direction of the flow of mother solution is deduced.

The interpolation of atomic scattering factors is discussed. Analytical approximations by the method of orthogonal polynomials and by Lagrangian interpolation are presented and are compared with methods reported previously. The accuracy of interpolation is estimated and an application of the technique in the calculation of structure factors is suggested, particularly with reference to small computers. The working range is restricted to values of (sin θ )/ λ between 0 and 0.7.

Two new types of discrepancy factors which are mean values of fractional discrepancies of observed and calculated quantities are suggested. One of them is defined by [unk] where | F o | and | F c | are the observed and calculated structure factors, N is the total number of reflections and ‘ or ’ in the denominator means | F o | or | F c | whichever is larger, is to be used in the summation. The other index R ″ is similar to R ′ but uses intensities instead of amplitudes. Theoretical values of R ′ and R ″ are derived for a completely random structure, both for centrosymmetric and non-centrosymmetric cases. Tests on a practical case and comparison with the conventional discrepancy index are also reported.

A method for the computation of stereographic projections for crystals of low symmetry is described.

The structure of the nickel cyanide ammoniate complex has been determined by x-ray diffraction methods. The cell is tetragonal with a = 7.222 Å and o = 17.42 Å. The nickel and cyanide atoms pack in extended square arrays perpendicular to the c axis. The structure is compared with the structures of its hydrate and benzene clathrate which have been determined by other workers.

Retaining the original orientation, thallium carbonate has a = 7.55 Å, b = 5.38 Å, c = 10.58 Å, β = 94°47′. The space group is I 2/m and the cell contains 4 Tl 2 CO 3 . Indexed x-ray powder data are given.