Volume 206, Issue 1

TlCr 3 S 5 crystallizes in the monoclinic space group C 2/ m with the lattice parameters a = 19.293(4), b = 3.4653(9), c = 11.962(2) Å, β = 122.18(3)°, V = 676.8(3) Å 3 , Z = 4. The structure was refined to R = 0.067 and R w = 0.048. CrS 6 octahedra are joined by common edges and faces leading to a three dimensional network with channels parallel to the crystallographic b -axis. The Tl atoms are confined within these channels.

In the natrochalcite-type compounds NaCu 2 (H 3 O 2 )(SO 4 ) 2 and RbCu 2 (H 3 O 2 )(SeO 4 ) 2 the lengths of the two different hydrogen bridges vary widely: 2.44 Å and 2.70 Å in the Na-S member, and 2.60 Å and 3.03 Å in the Rb-Se member. Both structures were refined by single-crystal neutron diffraction methods. Least-squares refinements (harmonic vibrations for all atoms) resulted in a splitting of the H atom positions of the short bridges by 0.265(13) Å in the Na-S member, and by 0.472(6) Å in the Rb-Se member. In the two compounds no clear evidence for an ordering of the split position of the H atom was found.

Application of an external electric field of 1300 V/mm ∥ c induced the displacement of ∼3% of the chloride ions in the channels of a chlorapatite crystal with composition Ca 10− x (PO 4 ) 6 Cl 2−2 x □ 2 x ( x = 0.03). The percentage of the displaced ions corresponds to the concentration of the vacancies in the channels. The displacements bring about a partial reversal of the elementary dipoles in the ideally antiferroelectric structure and thus lead to a ferrielectric phase. At a field strength of 5000 V/mm chlorapatite became optically uniaxial and possibly ferroelectric.

The structure of K 2 ZnSi 3 O 8 has been determined using Rietveld analysis of X-ray powder diffraction data ( R p = 3.99%). With space group Imam and cell dimensions a = 5.285(1), b = 9.058(1), c = 8.535(1) Å, tetrahedrally coordinated Zn and Si atoms are disordered over one crystal-lographic site forming at tecto-(zincio)silicate structure with 8-coordinated K atoms in intervening large cavities. This is the first example of the aristotype of the NH 4 LiSO 4 family of general formula, AT 2 O 4 , where A is a large alkali (or pseudo-alkali) atom and T represents tetrahedrally-coordinated atoms including: Al, Be, Co, Li, Mg, P, S, Si, and Zn.

Application of Direct Methods to reflection intensities obtained by powder diffraction is often obstructed by the fact that not all intensities are reliable. In this paper a weighting scheme is proposed to overcome this problem. The scheme is implemented in the Direct Methods program SIMPEL88 in such a way that phase relations containing overlapping reflections are less probable than they would have been containing very reliable reflections only. Preliminary test results show that this way of tackling the problem is very promising.

The method of single-wavelength anomalous scattering (SAS) allows the determination of structure-factor phases apart from a two-fold phase ambiguity. A statistical procedure is described, based on the joint probability distribution of three structure-factors, in which a priori the influence of partial structural information is introduced (Giacovazzo, C., Acta Crystallogr. A39 (1983) 685–692), which permits to calculate the relative probabilities of the two SAS solutions for each phase. The relation of this procedure with the sparse density principle (i.e. the assumption that ϱ ( r ) ϱ (− r ) ≈ 0) is indicated. A possible application is the use of extra structural information, e.g. information on atomic positions in a protein backbone, in the course of a protein crystal structure determination.

A direct method was tested in solving the structure of a 3-dimensional quasicrystal in 6-dimensional space. Theoretical 3-dimensional diffraction data were used which contain Gaussian distribution errors with a mean error of about 20% for intensities. The diffraction data were firstly converted to a set of 6-dimensional structure factors. The window fuhction used for the conversion was measured from the Patterson origin peak in pseudo space. A direct method was then applied to solve the phase problem in 6-dimensional space. Test results showed that the procedure is very efficient.

Crystal data: ( I ) Methylammonium hexachloromolybdate(III) Chloride, (CH 3 NH 3 ) 4 [MoCl 6 ]Cl, M r = 472.38, orthorhombic, Pbam ; a = 12.596(3), b = 9.923(2), c = 7.333(2) Å, V = 916.6(7) Å 3 , T = 293(2) K, D x = 1.711(1) g cm −3 , D m = 1.70(2) g cm −3 , Z = 2, F (000) = 474, Mo Kα radiation, λ = 0.71069 Å, μ = 17.16 g cm −1 , final R = 0.043 and R w = 0.048 for 1325 observed reflections [ I > 3 σ ( I )]. ( II ) Dimethylammonium hexachloromolybdate(III) chloride, [(CH 3 ) 2 NH 2 ] 4 [MoCl 6 ]Cl, M r = 528.48, orthorhombic, P 2 1 2 1 2; a = 12.867(2), b = 10.158(2), c = 8.719(2) Å, V = 1145.3(7) Å3, T = 293(2) K, D x = 1.532(1) g cm −3 , D m = 1.53(2) g cm −3 , Z = 2, F (000) = 538, Mo Kα radiation, λ = 0.71069 Å, μ = 13.81 cm −1 , final R = 0.019 and R w = 0.020 for 1548 observed reflections [ I > 3 σ ( I )]. There exist alternate layers of [Cation/[MoCl 6 ] 3− ] and [Cation/Cl − ] parallel to the ( a , b ) plane in both structures interlinked by hydrogen bonds N–H…Cl. The space group of compound ( II ) is a subgroup of the space group of compound ( I ). The symmetry reduction ( Pbam → P 2 1 2 1 2) results from the orientation of the [Me 2 NH 2 ] + ions in compound ( II ).

The structure of terguride monohydrate (C 20 H 28 N 4 O · H 2 O) was solved by direct methods and refined anisotropically to R = 0.069 for 3040 unique observed reflections. The title compound crystallizes in space group C 2 with lattice parameters a = 38.262(4), b = 7.834(3), c = 13.497(2) Å, β = 101.69(2)°, Z = 8. Two independent terguride molecules (denoted as A and B) were found in asymmetric unit, each includes tetracyclic ergoline ring system (one five-membered and three six-membered rings). The pyrrole moiety is nearly planar, the ergoline C ring has half boat conformation (in both molecules). Conformation of the ergoline D ring can be described as almost regular chair for A molecule, whereas it is considerably distorted for B molecule. The three-dimensional hydrogen bond network is built through water molecules only.

Powder neutron diffraction studies of solid CF 3 I indicate that there are two solid-state phases with a transition temperature of 115 K. The structure of the higher-temperature phase I, which melts at approximately 143 K, has been determined from high-resolution powder diffraction data. The structure is orthorhombic, space group Cmca , Z = 8, with a = 7.5354(7) Å, b = 5.8491(6) Å, c = 19.323(1) Å at 130 K. The structure consists of bilayers of iodine atoms interspaced with bilayers of CF 3 groups.