Volume 217, Issue 9

Theoparacelsite is a recently discovered orthorhombic copper arsenate hydroxide mineral with the ideal chemical formula Cu 3 As 2 O 7 (OH) 2 , but because of disorder only two smaller formula units CuAs 2/3 H 2/3 O 3 are established in the orthorhombic unit cell with lattice parameters a = 4.6644(5) Å, b = 8.3212(8) Å and c = 2.9377(3) Å, and space group Pbmm . We show that this rare mineral is iso-typic to the unique inorganic spin-Peierls phase CuGeO 3 ( T SP = 14.3 K) and to the recently discovered copper polysilicate, CuSiO 3 , which is also a quasi-one-dimensional S = 1/2 anti-ferromagnetic chain compound ( T N = 7.9 K). We discuss in detail the previously reported crystallographic data of the phases at room temperature in view of its ability to attain similar low-dimensional physical properties and conclude that the arsenate with its S = 1/2 spin chains of edge-sharing CuO 4+2 octahedra may be a further inorganic compound that could show the spin-Peierls effect. Although the Cu-O(2)-Cu angle, which is considered to be important for the magnetic interaction and ground state properties, has been found for the arsenate (98.8°) almost identical to that of the germanate (99.1°), the modified inter-chain coupling of the arsenate as a defect variant of the CuGeO 3 structure type may suppress the spin-Peierls effect and favour other states of magnetic order (chiral spin chains, AF order). With respect to the rarity of the mineral some remarks on synthesis routes are made.

The crystal structure of kröhnkite [Na 2 Cu(SO 4 ) 2 ·2H 2 O], which contains infinite chains composed of CuO 6 octahedra corner-linked with SO 4 tetrahedra, was originally determined 1952 by Dahlman. Since then, a large number of both minerals (e.g., collinsite, fairfieldite, etc.) and synthetic compounds with closely related struc-tures have been investigated, but the structural relation was not always recognised. In the present review we compare the structures of A n M ( X O 4 ) 2 ·2H 2 O ( A = mono- or divalent cation, n = 1, 2; M = di- or trivalent cation; X = penta- or hexavalent cation) compounds with kröhnkite-type infinite chains, and propose a structural classification into six types. The rod group symmetries of the chains are compared. Furthermore, the linkage of these chains to double chains (in krausite-type compounds) and sheets (in yavapaiite-, merwinite-, bafertisite-type and similar structures) is discussed, and the occurrence of octahedral-tetrahedral chains topologically identical to those in kröhnkite (referred to as kröhnkite-type) and similar chains (referred to as kröhnkite-like) in several related compounds is pointed out.

The crystal structure of the α-Na 2 B 8 O 13 stable modification has been refined by single crystal X-ray diffraction at 293, 573 and 773 K. The compound is monoclinic, P 2 1 / a space group, unit-cell parameters at 273 K are: a =6.507(3), b =17.796(8), c =8.377(5) Å, β =96.60(4)°, Z =4. The structure contains two interlocking boron-oxygen frameworks, each of them consisting of alternating single and double rings composed of two triangles and a tetrahedron so called triborate and pentaborate groups. Two non-equivalent sodium atoms linking two frameworks are coordinated by 8 oxygens. They are connected into finite chains consisting of four NaO 8 polyhedra by sharing oxygen edges. The α-Na 2 B 8 O 13 thermal expansion investigated by high temperature powder X-ray diffraction has sharply anisotropic character including negative thermal expansion: the coefficients of a thermal expansion tensor in temperature region 273–1000 K are α 11 = 55–0.042·(273+ t ), α 22 = 11, α 33 = -15 + 0.032·(273+ t ) (×10 –6 K –1 ), μ = (c^α 33 ) = 42°. That high anisotropy is caused by anisotropic thermal vibrations of the Na atoms. Under heating a high stability of the B-O rigid groups and a weak contraction of some B-O bond lengths were revealed. In response to anisotropic thermal behaviour of Na atoms rigid B-O groups of the anion framework rotate relative to each other as a three-dimensional hinge.

The crystal structure of Pb 10 O 7 (OH) 2 F 2 (SO 4 ) (triclinic, P 1̅, a = 9.6637(16), b = 10.1922(17), c = 11.1407(18) Å, α = 110.978(3), β = 91.795(3), γ = 107.020(3)°, V = 968.2(3) Å 3 ) has been solved by direct methods and refined to R 1 = 0.050 ( wR = 0.089). There are ten symmetrically unique Pb 2+ cations in the structure all of which have highly asymmetric coordination polyhedra owing to presence of stereoactive 6 s 2 lone pairs of electrons. The structure is based upon [O 7 Pb 10 ] zigzag chains of edge-sharing OPb 4 oxocentered tetrahedra. The observed [O 7 Pb 10 ] chain is the most complex chain of oxocentered tetrahedra known so far and was not observed previously in structures of inorganic compounds. The chains run along [011] and their plane is parallel to (111). The chains form sheets parallel to (111). The SO 4 ) tetrahedra, F – anions and OH groups are between the sheets providing their linkage into three-dimensional structure. A comparison of the structure with other Pb oxysulfates is given in the framework of theory of complexes of anion-centered tetrahedra.

Langasite (La 3 Ga 5 SiO 14 ) crystallizes in the polar space group P321. Here we report the pressure dependence of the lattice parameters at high hydrostatic pressures up to 10.2 GPa. The results presented in terms of compressibility agree well with results from measurements of the elastic constants. According to a Birch-Murnaghan equation of state, the bulk modulus and its pressure derivative are 134.6(2) and 2.8(1) GPa, respectively. The compression of langasite crystals is anisotropic, the c / a -ratio appears to be a linear function of the pressure applied according to c / a = 0.62460(6) + 0.00055(1)* p /GPa. Altogether four single crystals have been investigated. Two of them showed a splitting of reflections ( hhl ). This splitting reaches a maximum at approximately 9 GPa and proved to be reversible.

Topological analysis and comparing of the crystal structures of anhydrous inorganic salts containing triangular oxo-anions (borates, carbonates and nitrates) was performed using the program package for multipurpose crystallochemical analysis TOPOS. The examples of topological similarities between these classes of substances and 2920 framework binary compounds A y X z were revealed and discussed. It is shown that in most cases the ionic arrays constructed according to one of close packings or to a b.c.c. motif can be selected in the crystal structures of anhydrous inorganic salts. The conclusions were made about the structure-forming role of atoms and atomic groups in these substances. The crystal structures of anhydrous salts are substantiated to be correctly considered as packings of metal cations and quasi-spherical oxo-anions.

Solid (NO) 2 TiCl 6 undergoes two phase transitions closely related to the order-disorder behaviour of the NO + species present in this salt-like compound. Starting from ambient conditions the sequence of space groups is P 4/ mnc → C 2/ c → P 2 1 / n when the temperature is lowered. At T c 1 = 199 K the formation of quadruplet domains is observed, while at T c 2 = 176 K superstructure reflexions appear due to decentring of the unit cell. In the tetragonal high-temperature form the orientational disorder of the nitrosonium groups is fourfold, partial order then starts to develop in the intermediate phase, whereas almost complete order is reached at 123 K in the second monoclinic phase. This two-step symmetry reduction was studied by DSC, powder diffraction, and by single-crystal measurements including complete structure determinations at 291, 203 and 123 K. In P 2 1 / n the selection of just one NO + orientation per site is coupled to a characteristic displacement pattern of the titanium atoms. For electrostatic reasons the latter show a response to short N···Cl contacts by an off-centre shift (ca. 11pm) inside their 'octahedral' chlorine cages. The stereochemical influence of antibonding NO + orbitals determines the inclination of the cation towards the nearest chlorine neighbours.

In this paper we report the crystal structure solution of a meta-stable phase of phenyl salicylate (salol). This modification, which is obtained from a super-cooled melt, has been observed before, but the structure has not been previously determined. Structure solution was achieved by a procedure combining a computer based systematic search method with a subsequent Rietveld refinement of experimental X-ray powder diffraction data. A clustering procedure to group similar trial structures based upon energy minimisations is discussed. The best trial structure identified by the combination of a systematic search and clustering was refined using the Rietveld method and a final structure with a weighted pattern index R wp of 0.0513 (excluding background) was obtained. The meta-stable phase of salol crystallises with a monoclinic unit cell with cell parameters a = 17.4047(8) Å, b = 5.9800(5) Å, c = 10.6882(7) Å, and β = 108.988(5)º. The space group is P 2 1 / n with four molecules in the unit cell. A structural comparison of the stable and meta-stable modifications is given.

A novel case of polymorphism in platinum(II) complexes is reported. Single crystals of [PtBr 2 (2,9-dimethyl-1,10-phenanthroline)] were obtained at room temperature from a dilute dichloromethane/pentane (1:3, v/v) solution of the title compound in the presence of guanosine. The space group resulted to be Pbca [ a = 20.4024(3), b = 20.3638(3), and c = 13.9522(2) Å, V = 5796.7(1) Å 3 , Z = 8]. Crystals of the same compound, obtained in slightly different conditions, had already been investigated by X-ray diffraction and found to have P 2 1 / n space group, [ a = 12.155(4), b = 7.933(2), and c = 14.522(2) Å, β = 93.74(2)°, V = 1397.3(6) Å3, Z = 4]. Comparison of the two polymorphous forms reveals huge differences in the crystal packing and the important role of the stacking interactions between phenanthroline ligands.