Zeitschrift Fur Kristallographie-new Crystal Structures 

Abstract: The structure of a cubic A1 69.5 Pd 23.0 Mn 6.2 Si 1.3 phase, which is referred as a 2/1 approximant of an icosahedral phase, was determined by means of single crystal X-ray structural analysis: space group Pm3 (No. 200), a = 20.211(2) A, V = 8255.9(14) A 3 , MoK a (λ = 0.71069 A); refined as A1 69.6 Pd 24.3 Mn 6.1 , atom/cell = 512.83, F(000) = 11154.6, μ = 8.72 mm -1 , D calc = 4.95 Mgm- 3 , R(wR2) = 0.0623(0.1691) for 2597 reflections with I > 2.0σ(J) and goodness of fit S = 1.096 for 165 variables. In the cubic 2/1 approximant structure, an atom cluster with about 20 A in diameter locates at the origin of the unit cell. This large cluster can be described by nine atomic shells with icosahedral symmetry, where the atomic arrangements similar to the Bergman or double Mackay clusters are clearly identified together with the τ-inflated relationships in their sizes. The icosahedral quasicrystals in Al-Pd-Mn system are considered to be realized by a quasi-periodic arrangement of the similar atom clusters.

Abstract: The structures of hexagonal μ-MgZnRE phases of Mg 28.3 Zn 65.2 Sm 6.5 and Mg 28.6 Zn 63.8 Gd 7.7 , were determined by single crystal X-ray structural analysis: μ-MgZnSm, P6 3 /mmc (No. 194), a = 14.619(1) A, c = 8.708(1) A, atoms/cell = 92, F(000) = 2484, μ = 27.44 mm -1 , D calc = 5.621 Mg m -3 , R = 0.0201 for the observed 1110 reflections with F obs > 4.0σ(F obs ) and μ-MgZnGd, P6 3 /mmc (No. 194), a = 14.633(2) A, c = 8.761(2) A, atoms/cell = 92, F(000) = 2527, μ = 28.51 mm -1 , D calc = 5.707 Mg m -3 , R = 0.0210 for the observed 1084 reflections with Fobs > 4.0σ(F obs ). The structure of μ-MgZnRE is closely related to that of the MgZn 2 Laves phase (P6 3 /mmc, a = 5.223 A, c = 8.566 A). The introduction of large RE elements increases the distortion of the icosahedral atomic arrangement around Zn and this produces a variety of icosahedral linkages in the μ-MgZnRE structure.

Abstract: CaH406S, monoclinic, C12/cl (No. 15), a = 6.284(1) Â, b = 15.200(1) Â,c = 6.523(1) λ,β = 127.41(1)°, V= 494.9 Â, Z = 4, Rp(F) = 0.028, wtfrefiF) = 0.079, T= 293 K. Source of material The crystallising solution was made up from commercial 0.5 M aqueous solutions of A. R. grade. Discussion In order to standardize all entries of powder diffraction patterns for gypsum and synthetic calcium sulphate dihydrate, the JCPDS [ 1,2] uses the reduced cell: a, b, c = 5.68 Â, 6.29 Â, 8.22 Â; α,β, γ = 67.5°, 81.0°, 65.8°, and indexing based on the monoclinic cell: a\ b\ c' = 6.29 Â, 15.18 A, 5.68 k\s = 114.2°, and space group CUc. This now seems to be generally accepted as the standard setting, but although it has the correct systematic absences, the symmetry is wrong, and according to International Tables [3] this is cell choice 2 (A2/n) in the setting C2Jn. This incorrect assignment was noticed in the course of the present work, only after data collection according to this cell choice and then transformed to the correct C2/ccell witha,b,c = 6.28 Â, 15.20Â, 6.52 k\s= 127.4°, where ai + bj + ck = -a'i b'j + (a' + ?>')k ; with i, j, k being the unit vectors. This choice of unit cell [3] has the correct setting of the space group, but being less eccentric, is now recommended as the standard setting. In the lower figure, a projection along the b axis of the actual unit cell (drawn to scale) is shown together with all previous choices. The unique b axis is common for all cell choices. The space group symmetry for the standard cell is also shown as a guide to the different specifications of the space group. A comparison of the crystal structures of the mineral gypsum with the synthetic material confirmed that there are no differences between the structures. The agreement with the neutron results is particularly striking, with the only significant difference related to the position of H(2), which is clearly wrong in the present work. It is still an improvement over the previous best X-ray study of Cole & Lancucki [4] in which hydrogen position could not be refined against the data. Important issues for discussion are the shape of the sulphate ion and the hydrogen bonding. The known deviation of the sulphate geometry from a symmetrical tetrahedron is convincingly confirmed, while the observed hydrogen-bonding is essentially the same as observed before. In the final analysis the structure of pure gypsum is now known in sufficient detail from X-ray diffraction to allow an analysis of distortions, introduced by dopants, such as lanthanide ions. * Correspondence author (e-mail: jboeyens@postino.up.ac.za) Brought to you by | University of Pretoria Authenticated Download Date | 9/26/14 8:16 AM 1 0 Calcium sulphate dihydrate Table 1. Data collection and handling. Table 2. Atomic coordinates and displacement parameters (in Â"). Crystal: Wavelength: MDiffractometer, scan mode: 2 0 m a v Wl*/.)measured, NfhUhmque: Criterion for N(hkl)gt: N(param)„ fined: Program: clear prism, size 0.08 χ 0.16 χ 1.2 mm Mo Ka radiation (0.71073 λ ) 16.32 cm" Enraf Nonius CAD4, ω/20 69.9° 4287, 109 lobi > 2 Oliate), 971 46 SHELXL-93 [5] Atom Site .r V Uiso H(l) 8/ 0.749(3) 0.162(1) 0.251(4) 0.031(8) H<2) 8/ 0.584(5) 0.235(2) 0.073(5) 0.057(7) Table 3. Atomic coordinates and displacement parameters (in Â). Atom Site X y ζ U M t / 2 2 t /33 U ,2 U 1 3 t / 2 3 Ca 4e 1/2 0.07956(1) 3/4 0.0105(1) 0.0124(1) 0.0104(1) 0 0.0057(1) 0 S(2) 4e 0 0.07724(2) 3/4 0.0084(2) 0.0108(2) 0.0090(1) 0 0.0046(1) 0 0(1) 8/ 0.0372(1) 0.13198(5) 0.5872(1) 0.0163(3) 0.0187(3) 0.0145(3) 0.0006(2) 0.0093(2) 0.0052(2) 0(2) 8/ 0.2424(1) 0.02211(4) 0.9092(1) 0.0115(3) 0.0179(3) 0.0172(3) 0.0052(2) 0.0075(2) 0.0068(2) OW 8/ 0.6202(2) 0.18197(5) 0.0784(2) 0.0304(4) 0.0182(3) 0.0157(3) 0.0013(3) 0.0099(3) -0.0028(2) Acknowledgments. We gratefully acknowledge Quirina Roode for growing the single crystals and Leanne Cook for data collection at the University of the Witwatersrand.

Abstract: Ba6Ge25, cubic, P4i32 (No. 213), a = 14.5564(2) A, V = 3075.8 A, Ζ = 4, pm = 5.698 g -cm", R g t ( F ) = 0.089, wR(F) = 0.086, Γ =293 Κ. Source of material The compound was prepared from the elements by melting in an open glassy carbon crucible (HF furnace, argon atmosphere) and annealing at 923 Κ (12 d). Ba6Ge25 is silvery metallic, brittle, stable in air and moisture. Discussion BaaGe25 belongs to the chiral clathrate structure family cP 124, being the second binary prototype of the BaoIn4Ge2i type [1-4]. The structure is characterized by a chiral 3D framework of condensed Ge2o pentagondodecahedra (pdod) embedded in a 3D channel labyrinth. Each pdod ist centered by Bal and the rest Ba atoms occupy the cavities in the zeolite-like labyrinth. The rather large elongation of the displacement ellipsoid of Ba2 (M2) is typical for the structure family. The Ge—Ge bond lengths vary between 2.47 A and 2.56 A. Per formula unit, there are seventeen fourfold bonded (4b) Ge° atoms and eight Ge are threefold bonded (3b)Ge : (Ba)6((3b)Ge")8((4b)Ge°)i7(4el. There are no indications of vacancies in the Ge framework. The compound is a Zintl phase with few conduction electrons. Below 180 K, the compound is a semiconducting Zintl phase with 4e in localized antibonding states of the Ge framework and it becomes metallic above 250 Κ [1], Table 1. Data collection and handling. Crystal: silvery block, size 0.10 χ 0.08 χ 0.02 mm Wavelength: Mo Ka radiation (0.71073 A) μ·· 315.23 cm" Diffractometer, scan mode: Nicolet R3m/V, ω 20max: 65° N(hkl)„,easured, N(hkl)u„ique: 1750, 1405 Criterion for F0bs, N(hkl)gC. Fobs > 4 o(Fobs), 630 N(param)Kiinea'· 49 Programs: SHELXTL-plus [5], ATOMS [6] Table 2. Atomic coordinates and displacement parameters (in A). Atom Site χ y ζ U11 U22 C/33 U12 U13 i/23 Ba(l) 8c 0.0607(2) X X 0.017(1) U11 Uu 0.002(1) U12 U12 Ba(2) 12d 1/8 0.1887(3) y+1/4 0.107(6) 0.031(2) U21 0.276(3) 0.005(3) -U13 Ba(3) 4 a 3/8 3/8 3/8 0.025(2) Uu Uu 0.010(2) Uu U12 Ge(l) 24e 0.2929(4) 0.9538(4) 0.7501(4) 0.016(3) 0.015(2) 0.017(3) 0.002(2) 0.003(2) 0.001(2) Ge(2) 8c 0.9251(4) X X 0.015(2) Uu υ η 0.002(2) U\2 U12 Ge(3) 8c 0.2182(5) X X 0.027(2) V11 Uu 0.005(3) Un Un Ge(4) 12d 1/8 0.8309(4) y+1/4 0.010(3) 0.017(2) U22 0.156(2) -0.004(2) U n Ge(5) 24e 0.9166(4) 0.0847(4) 0.8532(4) 0.015(3) 0.020(3) 0.015(2) 0.001(2) 0.001(2) 0.000(2) Ge(6) 24e 0.1853(4) 0.9899(3) 0.8767(4) 0.015(2) 0.013(2) 0.017(3) -0.004(2) -0.003(2) 0.003(2) * Correspondence author (e-mail: carrillo@cpfs.mpg.de) 10.1515/ncrs-2000-0208 Downloaded from PubFactory at 09/02/2016 06:03:31AM via free access 208 Hexabarium pentacosagermanide

Abstract: Abstract C17H16Cl2N2O2, monoclinic, P21/c (no. 14), a = 9.1238(5) Å, b = 13.0575(4) Å, c = 14.3740(5) Å, β = 104.616(4)°, V = 1657.02(11) Å3, Z = 4, Rgt(F) = 0.0460, wRref(F2) = 0.1120, T = 293(2) K.