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JournalISSN: 1433-7266

Zeitschrift Fur Kristallographie-new Crystal Structures 

De Gruyter
About: Zeitschrift Fur Kristallographie-new Crystal Structures is an academic journal published by De Gruyter. The journal publishes majorly in the area(s): Crystal structure & Monoclinic crystal system. It has an ISSN identifier of 1433-7266. It is also open access. Over the lifetime, 7206 publications have been published receiving 9744 citations. The journal is also known as: New crystal structures (Print) & Zeitschrift für Kristallographie. New crystal structures (Print).


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TL;DR: The structure of a cubic A1 69.3 phase, referred as a 2/1 approximant of an icosahedral phase, was determined by means of single crystal X-ray structural analysis as discussed by the authors.
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.

45 citations

Journal ArticleDOI
TL;DR: The structure of hexagonal MgZnRE phases of Mg 28.3 Zn 65.2 Sm 6.7, were determined by single crystal X-ray structural analysis.
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.

37 citations

Journal ArticleDOI
TL;DR: In this paper, the authors proposed the use of the reduced cell instead of the C2Jn unit cell in the standard setting, which has the correct systematic absences, but the symmetry is wrong.
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.

32 citations

Journal ArticleDOI
TL;DR: In this article, a more precise refinement of the atomic as well as the displacement parameters of the previously reported model was obtained on the basis of photographic single crystal data obtained from ground single crystals of Pd2Ga and X-ray powder diffraction.
Abstract: GaPd2, orthorhombic, Pnma (no. 62), a = 5.4829(8) A, b = 4.0560(4) A, c = 7.7863(8) A, V = 173.2 A, Z = 4, Rgt(F) = 0.022, wRref(F) = 0.029, T = 295 K. Source of material Starting from powder of Pd (99.9 %, Chempur) and granules of Ga (99.9999%, Chempur) in a ratio of 2:1 and 1.5 mg/ml GaI3 as transport agent (99.999%, Chempur), needle-shaped single crystals of Pd2Ga were synthesized by an exothermal chemical transport reaction in a temperature gradient from 673 K (source) to 873 K (sink) [1]. Experimental details Lattice parameters of the title compound were determined by least-squares fitting of 24 reflections from powder X-ray diffraction data obtained from ground single crystals (Huber Image Plate Guinier camera G670, CuK41 radiation, , = 1.54056 A, LaB6 as internal standard, a = 4.15692 A). Due to the significant homogeneity range reported for Pd2+xGa1–x [2], the occupancy values together with the anisotropic displacement parameters were allowed to vary in separate series of the structure refinement while the overall scale factor was fixed. The resulting occupancies were equal to unity within one e.s.d. 0.998(4), 1.003(4) and 1.006(5) for Pd1, Pd2 and Ga sites, respectively, confirming the 2:1 composition of the investigated specimen. In the final refinement series full occupancies were assumed for all positions. Discussion The crystal structure of Pd2Ga adopts an atomic arrangement of theCo2Si type of structure [3]. The present investigation provides a more precise refinement of the atomic as well as the displacement parameters of the previously reportedmodel obtained on the basis of photographic single crystal data [4] and X-ray powder diffraction [2]. The title compound represents the stoichiometric (x = 0) composition of Pd2+xGa1–x and thus falls into the reported homogeneity range of –0.04 ( x ( 0.02 at 873 K [2,4]. All atoms in the crystal structure of Pd2Ga are situated on one of the two mirror planes perpendicular to [010]. A clear gap at !3.00 A separates the first coordination sphere of each atom in the structure of Pd2Ga. Each Pd1 atom has distorted tetrahedral coordination by four Ga atoms with distances varying between 2.54 A and 2.56 A. One additional Ga atom is quite far away at a distance of 2.96 A. The environment of Pd2 is different: nonplanar trigonal coordination by three Ga atomswith d(Pd—Ga) = 2.56A – 2.62A and two other Ga atoms at a considerably longer distance of 2.84 A. The closest Pd—Ga contacts are comparable with the sum of the single bond radii of Pd (1.28 A) and Ga (1.25 A) [5]. The coordination sphere of each palladium site is completed by eight Pd atomswith d(Pd—Pd) between 2.82A and 2.99A, thus increasing the coordination number of both positions to 13. These contacts are slightly longer than the interatomic distance of 2.75A in ccpPdmetal [6]. Gallium atoms are surrounded exclusively by ten palladium species. The shortest Ga···Ga distance of 3.43 A is significantly longer than the average interatomic distance of 2.70 A in the 4-modification of Ga [6]. The environment of gallium atoms is formed by seven Pd atoms in the range of 2.54 A – 2.62 A and three Pd atoms at distances 2.84 A (2×) and 2.96 A (1×). Z. Kristallogr. NCS 223 (2008) 7-8 / DOI 10.1524/ncrs.2008.0004 7 © by Oldenbourg Wissenschaftsverlag, Munchen

28 citations

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2023208
2022165
2021400
2020376
2019437
2018435