Correct Space Group Research Articles

Crystal structure of the new metallic copper oxide La 2Sr 6Cu 8O 18-δ

The structure of metallic copper oxide La 2Sr 6Cu 8O 18-δ (δ≈0.4) has been determined by electron diffraction and neutron powder diffraction. The correct space group was found to be P 4/mbm with lattice parameters a=10.8502 (3) Å≈2√2a p and c=3.7658 (1) Å≈a p, where a p is the lattice parameter of the cubic perovskite. The oxygen vacancies are ordered both in the basal plane, along [001], and in the z= 1 2 plane, along [110]. The idealised framework of [Cu 8O 18] consists of square planar CuO 4 groups and CuO 5 square pyramids by sharing their corner oxygen, leading to the formation of hexagonal tunnels along the c-axis. La 3+ and Sr 2+ ions are located in these tunnels in a disordered manner.

Crystal structure of polyacetylene revisited: An x-ray study

We present new x-ray diffraction data on trans-( CH) x which shows that the blond alteration is out of phase on adjacent chains, and therefore the correct space group is P2 1 n . Previous attempts to resolve this question by analyzing only the (00L) intensities have yielded ambiguous results; all experiments show some (001) intensity which is not allowed P2 1 n . By analyzing indidual off-axis peak intensities, we show unambiguously that the bulk 3-D structure is inconsistent with P2 1 a (in-phase bond alternation), and therefore the weak, sample-dependent (001) intensity probably results from uncorrelated local defect regions with in-phase bond alternation on adjacent chains.

Structure of (aniline)(chloro)bis(ethylenediamine)cobalt(III) chloride monohydrate

C10H23C1CoNsZ+.2C1-.HzO, Mr=396 .64 , triclinic, P1, a = 8 . 1 8 9 ( 3 ) , b=9 .502 (4 ) , c = 12.051 (5) A, a = 104.12 (2), /3 = 101.89 (2), y = 100.68 (2) °, V = 862.0 (1) A 3, Z = 2, Dx = 1.53 g cm -3, A(Mo Ka) = 0.71069 A,/~ = 14.7 c m 1, F(000) = 412, room temperature, final R = 0.054 for 1893 observed reflections [1>3o-(/)] out of 3540 independent reflections. The aniline and chloride are in cis positions. The aniline nitrogen is sp 3 hybridized; the aniline N C o distance is 0.07 A longer than the ethylenediamine N C o bonds. The aniline ring tilts away from the ethylenediamine chelate rings in contrast to the situation found in aniline complexes of organometallic cobalt dimethylglyoxime derivatives. An extended network of hydrogen bonds is formed by the chlorides, water, and one of the ethylenediamine N atoms. The Co--N(anil ine) bonds in Co m aniline complexes fall into long (2.156 A av.) and short (2.022/~ av.) groups. The long Co--N(aniline) bonds are trans to alkyl groups; the short bonds are trans to N, C1or I . Experimental. Title compound prepared as reported previously (Meisenheimer & Kiderlen, 1924; Bailar & Clapp, 1945; Chawla, Lambert & Jones, 1967). Chromatography on Dowex 50-X8 (200-400) mesh, elution with 3 M HC1, crystals obtained by vapor diffusion of ethanol into an aqueous solution of the title compound. Dark red crystals, 0.25 x 0.22 x 0.35 mm; Nicolet P3m diffractometer; graphite-monochromatized M o K a radiat ion; o.r--2O scan. Scan speed 415 ° (0 )min -1. Indicated Laue symmetry 1. Correct space group P1 confirmed by subsequent calculations. Cell parameters from setting angles of 15 reflections between 30 and 35 ° . Data corrected for Lorentz and polarization effects; no correction made for absorption due to regular shape of crystal and * Work performed under the auspices of the US Department of Energy. t Author to whom correspondence should be addressed. small absorption coefficient. Intensities of three standard reflections measured after every 97 reflections did not show any systematic variation during data collection; 3540 unique reflections measured between 4 30-(/) used in refinement. Structure solved by direct methods (Karle & Karle, 1966) as implemented in the TEXRAY234 (Molecular Structure Corporation, 1985) crystallographic computing package. Non-H atoms refined with anisotropic themal parameters, all H atoms, located from subsequent difference syntheses, refined isotropically. Structures refined to convergence using 281 parameters by full-matrix least-squares methods (Molecular Structure Corporation, 1985) on a PDP 11/73 minicomputer. Y w(IFol -IF~[) 2 was minimized, w 1 = 0-2(IFol ) + 0.04(IFol) 2. Final R = 0.054, wR= 0.057, S = 1.24, (A/0-)max = 0.07, 0 . 2 4 < Ap < 0.31 e A -3. Atomic scattering factors used were those from International Tables for X-ray Crystallography (1974, Vol. IV). Fig. 1 is a view of the molecular ion showing the numbering scheme. Table 1 lists the refined coordinates and equivalent isotropic thermal parameters for the heavy atoms in the molecule.:]: Table 2 lists the bond distances and angles in the molecular ion. The coordination sphere of the Co nI ion is completed by the four N atoms of the two ethylenediamine ligands, N from the aniline molecule, and the C1ion to form an octahedral structure. Fig. 1 shows that the conformation of the complex is such that the aniline ring tilts away from the chelate ring made up of N( l l ) , C(7), C(8), and N(12), in contrast to the situation in the Co m dimethylglyoxime (dmg) com:~ Lists of structure factors, equivalent isotropic thermal parameters for C, N, O, CI, Co, anisotropic thermal parameters, H-atom parameters, bond lengths involving H, and selected bond length and angle comparisons for thirteen Co m aniline complexes have been deposited with the British Library Document Supply Centre as Supplementary Publication No. SUP 54747 (16 pp.). Copies may be obtained through The Technical Editor, International Union of Crystallography, 5 Abbey Square, Chester CH1 2HU, England. 0108-2701/92/061122-03506.00 © 1992 International Union of Crystallography R. J. BUTCHER, H. H. HAYDEN, M. M. D. DEYRUP AND C. B. STORM 1123 Table 1. Final least-squares positional parameters for C, N, C1 and Co (x 104)

CuO: X-ray single-crystal structure determination at 196 K and room temperature

An X-ray single-crystal determination of the CuO structure has been made at 196 K, i.e. below the Neel temperature 230 K, and, as a check, the crystal structure at room temperature was also determined. The correct space group for the structure at both temperature was found to be Cc. Earlier results of magnetic and neutron diffraction measurements can be explained as antiferromagnetic coupling between copper atoms via oxygen (superexchange) in chains running in the (1 0-1) direction. The structural results show changes with temperature in Cu-O distance in these chains: in each -Cu-O-Cu-group the longer is increased and the shorter decreased when passing from 196 K to room temperature. This implies a weaker antiferromagnetic coupling at room temperature. The refinement of CuO-structure at room temperature reported earlier by Asbrink and Norrby (1970) showed the symmetry to be C2/c. An attempt to refine CuO in the space group Cc with the old data was not successful. The different results obtained with different crystals are tentatively explained from published observations regarding valence fluctuations in CuO and non-stoichiometry caused by cation vacancies.

La2 –xSrxCuO4 –δ: structural, magnetic and transport measurements on antiferromagnets, insulators and superconductors

The structural, magnetic and conducting properties of the La2 –xSrxCuO4 –δ system (0≤x≤ 0.13, 0.01 ≤δ≤ 0.04) are examined as functions of temperature, and combined with assignment of the formal Cu oxidation state. High-resolution powder neutron structural results confirm that the correct space group is Abma, ruling out charge-density wave behaviour as well as other Fermi-surface instability mechanisms, as being responsible for the tetragonal-to-orthorhombic phase transition. The reduced magnitude and eventual disappearance of the orthorhombic distortion as the formal Cu oxidation state increases is well described by the better matching of the (La,Sr)—O and Cu—O bond distances in the two layers, (Ln2O2) and (CuO2), resulting in the tolerance factor increasing into the stability range of the K2NiF4 structure. The structural data reveal that there is a smooth variation of all structural parameters as the formal Cu oxidation state increases, except in the bond-length ratio (dax–deq)/(dax+deq), where dax and deq are the axial and equatorial Cu—O bond distances of the severely elongated CuO6 octahedra. This ratio peaks at the formal Cu oxidation state nearest to + 2 in our series (i.e.+ 2.01) to a value of 0.1207(3), decreasing at both higher and lower oxidation states. An electronic Jahn–Teller mechanism is found superior to a superexchange mechanism in explaining the orbital ordering, indicating that the holes formed on Sr2+ doping have σ* character. Analysis of the magnetic Bragg scattering resulting from antiferromagnetic ordering of the copper spins leads to the evaluation of the staggered Cu moment which is reduced on doping and disappears at a formal Cu oxidation state of between +2.01 and +2.04. The data support localised models for the Cu spin system and provide no evidence for spin-density wave behaviour. Analysis of resistivity data at low temperatures shows the occurrence of variable-range hopping even after the long-range magnetic order has disappeared, demonstrating that the electronic states at the Fermi level are localised. Doping by oxygen vacancies and/or Sr2+ cations leads to the introduction of impurity states into the Mott–Hubbard gap. Each carrier extends over two or three copper sites, with the localisation length ξ increasing to beyond the mean interdopant separation on approaching the metal–insulator transition. As the temperature increases, there is a crossover to a transport mechanism based on excitation to the mobility edge in the overlapping upper-Hubbard impurity and valence bands with the metal–insulator transition being of the Anderson type.

Structural Aspects of the Phase Separation in La2CuO4.032

AbstractThe average structure of superconducting La2CuO4.032 has been determined by single crystal neutron diffraction data. The excess oxygen is located between two adjacent LaO layers. Its presence distorts the apical‐oxygen sublattice in such a way that a short O‐O bond is formed (1.64Å). By scanning several hkl reflexions, we have confirmed that a phase separation occurs below room temperature. The peaks of the phases are in agreement with the unit cells proposed by Jorgensen et al. and Zolliker et al. However, Cmca seems to be the correct space group for both phases. La2CuO4+მ and the Ni counterpart are, thus, not isostructural.

Synthesis and structure of (cis )-[l-ferrocenyl-2-(4-nitrophenyl)ethylene], an organotransition metal compound with a large second-order optical nonlinearity

Nature 330, 360–362 (1987) THIS organotransition metal compound was thought to belong to the monoclinic space group Cc. Now the structure has been re-refined in the orthorhombic space group F2dd, and this is found to be the correct space group using fewer parameters to define the model. In the original paper the crystal data section (Fig.

Conformation of ethylene glycol and phase change in silica sodalite

The crystal structure of silica sodalite, including possible locations for encapsulated ethylene glycol, was determined at room temperature by using a combined single-crystal X-ray and powder neutron diffraction analysis. Unit cell composition: Si/sub 12/O/sub 24/ x 2C/sub 2/H/sub 4/(OH)/sub 2/, M/sub r/ = 845, cubic, Im3m, a = 8.830 (1) A (X-ray), a = 8.8273 (1) A (neutron). These refinements reveal that the correct space group for silica sodalite is Im3m (rather than I43m) and, therefore, that the sodalite framework is fully expanded. At room temperature, each sodalite cage contains one ethylene glycol molecule which has a range of geometrical positions. Intramolecular distances for the ethylene glycol molecule are (X-ray) C-C 1.78 (4), C-O 1.30 (6) A, (neutron) C-C 1.70 (3), C-O 1.25 (3) A. The shortest distances (3.4 A) between oxygens of the framework and molecule are consistent with weak hydrogen bonding. From the neutron diffraction data it was found that a sluggish phase change from cubic to lower symmetry occurs upon cooling below 200 K. At 10 K, silica sodalite appears to be monoclinic with approximate cell parameters a = 12.250 (8) A, b = 12.471 (8) A, c = 8.512 (6) A, ..beta.. = 91.37 (6)/sup 0/,more » based on an indexing of 12 peaks, but the precise symmetry is as yet unknown.« less

The Space Group of Sodium Nickel Arsenate, NaNiAs04

Abstract The correct space group of NaNiAsO4 is R3̅ and not R3 as previously reported

Effect of inaccessible volumes accompanied by symmetry elements on cumulative distribution functions

Abstract Symmetry elements are accompanied by volumes that cannot be occupied by the centres of atoms in the unit cell. The effect of these inaccessible volumes on cumulative distribution functions of normalised intensity has been investigated in space groups belonging to crystal classes 2, m, 2/m. The modified distribution functions have been compared with the corresponding distributions based on three published structures. The validity of these distribution functions in detecting the correct space group has been demonstrated by a reasonable good agreement between theoretical and experimental plots.

Hydrate der Hexachloroantimon(V)säure

The crystal structure of HSbCl6 · 4½H2O has been determined from three-dimensional X-ray data collected with a diffractometer at 180(± 5) K and refined to R = 0.023 (MoKα, radiation, 2169 reflexions). The correct space group is I112/b with a = 12.42(1), b = 16.65(1), c = 11.93(1) Å and γ = 92.44(5)° observed at 180 K. Pairs of asymmetrically coordinated H5O2+ ions [O – H⋯O 2.413(3) Å] are bridged by water molecules [O – H⋯O 2.682(2), 2.737(3) Å] to give centrosymmetric six-membered rings H14O62+. With the remaining water molecules attached to these rings [O – H⋯O 2.707(2), 2.791(3) Å] infinite chains along [010] are formed. The SbCl6− ions build up a pseudocubic subcell with Z = 2. To accommodate the protonated water chain the anions are shifted apart and rotated with regard to one of the ‘cubic’ diagonals, thus explaining the observed reduction of symmetry and a unit cell with Z = 8.

Magic‐Angle‐Spinning NMR (MAS‐NMR) Spectroscopy and the Structure of Zeolites

AbstractAfter outlining the chemical features and properties which make zeolites such an important group of catalysts and sorbents, the article explains how high‐resolution solid‐state NMR with magic‐angle spinning reveals numerous new insights into their structure.29Si‐MAS‐NMR readily and quantitatively identifies five distinct Si(OAl)n(OSi)4‐nstructural groups in zeolitic frameworks (n = 0, 1,….4), corresponding to the first tetrahedral coordination shell of a silicon atom. Many catalytic and other chemical properties of zeolites are governed by the short‐range Si, Al order, the nature of which is greatly clarified by29Si‐MAS‐NMR. It is shown that, as expected from Pauling's electroneutrality principle and Loewenstein's rule, both in zeolite X and in zeolite A (with Si/Al = 1.00) there are no AlOAl linkages. In zeolite A and zeolite X with Si/Al = 1.00 there is strict alternation of Si and Al on the tetrahedral sites. Ordering models for Si/Al ratios up to 5.00 (in zeolite Y) may also be evaluated by a combination of MAS‐NMR experiments and computational procedures.29Si‐MAS‐NMR spectra reveal the presence of numerous crystallographically distinct Si(OSi)4sites in silicalite/ZSM‐5, suggesting that the correct space group for these related porosilicates is not Pnma.27Al‐MAS‐NMR clearly distinguishes tetrahedrally and octahedrally coordinated aluminum, proving that, contrary to earlier claims, Al in silicalite is tetrahedrally substituted within the framework. In combination,29Si‐ and27Al‐MAS‐NMR is a powerful tool for monitoring the course of solid‐state processes (such as ultrastabilization of synthetic faujasites) and of gas‐solid reactions (dealumination of zeolites with silicon tetrachloride vapor at elevated temperatures). They also permit the quantitative determination offrameworkSi/Al ratios in the region 1.00 &lt; Si/Al &lt; 10 000. Since most elements in the periodic table may be accommodated within zeolite structures, either as part of the exchangeable cations or as building units of the anionic framework, there is immense scope for investigation by MAS‐NMR and its variants (cross‐polarization, multiple pulse and variable‐angle spinning) of bulk, surface and chemical properties. Some of the directions in which future research in zeolite science may proceed are adumbrated.

The zeolite A debate: how wrong is a nearly correct space group?

The apparent contradiction between NMR and X-ray results concerning the Si, A1 distribution in zeolite A is quantitatively analysed. It is concluded that the Fm{\bar 3}c structure model, based on X-ray refinements, is compatible with the NMR data if a cautious chemical-shift assignment is applied. The appropriate use of crystallographic approximations and their symmetry is discussed.

The single crystal polarized vibrational spectra of NaMnF3 and NaCoF3 and the assignment to their correct space group

The single crystal polarised IR reflectance spectra of NaMnF3 and NaCoF3 and the single crystal Raman spectra of NaMnF3 have been discussed. The reflectivity data was subjected both to K-K and classical dispersion analysis to obtain the dispersion parameters. From consideration both of the factor group predictions and Raman and IR data, the centrosymmetric space group D2h16 is assigned for the NaMnF3 and NaCoF3 complexes.

Thermal and dielectric properties of LiKSO 4 and LiCsSO 4

Measurements of the thermal and dielectric properties of single crystals of the double sulfates LiKSO 4 and LiCsSO 4 are reported. Uncertainty in the determination of the space group of LiCsSO 4 is resolved on the basis of optical second harmonic generation and pyroelectric measurements; the correct space group is Pcmn. Measurements of the pyroelectric coefficient, dielectric constant, and specific heat of LiKSO 4 permit an assessment of the suitability of this material for use in pyroelectric detectors.

Neutron diffraction of the paraelectric and antiferroelectric phase of deuterated copper formate tetradeuterate

The crystal structure of the paraelectric and antiferroelectric phase (Tc = 246.1 K) of deuterated copper formate tetradeuterate was refined from neutron diffraction data. Final R factors were 0.022 and 0.033 respectively. Superstructure reflections in both phases indicate P1 as the correct space group. The refinement was carried out in the approximate space groups P21/a (296 K) and P21/n (120 K). The justification for this choice is discussed. The deuterium population parameters of the paraelectric phase are related to the hindered rotation of the water molecules. Accurate positional and thermal parameters for the antiferroelectric phase are presented and a value for the dipole moment was calculated from the refined ordering of the water layers.

Crystal structure studies of Group V chalcogenide compounds. I. The structure of tricyclohexylphosphine sulphide

Crystals of tricyclohexylphosphine sulphide, C18H33PS, are orthorhombic, a = 10.906(2), b = 15.836(2), c = 10.362(2) Å, Z = 4, space group Pn21a. The structure was solved by direct methods and refined by full-matrix least-squares procedures to a final Rω of 0.058 for all 1209 reflexions with sin θ/λ ≤ 0.5377.Although second harmonic generation unambiguously established the correct space group as Pn21a, parameters reported here refer to the centrosymmetric space group Pnma. The geometry at phosphorus is approximately tetrahedral with an average P—C distance of 1.838(2) Å. Angles at phosphorus range from 105.4° to 113.2°. The P=S bond length of 1.966(2) Å is one of the longest bonds of this type so far reported. Rigid body analysis of thermal parameters suggests that the 'true' bond lengths are even longer.

Space group of the spinel structure: A neutron diffraction study of MgAl2O4

It has been suggested that the crystal structure of cubic spinel compounds should be referred to the non-centrosymmetric space group F43m instead of the customary space group Fd3m. The need for the new assignment arises from the possibility of an off-centre displacement of the octahedrally coordinated cations. The authors have examined MgAl2O4 carefully by powder neutron diffraction and find no evidence for such a displacement: it is concluded that the correct space group for this material is Fd3m.

Raman spectrum of a single crystal of 1,2,4,5-tetrachlorobenzene. Calculation of lattice vibrations.

Polarized Raman spectra of a single crystal of 1,2,4,5-C 6H 2Cl 4 at room temperature (β phase) in the lattice mode region have been obtained. The assignment of the bands to their proper factor group species allows a comparison with frequencies calculated using the atom-atom model. The agreement is good, supporting the transferability of the potential parameters. The lattice vibrations of the low-temperature α phase have also been obtained. The appearance of only five bands favours P 1 as the correct space group for this phase.

On the space group of MgAl2O4spinel

Abstract [001] electron diffraction patterns of MgAl2O4 spinel show (hk0) reflections with h + k = 4n + 2 ((200), (420), etc.) that violate the accepted Fd3m space group of this material. Since these ‘forbidden’ reflections cannot be attributed to double diffraction, non-zero Laue zones, or a second phase, this result is consistent with Grimes' recent suggestion that the correct space group for MgAl2O4 spinel is F43m.