Single-crystal Neutron Diffraction Data Research Articles

The structure of superconducting Pb 2Sr 2Y 1−xCa xCu 3O 8 by single crystal neutron diffraction data

The structures of two superconducting Pb 2Sr 2Y 1−xCa xCu 3O 8 crystals have been refined by single-crystal neutron diffraction data at room temperature. They have been found to belong to space group Pman. The calcium content of the two crystals, showing superconducting transitions at 65K and 38K, were x=0.27 and x=0.37, respectively. The comparison of the interatomic distances to the ones reported previously for the undoped non-superconducting compound indicates that the charge transfer takes place from the (Y,Ca) layers to the CuO 2 layers. The apical CuO distance is not implicated in the charge transfer, contrary to what occurs in YBa 2Cu 3O 6+x

Low temperature magnetic structure of erbium iron garnet

The magnetic and crystallographic structure of erbium iron garnet {Er3}[Fe2](Fe3)O12 has been refined simultaneously from powder and single crystal neutron diffraction data. At 5 K the ferrimagnetic structure has a spin direction parallel to [100]. Magnetic moments of iron and erbium and canting angles for the erbium sublattices were derived in space groupI41/acd for 5 K<T<65 K. The magnetic moments of the two erbium sites are 6.9μ B and 4.3μ B at 5 K. An indication of further symmetry reduction of the magnetic structure from tetragonal to orthorhombic below 5 K is discussed.

A neutron diffraction study on hydrogen bonding in the mineral mellite (Al 2[C 6(COO) 6] · 16H 2O) at 15 K

Lattice constants for tetragonal Al 2[C 6(COO) 6] · 16 H 2O at 15 K are a = 1555.3(5) and c = 2311.0(8) pm, Z = 8, space group I4 1 acd . Anisotropic refinement on single-crystal neutron diffraction data led to R w = 0.039. The crystal structure is made up by barely distorted Al(H 2O) 3+ 6 octahedra linked by strong asymmetric hydrogen bonds to the [C 6(COO) 6] 6− anions yielding a complicated three-dimensional framework. Additional water molecules not bound to Al 3+ participate in hydrogen bonding as well. Hydrogen bonds involving water molecules bound to Al 3+ as proton donors are markedly shorter (O ··· H 2O 257.6–266.4 pm) than those with noncoordinated water molecules (271.0 and 272.4 pm). OH bond lengths corrected for thermal motion range from 99.5 up to 101.3 pm. The hexaanion has 222 symmetry with CO bonds being equal (125.5–125.7 pm) within the limits of experimental error. The OCO angles are widened up considerably (124.96 and 126.30°). The C 6 ring of the anion deviates slightly, but significantly from an ideal hexagon.

The structure of superconducting Pb 2Sr 2Y 0.73Ca 0.27Cu 3O 8 by single-crystal neutron diffraction

The structure of superconducting Pb 2Sr 2Y 0.73Ca 0.27Cu 3O 8 has been refined by single-crystal neutron diffraction data. It has been found to belong to space group Pman with lattice parameters: a=5.3835(8), b=5.4091(8), c=15.784(2) Å determined by the use of a Guinier focusing camera, λ Fe Kα, and a silicon standard. The large single crystal used for the neutron experiment, prepared by the flux method, was twinned by reticular merohedry and contained two individuals related by the (110) pseudo mirror plane. The final R-factor based on F 2 was 6.33%. The most important structural feature of the superconducting Ca-doped compound with respect to the undoped non-superconducting one is the increase of the apical Cu(2)O(1) distance on going from the latter to the former. This is contrary to what occurs in YBa 2Cu 3O 6+ x where the appearance, as a function of x, of the superconducting state is accompanied by a decrease of the apical CuO distance indicating a charge transfer from the chain Cu cations to the planar ones. In Pb 2Sr 2Y 0.73Ca 0.27Cu 3O 8 the charge transfer takes place from the (Y,Ca) layers to the CuO 2 layers and the apical CuO distance is not implicated in the charge transfer.

Atomic structure of the icosahedral Al-Li-Cu quasicrystal

Using both powder and single crystal samples, neutron and X-ray diffraction data were obtained with quasicrystals of the AlLiCu system. Isotopic substitution on the Li and Cu atomic sites allowed amplitudes and phase shift of the partial structure factors to be determined. Using a high-dimensional crystallography approach results in the phases to be reconstructed and atomic densities were calculated. The six-dimensional periodic structure appeared as a primitive hypercubic lattice with mid-edge and vertex Al/Cu atomic surfaces plus a Li bodycentre site. The major drawbacks of the experimental approach are then bypassed by modelling details of the six-dimensional structure, still in agreement with diffraction data. The related three-dimensional quasiperiodic structure can be described in terms of connected clusters or, alternatively, families of atomic planes. Comparison with the structure of the crystalline R-phase is of interest.

Neutron diffraction study of the structure II clathrate hydrate: 3.5Xe.8CCl4.136D2O at 13 and 100 K

The crystal structure of the clathrate hydrate, 3.5Xe.8CCla.136D20, at 13 and 100K has been determined from single-crystal neutron diffraction data. The crystals are cubic, space group Fd3m, with ao = 17.192 (1)/~ at 13 K and 17.240 (2)/1 at 100 K. The structure was refined by least-squares methods with expansion coefficients of spherical harmonics as parameters describing rotational disorder of the CC14 molecules. The wR(F 2) indices are 0.040, 0.039 (13, 100 K) for 524, 522 reflections and 64, 61 parameters. The D20 molecules are disordered at 13 and 100 K in six hydrogen-bonded orientations of equal statistical weights, as in ice Ih. There are six nonequivalent hydrogen-bond interactions with O...D distances (at 13 K) between 1-738 (3) and 1-802 (1) ,~ and O...D--O angles between 174.8 (1) and 180 °. The covalent O---D lengths (uncorrected for thermal motion) are in the range 0-986 (1)-1.001 (2),~ and are decreased significantly from 13 to 100 K. The Xe atoms occupy statistically 22% of the dodecahedra and vibrate about the cage centers with r.m.s, displacements of 0.137 ,~ at 13 K and 0.179 ,~ at 100 K. The CC14 molecules exhibit large-amplitude libration motion about the C atom located at the center of the hexakaidecahedron. There are seven preferred molecular orientations with C--CI bonds directed toward the O vertices of the (D20)28 polyhedron. The C---C1 bond length from the radius parameter of the spherical distribution is 1.765 (2) ,/k at 13 K and 1-762 (3)/~ at 100 K, compared with the gas-phase value of 1.766 (3) ,~.

Magnetic and neutron diffraction studies on (Co1-xMnx)2P

Magnetic and neutron diffraction studies on the series of mixed (Co1-xMnx)2P phosphides are described. The magnetisation of CoMnP has been measured as a function of field at 4.2 K. The temperature dependence of the magnetisation along the b axis of the compounds with x=0.2, 0.5, 0.63 and 0.65 has been measured. Single-crystal neutron diffraction data at room temperature were used to refine the structure of two compositions with x=0.5 and 0.62. Polarised neutron measurements on CoMnP are interpreted in terms of antiferromagnetic-like domains associated with the orientation of the cobalt moments. The spin wave dispersion relations of CoMnP and (Co1-xMnx)2P, x=0.65, have been determined at 300 K and in a low-temperature helical phase for the latter. A theoretical understanding of these results is possible in terms of the canting of cobalt moments due to a Dzyaloshinski-Moriya interaction.

Neutron diffraction study of the modulated structure of Bi 2(Sr, Ca) 3Cu 2O 8+y

The modulated structure of the 80 K superconductor Bi 2(Sr, Ca) 3Cu 2O 8+y is refined from the single-crystal neutron diffraction data on a monodomain block of dimensions 1.5 × 1.0 × 0.03 mm 3. The modulation is recognized as commensurate with period 4.75 a, resulting in a supercell 19a × b × c (a = 5.397 (1), b = 5.401 (1), c = 30.716 (3) A ̊ ) and symmetry Pnaa. The essential feature of the structure is the periodic insertion of additional oxygen rows into BiO planes, which in turn causes large displacive modulation in all layers. The ideal oxygen stoichiometry is y = 4 19 ∽ 0.21 . Bonds in the BiO and SrO planes form characteristic chains running along the a-axis. The coordination of the Bi 3+ cations is uniform and consists of three mutually nearly perpendicular bonds to oxygens of 2.0–2.2 Å lengths. The Sr 2+ cations are bonded only to eight oxygen atoms at the most. About 7% of the Sr sites, located in the vicinity of the inserted oxygen, are possibly vacant. The Ca site situated between the CuO 2 planes is substituted to 30% by strontium.

Structural aspects of the phase separation in La 2 CuO 4.032

The average structure of superconducting La 2 CuO 4.032 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 reflections, we have confirmed that a phase separation occurs below room temperature. The peaks of one phase are in agreement with the space group Cmca and the unit cell proposed by Jorgensen et al. and Cox et al.. However, a monoclinic unit cell is needed to index the second phase reflections.

The crystal structure of superconducting La 2CuO 4.032 by neutron diffraction

The crystal structure of superconducting (37.5 K) La 2CuO 4.032 has been refined from single-crystal neutron-diffraction data at room temperature and 15 K. At both temperatures it exhibits the orthorhombic symmetry Cmca. The extra oxygen O4 atoms occupy the special positions ( 1 4 y 1 4 with y=0.243 at 15 K). They are located between two successive LaO layers and are surrounded by distorted cubes built up of two interpenetrated tetrahedra, one comprising four La atoms and the other four O1 atoms (the apical oxygen atoms of the CuO 6octahedra). The O4 insertion causes a 0.75 Å displacement of 4.8% of the O1 atoms towards new O3 positions ( x=0.030(5), y=0.182(2), z=0.100(5)). From the refined values of the occupancy factors at 15 K, it is deduced that for each extra O4 three O1 are displaced to O3 with one short O3–O4 distance of 1.64(3) Å. This value indicates the formation of a strong O-O covalent bond of peroxide type with a formal 2-valence. Since the La and Cu sublattices have been found to be fully occupied and the doping does not change the oxygen charge, the La, Cu and O sublattices have the formal valences 6+, 2+, and 8−, respectively. However, the increase in La coordination and the consequent La-O distance readjustment indicate, when compared to the undoped compound structure, that a charge transfer occurs in La 2CuO 4.032, with the excess positive charge going either to the La or to the O sublattice. In the latter case it would correspond to the formation of holes in the O 2p band.

Complexes with substituted 2,5-dihydroxy- p-benzoquinones. Hydrogen bonding of Ca[C 6(C 2H 5) 2O 4] · 3H 2O: A neutron diffraction study

Orthorhombic crystals of Ca[C 6(C 2H 5) 2O 4] · 3H 2O were grown in an aqueous silica gel. Lattice constants were a = 503.1(1) pm, b = 1561.9(4) pm, c = 1613.6(4) pm, Z = 4, space group P2 12 12 1. Anisotropic refinement on single-crystal neutron diffraction data led to R w = 0.038. Infinite, corrugated chains formed by Ca 2+ and the bis-chelating [C 6(C 2H 5) 2O 4] 2− ions are the main feature of the crystal structure. These chains extend along [010] and are linked to each other by asymmetric hydrogen bonds. Ca 2+ is coordinated by three water molecules and four oxygen atoms of the [C 6(C 2H 5) 2O 4] 2− ions. Six CaO distances are within the range from 237.7 to 243.0 pm. The seventh oxygen atom of the coordination sphere is more remote from Ca 2+ (CaO(1) 254.9(5) pm). Although the oxygen atoms of the dianion are not equally bound to Ca 2+ CO and CC bond lengths do not indicate a severe perturbation of the π electron delocalization. In one-half of the dianion the CO bond lengths are nearly identical (C(2)O(2) 126.1(4) and C(4)O(4) 125.5(4) pm) and the CC bonds differ by only 1.6 pm (C(2)C(3) 141.2(4) and C(3)C(4) 139.6(4) pm). In the other half containing O(1) which is more weakly bound to Ca 2+ than the other oxygen atoms of the dianion there is a very slight tendency toward a system of alternating double bonds (C(1)O(1) 128.4(4), C(1)C(6) 138.2(4), C(6)C(5) 142.3(4), and C(5)O(5) 124.7(4) pm). Obviously hydrogen bonding prevents a greater perturbation of resonance stabilization of the dianion by compensating for deficient coordination by Ca 2+ ions of the partially negatively charged oxygen atoms, since O(1) is involved in three hydrogen bonds, whereas each of the three other oxygen atoms of the dianion which are more tightly bound to Ca 2+ occurs as proton acceptor only in one hydrogen bond.

Single crystal neutron diffraction analysis (15 K) and ab initio molecular orbital calculations for ethylene carbonate

The crystal structure of ethylene carbonate, (1,3-dioxolan-2-one), C 3H 4O 3, has been refined from single-crystal neutron diffraction data at 15 K. The crystal data are C2/ c, Z = 4, molecular symmetry C 2, a = 8.817 (2), b = 6.243 (2), c = 6.644 (2) Å, β = 99.31 (1) °. The dioxolan ring has the twist (T) conformation with Cremer-Pople puckering parameters q = 0.2484 (3) Å, φ = 270 °. When corrected for thermal motion, the bond lengths are C-C 1.522 A, C-O 1.457 and 1.342 Å, CO 1.203 Å, C-H 1.091 and 1.097 Å. The O-C-C-O torsion angle is 24.8°. Ab initio molecular orbital calculations for the isolated molecule, at rest, using GAUSSIAN-82 with HF/3-21G, 6-31G and 6-31G * bases, also predicted the twist conformation. The 3-21G basis gave the best agreement for the puckering parameter ( q = 0.215 Å). The difference between calculated and thermally corrected experimental bond lengths ranged between 0.05 Å (C-O, 6-31G *) and 0.006 Å (C-C, 6-31G *), depending on the basis set used. A calculation of the potential energy surface at HF/3-21G indicates that the transition state is planar and therefore the molecule is not a pseudorotor. A small difference in the methylene C-H bond lengths, which is both observed and predicted by theory, is interpreted as evidence for “lone-pair hyperconjugation” between the nearly axial C-H σ * antibonding orbital and the carbonate π bonding orbital.

Single-crystal neutron studies of La1.7Sr0.16Pb0.14CuO4 and La1.7Sr0.3CuO4

Abstract Single-crystal neutron diffraction data indicate that in the low temperature orthorhombic phase crystals of La 1.7 Sr 0.3 CuO 4 and La 1.7 Sr 0.16 Pb 0.14 CuO 4 are composed of twinned domains of space group Bmab. The temperature dependence of the orthorhombic-tetragonal structural phase transition is similar for both materials. La 1.7 Sr 0.16 Pb 0.14 CuO 4 undergoes a metal-to-insulator transition at 5 K, although it is nearly isostructural to the superconductor La 1.7 Sr 0.3 CuO 4 , with T c =16 K.

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.

Water of crystallization and its hydrogen-bonded crosslinking in vivianite Fe3(PO4)2 · 8H2O; a neutron diffraction investigation

All the protons of the water of crystallization of vivianite Fe3(PO4)2 · 8H2O participate in bent hydrogen bonds of normal strength. Their geometry has been calculated from single crystal neutron diffraction data.

The Crystal Structure of Superconducting La2CuO4.032 by Neutron Diffraction

The structure of superconducting La2CuO4.032 has been determined by single-crystal neutron diffraction data. The excess oxygen, O4, is located between two adjacent LaO1 layers. The presence of O4 distorts the apical-oxygen O1 sublattice in such that each O4 forms a short bond (1.64 Å) with one O1, which indicates the formation of a strong covalent bond between these two atoms. The Cu and O2 sublattices as well as that of La are not affected by O4, however, 1.6% of the La cations increase their coordination from 9 to 10.

Hydrogen bonding in the chain-like coordination polymer ZnC 4O 4·4H 2O: A neutron diffraction study

Monoclinic crystals of ZnC 4O 4·4H 2O were grown in an aqueous silica gel: lattice constants a = 901.2(2) pm, b = 1333.6(3) pm, c = 674.6(2) pm, β = 99.33(2)°, Z = 4, space group C2 c . Anisotropic refinement on single-crystal neutron diffraction data led to R w = 0.0294 including anharmonic thermal parameters for hydrogen atoms and oxygen atoms of the water molecules. The coordination polyhedron of Zn 2− is a slightly distorted octahedron formed by four water molecules and two O squarate atoms which are bound in the trans position to the C 4 ring of the dianion (ZnO, 207.6(1)–212.6(1) pm). Infinite chains consisting of Zn 2+ and C 4O 2− 4 are the main feature of the crystal structure. Adjacent chains are connected by asymmetric hydrogen bonds between H 2O and the O squarate atoms. The squarate dianion is ideally planar. The crystallographically independent CO bond lengths are identical (2 × 125.0(1) pm) although the O squarate atoms are differently coordinated by Zn 2+ and H. The CC bond lengths are slightly different (146.4(1) and 146.7(1) pm).

Yeast tRNA(Asp)-aspartyl-tRNA synthetase complex: low resolution crystal structure.

Yeast aspartyl-tRNA synthetase, a dimer of molecular weight 125,000, and two molecules of its cognate tRNA (Mr = 24160) cocrystallize in the cubic space group I432 (a = 354 A). The crystal structure was solved to low resolution using neutron and X-ray diffraction data. Neutron single crystal diffraction data were collected in five solvents differing by their D2O content in order to use the contrast variation method to distinguish between the protein and tRNA. The synthetase was first located at 40 A resolution using the 65% D2O neutron data (tRNA matched) tRNA molecules were found at 20 A resolution using both neutron and X-ray data. The resulting model was refined against 10 A resolution X-ray data, using density modification and least-squares refinement of the tRNA positions. The crystal structure solved without a priori phase knowledge, was confirmed later by isomorphous replacement. The molecular model of the complex is in good agreement with results obtained in solution by probing the protected part of the tRNA by chemical reagents.

Crystal Data for Pb7Bi3, a Superconducting ε-Phase in the Pb-Bi System

AbstractThe ε-phase is hexagonal, space group P63/mmc. For the composition Pb7Bi3the following data were determined, a = 3.5058(1) Å c = 5.7959(5) Å, Vol. = 61.687 Å3(5), Dc= 11.17 Mg m−3. Filings from single crystal material were used to obtain powder data by the Guinier method and the X-ray results were in accordance with single crystal neutron diffraction data. Transition to the superconducting state took place in the interval 8.3 K–8.55 K.

ChemInform Abstract: Neutron Diffraction Analysis at 15 K and ab initio Molecular Orbital Calculations for α‐Cyanoacetohydrazide: a Crystal Structure with a High Energy Conformer

Abstract The crystal structure of α-cyanoacetohydrazide, C 3 H 5 N 3 O, is refined using single-crystal neutron diffraction data at 15 K. Nuclear equilibrium geometries of the isolated molecules are calculated using GAUSSIAN-82 for the eight possible conformers having C s symmetry. The conformation observed in the crystal has the eighth highest calculated energy, 36.6 kJ mol −1 above the lowest energy conformer, and the largest calculated dipole moment, 8.6 D, with a HF/6-31G* basis. In the crystal the molecule is distorted from the ideal C s symmetry. These distortions add another 20.3 kJ mol −1 in energy, calculated with HF/6-31G*. All the hydrogens, including the CH, are involved in intermolecular hydrogen-bonding which is unusual in that it is formed entirely by three-center bonds. These bonds form a network which includes dimer and chain configurations.