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At least 1,207 records · Page 67Linked to original sources
PubMed2001

X-ray powder structure of a new two-dimensional nickel(II) coordination polymer with pyrazine-2,3,5,6-tetracarboxylic acid.

The new nickel(II) coordination polymer poly[diaquanickel(II)-mu-(pyrazine-2,3,5,6-tetracarboxylato)-tetraaquanickel(II)], [[[Ni(C(8)N(2)O(8))(H(2)O)(2)]Ni(H(2)O)(4)]](n), has been synthesized and characterized both spectroscopically and crystallographically, by X-ray powder diffraction analysis. In this two-dimensional coordination polymer, Ni(II) ions are bridged by pyrazine-2,3,5,6-tetracarboxylic acid, coordinating in a bis-bidentate manner, so forming one-dimensional polymeric chains. The chains are linked by a second Ni(II) ion, via an O atom of the coordinated carboxylate group, resulting in the formation of a two-dimensional layer-like polymer. The remaining coordination sites of the two independent octahedral Ni(II) ions are occupied by water molecules. The layers are connected via hydrogen bonds involving all six coordinated water molecules.

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PubMed2001

CdBiO2Cl: synthesis and powder structure solution.

The title compound, cadmium bismuth dioxide chloride, CdBiO2Cl, was obtained as a white powder by reaction of solid BiOCl with CdO at 973 K. Ab initio crystal structure determination was carried out using X-ray powder diffraction techniques, including direct methods for atom location and Rietveld fitting for the final refinement. Being monoclinic, the crystal structure can be related to tetragonal Sillen layered phases. The main structural elements present are CdBiO2+ pleated metal-oxygen layers alternating with Cl layers along the c axis, whereas along the b axis, all atoms are on mirror planes. The formation of a strong Cd-Cl bond draws the layers together, causing layer deformation and a monoclinic distortion in the layer arrangement.

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PubMed2002

In1.08Gd0.92Ge2O7: a new member of the thortveitite family.

Indium gadolinium digermanium heptaoxide, In(1.08)Gd(0.92)Ge(2)O(7), with a thortveitite-type structure, has been prepared as a polycrystalline powder material by a high-temperature solid-state reaction. As in the mineral thortveitite, the crystal structure belongs to the monoclinic system, with space group C2/m (No. 12). The precise structural parameters were obtained by applying the Rietveld method of refinement to the X-ray powder diffraction data. This layered structure presents, on one side, a honeycomb-like arrangement of the unique octahedral site, which is occupied randomly by In and Gd atoms, and, on the other side, sheets of isolated Ge(2)O(7) diortho-groups made up of double tetrahedra sharing a common vertex and displaying C(2h) point symmetry. This compound showed a remarkable photoluminescence effect when it was irradiated with the X-ray beam during the X-ray diffraction measurements, and with the alpha beam during the Rutherford back-scattering spectrometry experiments employed to analyze the chemical stoichiometry.

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PubMed2002

LiNa2AlF6: a powder structure solution.

Lithium sodium aluminium fluoride was obtained as a white powder by melting a stoichiometric mixture of AlF(3), NaF and LiF at 1223 K, and then cooling to 923 K and sintering at this temperature for 4 h. The ab initio crystal structure determination was carried out using X-ray powder diffraction techniques. The monoclinic structure of LiNa(2)AlF(6) can be related to cubic elpasolite. The Li and Al atoms lie on inversion centres. The main octahedral AlF(6) structural elements are not deformed, but are rotated slightly with respect to the unit-cell axes. The Li atoms have octahedral coordinations, whereas the Na atoms have cubo-octahedral coordinations. The Na coordination polyhedron is distorted in comparison with that of elpasolite.

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PubMed2003

Ca2MgWO6 from neutron and X-ray powder data.

The room-temperature structure of the B-site-ordered complex perovskite dicalcium magnesium tungstate, Ca(2)MgWO(6), has been determined by simultaneous Rietveld refinement of neutron and X-ray powder diffraction patterns. Ca(2)MgWO(6) is characterized by B-site ordering and an a(-)a(-)c(+)-type BO(6) octahedral tilt mechanism.

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PubMed2003

Partial Sn-atom ordering in Sm3Ga0.80-2.48Sn4.20-2.52.

Trisamarium digallide tristannide crystallizes with a partially ordered Pu(3)Pd(5)-type structure in space group Cmcm. In a single crystal of Sm(3)Ga(1.89(4))Sn(3.11(4)), the 8g position is mostly occupied by Sn atoms (93% Sn and 7% Ga), while the 4c and 8f positions are occupied by a Ga/Sn statistical mixture. The evolution of the structure as a function of the Ga content has been studied by X-ray powder diffraction on ten Sm(3)Ga(5-x)Sn(x) samples. It is shown that the 8g position remains occupied essentially exclusively by Sn atoms within the whole homogeneity range, with x ranging from 2.52 to 4.20.

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PubMed2004

Tetragonal CeNbO(4) at 1073 K in air and in vacuo.

The structure of the high-temperature scheelite-type polymorph of cerium niobium tetraoxide, CeNbO(4), has been determined using time-of-flight neutron powder diffraction data collected both in situ at 1073 K in air and in vacuo. In both cases, the structure was found to be tetragonal, with I4(1)/a symmetry and without any significant deviation from the stoichiometric composition.

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PubMed2005

Reinvestigation of Ge4Se9 based on single-crystal data.

Tetragermanium nonaselenide, Ge4Se9, adopts a two-dimensional layered structure. The layer is made up of infinite chains of corner-sharing GeSe4 tetrahedra and the chains are connected via the Ge2Se7 unit to form the two-dimensional layer. These layers are stacked to form the three-dimensional structure with a van der Waals gap. A previous structure report on Ge4Se9 based on powder diffraction data [Fjellvåg, Kongshaug & Stølen (2001). J. Chem. Soc. Dalton Trans. pp. 1043-1045] is comparable with our results except for the absolute structure determination.

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PubMed2006

PbZn(1/3)Nb(2/3)O3 at 4.2 and 295 K.

The structure of the relaxor ferroelectric lead zinc niobium trioxide, Pb(Zn(1/3)Nb(2/3))O3, known as PZN, was determined at 4.2 and 295 K from very high resolution neutron powder diffraction data. The material is known for its extraordinary piezoelectric properties which are closely linked to the structure. Pseudo-cubic lattice parameters have led to considerable controversy over the symmetry of the structure, which was found to be rhombohedral in the space group R3m at both temperatures. Atomic coordinates have been determined for the first time. They show that, whereas the deviation of the rhombohedral angle from 90 degrees approaches zero at 295 K, the atomic coordinates do not approach typical cubic positions and hence the polarization remains high.

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PubMed2006

An X-ray powder investigation of catena-poly[copper(II)-di-mu-chloro-kappa41:2Cl-mu-1,5-dimethyl-1H-tetrazole-kappa2N3:N4].

The crystal structure of the polymeric title complex, [CuCl2(C3H6N4)]n, has been solved from laboratory X-ray powder diffraction data collected at room temperature. The structural model obtained was refined with the Rietveld method using geometric soft restraints. There are two Cu atoms, two Cl atoms and one 1,5-dimethyltetrazole ligand in the asymmetric unit. Both Cu atoms lie on inversion centres and adopt essentially elongated octahedral coordination. Within the octahedra, the elongated axial positions are occupied by Cl atoms, while two Cl and two N atoms (N3 and N4 of the tetrazole ring) are in equatorial sites. Each Cl atom forms an asymmetric bridge between neighbouring Cu atoms, which are also bridged via the N3-N4 bond of the tetrazole ring. These bridges result in the formation of polymeric chains, running along the a axis, with weak C-H...Cl hydrogen bonds crosslinking the chains.

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PubMed1991

A multisolution method of phase determination by combined maximization of entropy and likelihood. V. The use of likelihood as a discriminator of phase sets produced by the SAYTAN program for a small protein.

The use of a likelihood criterion associated with maximum-entropy (ME) extrapolation for selecting phase sets as part of a new multisolution phasing strategy, already applied to solving small crystal structures from single-crystal data [Gilmore, Bricogne & Bannister (1990). Acta Cryst. A46, 297-308] and X-ray powder diffraction data [Gilmore, Henderson & Bricogne (1991). Acta Cryst. A47, 830-841], has been tested on the small protein avian pancreatic polypeptide (APP) with 301 non-H atoms in the asymmetric unit in space group C2. A collection of 50 phase sets for APP were provided by Woolfson & Yao. They had been generated from random starting phases by the SAYTAN procedure [Woolfson & Yao (1990). Acta Cryst. A46, 409-413] using data to a resolution of 0.98 A. Six of these had an unweighted mean absolute phase error, mean value of magnitude of delta phi, of less than 50 degrees, the remainder having phase errors of 60 degrees or more. However, none of the conventional figures of merit were able to identify these preferred sets. Each phase set was subjected to our standard procedure of entropy maximization and of evaluation of the log-likelihood gain resulting from the associated ME extrapolation. With only a small subset of data (to 2 A resolution), the likelihood criterion identified unambiguously the phase sets with mean value of magnitude of delta phi less than 50 degrees. In contrast, conventional figures of merit showed no such ability.(ABSTRACT TRUNCATED AT 250 WORDS)

Pancreatic Polypeptide↗
PubMed1999

A multisolution method of phase determination by combined maximization of entropy and likelihood. VI. The use of error-correcting codes as a source of phase permutation and their application to the phase problem in powder, electron and macromolecular crystallography.

The use of error-correcting codes as a source of efficient designs of phase permutation schemes is described. Three codes are used, all taken from the Bricogne BUSTER program [Bricogne (1993). Acta Cryst. D49, 37-60]: the Hamming [7, 4, 3], the Nordström-Robinson (16, 256, 6) and the Golay [24, 12, 8] or its punctured [23, 12, 7] form. These are used in a maximum-entropy-likelihood phasing environment to carry out phase permutation of basis-set reflections instead of the usual quadrant permutation or magic integer approaches. The use of codes in this way inevitably introduces some errors in the phase choices, but for most structures this is not significant especially when the gain in sampling efficiency is considered. For example, the Golay [24, 14, 8] allows the permutation of 24 centric phases in such a way that only 4096 phase sets are produced instead of 2(24) = 16777216, and one of these sets has, at most, only four wrong phases. The method is successfully applied to three powder diffraction data sets of increasing complexity, and with increasing degrees of overlap {Mg(3)BN(3), Sigma-2 ([Si(64)O(128)].4C(10)H(17)N) and the NU-3 zeolite}, a sparse electron diffraction data set for buckminsterfullerene, C(60), and the small protein molecule crambin at 3 Å resolution where 42 reflections are phased with a Uweighted mean phase error of 58.5 degrees.

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PubMed2000

Neutron Rietveld refinement of the incommensurate phase of the ordered perovskite Pb2CoWO6.

The incommensurate structure of lead cobalt tungstate has been refined by the Rietveld method on neutron data collected at 250 K. The space group is planar monoclinic 12/m(alpha0gamma)0s [a = 7.9602 (4), b = 5.6779 (3), c = 5.6967 (3) A, beta = 90.047 (5)degrees, q(inc) = 0.9000 (9)a* + 0.1735 (6)c*]. The use of powder diffraction techniques to investigate ferroelastic modulated phases is discussed and compared with a previous polydomain single-crystal structural analysis. The modulated displacements of light atoms have been determined, allowing an accurate description of the modulation of both the cations and the O-atom framework. The refinement suggests a displacive model for the phase transition, involving significant atomic shifts for Pb atoms and a quite complex mixing of tilt and deformation of the oxygen octahedra. The average character of this modulated structure is antiferroelectric.

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PubMed2000

The tetragonal phase of Na(0.5)Bi(0.5)TiO3--a new variant of the perovskite structure

The structure of the tetragonal phase of the A-site-substituted perovskite sodium bismuth titanate, Na(0.5)Bi(0.5)TiO3, has been determined by neutron powder diffraction at 698 K. The structure was refined in space group P4bm with a (= b) = 5.5191 (1), c = 3.9085 (1) A, V= 119.055 (5) A3, Z = 2 and Dx = 5.91 Mg m(-3). The structure exhibits an unusual combination of in-phase (a0a0c+) tilts and antiparallel cation displacements along the polar c axis, which results in a new variant of the perovskite structure.

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PubMed2000

Structural phase transitions of HfV2 at low temperatures.

We report a high-resolution synchrotron X-ray powder diffraction study on HfV(2), hafnium divanadium, at low temperatures. In this work we show, for the first time, a complete sequence of structural phase transitions of HfV(2) from cubic (Fd3m) to tetragonal (I4(1)/amd) to orthorhombic (Imma) in succession as temperature decreases. Peak splitting and extra diffraction peaks owing to lattice distortion can be clearly distinguished for the low-symmetry phases. The atomic positions and lattice parameters were obtained by Rietveld refinement. The bond lengths and angles of the HfV(2) crystal structure at the low-symmetry phases were correctly determined from the structure refinement. The face-centered cubic (Fd3m) unit cell (Z = 24) transforms to a body-centered tetragonal (I4(1)/amd) phase with a 45 degrees rotation relative to the cubic cell and with a reduced number of atoms (Z = 12) in the unit cell at a temperature of T = 112 K. The orthorhombic phase occurs at T = 102 K and it keeps the body-centered symmetry (Imma) and Z = 12 in the unit cell. The refinement results indicate that there may be a small amount of untransformed cubic phase left over in the lower symmetry phases. The abnormal thermal contraction of both tetragonal phase and orthorhombic phase marks the significance of structural change in HfV(2).

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PubMed2001

High-pressure structures of alpha- and delta-ZrMo2O8.

In situ high-pressure synchrotron X-ray powder diffraction studies of trigonal alpha-ZrMo(2)O(8), zirconium molybdate, have been performed from ambient conditions to 1.9 GPa, over the alpha-delta phase transition at 1.06-1.11 GPa. The monoclinic structure of delta-ZrMo(2)O(8), stable between 1.1 and 2.5 GPa at 298 K, has been solved by direct methods and refined using the Rietveld method. Significant distortions of the ZrO(6) and MoO(4) polyhedral elements are observed for delta-ZrMo(2)O(8), as compared to the ambient conditions of the alpha-phase, while the packing of anions becomes more symmetric at high pressure.

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PubMed2001

Kinetic behaviour investigations and crystal structure of nitric acid dihydrate.

X-ray powder diffraction experiments are performed to prove the possible crystallization of nitric acid dihydrate (HNO(3).2H(2)O, further denoted NAD) and to determine the best thermal conditions for growing a single crystal. It is shown that the kinetic behaviour of NAD strongly depends on the preliminary thermal treatment. One good single crystal obtained by an in situ adapted Bridgman method procedure enabled determination of the crystal structure. The intensities of diffracted lines with h odd are all very weak. The H atom of nitric acid is delocalized to one water molecule leading to an association of equimolar nitrate (NO(3)(-)) and an H(5)O(2)(+) ionic group. The asymmetric unit contains two such molecules. These two molecules are related by a pseudo a/2 translation (with a 0.3 A mean atomic distance difference), except for one H atom of the water molecules (0.86 A) because of their different orientations in the two molecules. The two molecules, linked by very strong hydrogen bonds, are arranged in layers. Two layers which are linked by weaker hydrogen bonds are approximately oriented along the c axis. The structure may be described by translations of this set of two layers along the c axis without hydrogen bonds leading to a two-dimensional hydrogen-bond network. The structures of the monohydrate (NAM) and trihydrate (NAT) are re-determined for comparisons. These structures may be described by one- and three-dimensional hydrogen-bond networks, respectively.

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PubMed2001

Bulk modulus and non-uniform compression of Nb3Te4 and InxNb3Te4 (x < 1) channel compounds.

The crystal structures of Nb3Te4 and InxNb3Te4 [x = 0.539 (4)] are reported for a series of pressures between 0 and 40 GPa. Both compounds crystallize in space group P6(3)/m with a = b = 10.671 and c = 3.6468 A for Nb3Te4, and a = b = 10.677 and c = 3.6566 A for InxNb3Te4 at ambient conditions. Phase transitions were not observed. High-pressure X-ray powder diffraction was measured using a diamond anvil cell and synchrotron radiation. Full Rietveld refinements provided the values of the lattice parameters and the values of the atomic coordinates at each pressure. The bulk modulus is found as K(0) = 70 (5) GPa for Nb3Te4 and as K(0) = 73 (4) GPa for InxNb3Te4. The analysis of the pressure dependences of the detailed crystal structures shows that the compression along c involves the folding up of the quasi-one-dimensional zigzag chains of Nb. The compression perpendicular to c is entirely due to the reduction of the diameter of the channels. The presence of intercalated In atoms is found to have hardly any influence on the compression behaviour up to 40 GPa.

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