| 1. |
- Christensen, Jeppe, et al.
(författare)
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Vacancy Ordering Effects in AlB2-type ErGe2-x(0.4 < x < or = 0.5)
- 2008
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Ingår i: Acta Crystallographica Section B. - 0108-7681 .- 1600-5740. ; 64:3, s. 272-280
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Tidskriftsartikel (refereegranskat)abstract
- In the Er-Ge system, the compostion range ErGe2 to Er2Ge3 has been investigated. Eight samples were produced by arc melting of the elements, and analyzed using X-ray powder diffraction. Nine crystal structures were found to be present in the samples. The structures are described as a homologous series and presented within the superspace formalism using the superspace group X2/m(0)0s, X representing the centring vector (½, ½, 0, ½). In this description the modulation vector q = (a* + c*) is shown to be a direct measure of the Ge content as ErGe2 - ( falls in the range to ½). The large composition range is achieved by extended vacancy ordering in the planar 63 net of Ge with subsequent relaxation.
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| 2. |
- Dinnebier, R., et al.
(författare)
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Crystal structures of the trifluoromethyl sulfonates M(SO3CF3)(2) (M = Mg, Ca, Ba, Zn, Cu) from synchrotron X-ray powder diffraction data
- 2006
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Ingår i: Acta Crystallographica Section B. - 0108-7681 .- 1600-5740. ; 62, s. 467-473
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Tidskriftsartikel (refereegranskat)abstract
- The crystal structures of divalent metal salts of trifluoromethyl sulfonic acid ('trifluoromethyl sulfonates') M( SO3CF3)(2) (M = Mg, Ca, Ba, Zn, Cu) were determined from high-resolution X-ray powder diffraction data. Magnesium, calcium and zinc trifluoromethyl sulfonate crystallize in the rhombohedral space group R (3) over bar . Barium trifluoromethyl sulfonate crystallizes in the monoclinic space group I2= a(C2/c) and copper trifluoromethyl sulfonate crystallizes in the triclinic group P (1) over bar. Within the crystal structures the trifluoromethyl sulfonate anions are arranged in double layers with the apolar CF3 groups pointing towards each other. The cations are located next to the SO3 groups. The symmetry relations between the different crystal structures have been analysed.
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| 3. |
- Langer, Vratislav, 1949, et al.
(författare)
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Second-degree twinning and dynamic disorder in the crystal structure of deca-dodecasil 3R
- 2005
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Ingår i: Acta Crystallographica Section B: Structural Science. - 0108-7681 .- 1600-5740. ; B61:6, s. 627-634
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Tidskriftsartikel (refereegranskat)abstract
- The structure of deca-dodecasil 3R (DD-3R), Si120O240, a very well suited material for the synthesis of inorganic/organic composites structured on a nanometer level, has been investigated in detail. So far, a highly complicated twinning has hampered its structure description at a desirable level of accuracy. This twinning has now been resolved and a new structure determination is presented. Structure refinement in the R-3 space group revealed a large, unusually shaped atomic displacement ellipsoid for oxygen-bridging units (tetrahedra), bridging Si-O bonds shorter than expected and the linear Si-O-Si' bond angle dictated by special positions at a threefold axis. A structure model based on a statistically disordered bridging O atom improved the accuracy of the Si-O bonds of interest, but provided unacceptable O-O contacts. To solve this dilemma, ab initio NVT molecular dynamics calculations were performed to study the possible configurations. Wavelet analysis of the time variations of selected Si-O distances pointed to a synchronous shift of the whole building units (tetrahedra). Low-frequency features of the calculated phonon density of states agree well with the published INS (inelastic neutron scattering) spectra of several silica polymorphs, indicating that the nature of the disorder in DD-3R is dynamic rather than static.
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| 4. |
- Majerz, Irena, et al.
(författare)
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Asymmetric hydrogen bonds in a centrosymmetric environment. III. Quantum mechanical calculations of the potential-energy surfaces for the very short hydrogen bonds in potassium hydrogen dichloromaleate
- 2007
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Ingår i: Acta Crystallographica Section B. - 0108-7681 .- 1600-5740. ; 63, s. 748-752
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Tidskriftsartikel (refereegranskat)abstract
- In the crystal structure of potassium hydrogen dichloromaleate there are two short hydrogen bonds of 2.44 angstrom. The 'heavy-atom' structure is centrosymmetric ( space group P 3 (1) over bar) with centers of symmetry in the middle of the O-O bonds, suggesting centered hydrogen bonds. However, earlier unconventional types of refinements of the extensive neutron data taken at 30, 90, 135, 170 and 295 K demonstrated that the H atoms are actually non-centered in the hydrogen bonds, although the environment is centrosymmetric. Traditionally it has been assumed that the hydrogen distribution adopts the same symmetry as the environment. Reviewing these unusual results it was considered of great interest to verify that the non-centered locations of the H atoms are reasonable from an energy point of view. Quantum mechanical calculations have now been carried out for the potential-energy surfaces ( PES) for both the centered and non-centered locations of the H atoms. In all cases the non-centered positions are closer to the energy minima in the PES than the centered positions, and this result confirms that the structure is best described with noncentered H atoms. There is virtually perfect agreement between the quantum-mechanically derived reaction coordinates ( QMRC) and the bond-order reaction coordinates ( BORC) derived using Pauling's bond-order concept together with the principle of conservation of bond order.
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| 5. |
- Majerz, Irena, et al.
(författare)
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Comparison of the proton-transfer path in hydrogen bonds from theoretical potential-energy surfaces and the concept of conservation of bond order. II. (N—H…N)+ hydrogen bonds
- 2007
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Ingår i: Acta Crystallographica Section B. - 0108-7681 .- 1600-5740. ; 63:4, s. 650-662
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Tidskriftsartikel (refereegranskat)abstract
- The quantum-mechanically derived reaction coordinates (QMRC) for the proton transfer in (N—H—N)+ hydrogen bonds have been derived from ab initio calculations of potential-energy surfaces. A comparison is made between the QMRC and the corresponding bond-order reaction coordinates (BORC) derived by applying the Pauling bond-order concept together with the principle of conservation of bond order. We find virtually perfect agreement between the QMRC and the BORC for intermolecular (N—H—N)+ hydrogen bonds. In contrast, for intramolecular (N—H—N)+ hydrogen bonds, the donor and acceptor parts of the molecule impose strong constraints on the N—N distance and the QMRC does not follow the BORC relation in the whole range. The X-ray determined hydrogen positions are not located exactly at the theoretically calculated potential-energy minima, but instead at the point where the QMRC and the BORC coincide with each other. On the other hand, the optimized hydrogen positions, with other atoms in the cation fixed as in the crystal structure, are closer to these energy minima. Inclusion of the closest neighbours in the theoretical calculations has a rather small effect on the optimized hydrogen positions. [Part I: Olovsson (2006). Z. Phys. Chem.220, 797–810.]
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| 6. |
- Smrcok, Lubomir, et al.
(författare)
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On hydrogen bonding in 1,6-anhydro-β-D-glucopyranose (levoglucosan): X-ray and neutron diffraction and DFT study
- 2006
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Ingår i: Acta Crystallographica Section B: Structural Science. - 0108-7681 .- 1600-5740. ; B62:5, s. 912-918
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Tidskriftsartikel (refereegranskat)abstract
- The geometry of hydrogen bonds in 1,6-anhydro-β-D-glucopyranose (levoglucosan) is accurately determined by refinement of time-of-flight neutron single-crystal diffraction data. Molecules of levoglucosan are held together by a hydrogen-bond array formed by a combination of strong O-H...O and supporting weaker C-H...O bonds. These are fully and accurately detailed by the neutron diffraction study. The strong hydrogen bonds link molecules in finite chains, with hydroxyl O atoms acting as both donors and acceptors of hydroxyl H atoms. A comparison of molecular and solid-state DFT calculations predicts red shifts of O-H and associated blue shifts of C-H stretching frequencies due to the formation of hydrogen bonds in this system.
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| 7. |
- Zhang, Hongqiang, et al.
(författare)
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A structure model for τ(μ) phase in Al-Cr-Si alloys deduced from λ phase by the strong reflections approach
- 2006
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Ingår i: Acta Crystallographica Section B. - 0108-7681 .- 1600-5740. ; 62:1, s. 16-25
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Tidskriftsartikel (refereegranskat)abstract
- There are very obvious common features in the electron diffraction patterns of the λ and τ(μ) phases in the Al–Cr–Si system. The positions of the strong reflections and their intensity distributions are similar for the two structures. The relation of the reciprocal lattices of the λ and τ(μ) phases is studied. By applying the strong-reflections approach, the structure factors of τ(μ) are deduced from the corresponding structure factors of the known λ phase. Rules for selecting reflections for the strong-reflections approach are described. Similar to that of λ, the structure of τ(μ) contains six layers stacked along the c axis in each unit cell. There are 752 atoms in each unit cell, 53 of them are unique. The corresponding composition of the τ(μ) model is Al3.82 − xCrSix. Simulated electron diffraction patterns from the structure model are in good agreement with the experimental ones. The arrangement of interpenetrated icosahedral clusters in the τ(μ) phase is discussed.
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