Sven Lidin

The superstructure of domain-twinned η'-Cu6Sn5

The cell of the low-temperature modification of Cu 6 Sn 5 , the η-phase, has been determined by means of electron diffraction of single domains. The basic hexagonal NiAs (B8) type cell is pentupled, reflected in the reciprocal lattice by a fivefold superstructure running along [1121]. The extensive domain-twinning macroscopically gives rise to perfect hexagonal symmetry, which explains the previously proposed hexagonal cell [a=5a NiAs , c=5c NiAs ; Bernal (1928), Nature, 122, 54]. The structure was solved by occupying every fifth trigonal bipyramidal site of the NiAs-type structure and shifting the atoms surrounding these additional Cu atoms to form Edshammar 11 polyhedra with Cu-Sn and Cu-Cu distances in the range 2.60-2.74 A

The Samson phase, β-Mg2Al3, revisited

Co-Authors: Michael Feuerbacher, Carsten Thomas, Julien P. A. Makongo, Stefan Hoffmann, Wilder Carrillo-Cabrera, Raul Cardoso, Yuri Grin, Guido Kreiner, Jean-Marc Joubert, Thomas Schenk, Joseph Gastaldi, Henri Nguyen-Thi, Nathalie Mangelinck-Noël, Bernard Billia, Patricia Donnadieu, Aleksandra Czyrska-Filemonowicz, Anna Zielinska-Lipiec, Beata Dubiel, Thomas Weber, Philippe Schaub, Günter Krauss, Volker Gramlich, Jeppe Christensen, Sven Lidin, Daniel Fredrickson, Marek Mihalkovic, Wieslawa Sikora, Janusz Malinowski, Stefan Brühne, Thomas Proffen, Wolf Assmus, Marc de Boissieu, Francoise Bley, Jean-Luis Chemin, Jürgen Schreuer Abstract. The Al−Mg phase diagram has been reinvestigated in the vicinity of the stability range of the Samson phase, β-Mg2Al3 (cF1168). For the composition Mg 38.5 Al 61.5, this cubic phase, space group Fd-3m (no 227), a = 28.242(1) Å, V = 22526(2) Å3, undergoes at 214 °C a first-order phase transition to rhombohedral β′-Mg2Al3(hR293), a = 19.968(1) Å, c = 48.9114(8) Å, V = 16889(2) Å3, (i.e. 22519 Å3 for the equivalent cubic unit cell) space group R3m (no 160), a subgroup of index four of Fd-3m. The structure of the β-phase has been redetermined at ambient temperature as well as in situ at 400 °C. It essentially agrees with Samsons model, even in most of the many partially occupied and split positions. The structure of β′-Mg2Al3is closely related to that of the β-phase. Its atomic sites can be derived from those of the β-phase by group-theoretical considerations. The main difference between the two structures is that all atomic sites are fully occupied in case of the β′-phase. The reciprocal space, Bragg as well as diffuse scattering, has been explored as function of temperature and the β- to β′-phase transition was studied in detail. The microstructures of both phases have been analyzed by electron microscopy and X-ray topography showing them highly defective. Finally, the thermal expansion coefficients and elastic parameters have been determined. Their values are somewhere in between those of Al and Mg.

Chapter 3 – Molecular Forces and Self-Assembly

This Chapter outlines the nature, delicacy, and specificity of molecular forces, and the ways these forces work together with the geometry of molecules to organize self-assembled molecular aggregates. It discusses the meaning of the size and shape of a molecule and describes the forces that enable the curvature of aggregates of self assembled molecules to be changed. The acute sensitivity of many biological phenomena to temperature can in some cases be correlated with forces. A brief overview of ideas and the physical notions behind self-assembly of surfactant-water systems is presented. The classical intuition on molecular forces is embodied in the Derjaguin–Landau–Verwey–Overbeek theory of colloid stability. It blends two themes: (1) an intervening liquid can be thought of as a structureless continuum with bulk liquid properties, up to a molecular distance from the surface and (2) any object (surface) must perturb proximal liquid structure (density, dipolar orientation, and hydrogen bonding) so that the transmission of force is propagated via a stress field passing from molecule to molecule in much the same way that the electromagnetic field is carried through the vacuum or a dielectric in Maxwells theory of the electromagnetic field.

A general structure model for Bi–Se phases using a superspace formalism

Solid-state synthesis in the Bi-Se system produced both commensurate and incommensurate phases of compositions ranging from Bi2Se3 to Bi4Se3, all crystallising in rhombohedral or trigonal layer structures. The a parameters are very similar for all phases but the c parameters vary irregularly between 10 and 100 A. A general model for all these phases was developed, using single crystal X-ray diffraction and a four-dimensional superspace formalism. The 4D superspace group is P: R3: m11, with approximate cell parameters of a ≈ 4.2 A and c ≈ 5.7 A valid for all phases. The q vector value in this model, ranging from 1.70[001]* to 1.80[001]* is the only parameter that varies with the composition. Two different modelling options were developed for the structures. One of them was a single-atom refinement model consisting of harmonic modulation waves and the other was a two-atom model based on a displacive sawtooth modulation of the Se atom. Both models converged to low R-values (Rall < 0.07) and small residual electron density values for all phases. The q vector may be physically interpreted as arising from the average distance between homoatomic layers in the structures. (Less)

A low-spin Fe( iii ) complex with 100-ps ligand-to-metal charge transfer photoluminescence

Transition-metal complexes are used as photosensitizers, in light-emitting diodes, for biosensing and in photocatalysis. A key feature in these applications is excitation from the ground state to a charge-transfer state; the long charge-transfer-state lifetimes typical for complexes of ruthenium and other precious metals are often essential to ensure high performance. There is much interest in replacing these scarce elements with Earth-abundant metals, with iron and copper being particularly attractive owing to their low cost and non-toxicity. But despite the exploration of innovative molecular designs, it remains a formidable scientific challenge to access Earth-abundant transition-metal complexes with long-lived charge-transfer excited states. No known iron complexes are considered photoluminescent at room temperature, and their rapid excited-state deactivation precludes their use as photosensitizers. Here we present the iron complex [Fe(btz)3]3+ (where btz is 3,3′-dimethyl-1,1′-bis(p-tolyl)-4,4′-bis(1,2,3-triazol-5-ylidene)), and show that the superior σ-donor and π-acceptor electron properties of the ligand stabilize the excited state sufficiently to realize a long charge-transfer lifetime of 100 picoseconds (ps) and room-temperature photoluminescence. This species is a low-spin Fe(iii) d5 complex, and emission occurs from a long-lived doublet ligand-to-metal charge-transfer (2LMCT) state that is rarely seen for transition-metal complexes. The absence of intersystem crossing, which often gives rise to large excited-state energy losses in transition-metal complexes, enables the observation of spin-allowed emission directly to the ground state and could be exploited as an increased driving force in photochemical reactions on surfaces. These findings suggest that appropriate design strategies can deliver new iron-based materials for use as light emitters and photosensitizers.

Elucidation of the crystal structure of oxyapatite by high-resolution electron microscopy.

High-resolution electron-microscopy (HREM) images from different hydroxyapatite (OHAp) samples showed p3 projection symmetry along [001] instead of the p6 projection symmetry compatible with the space group P6_3/m of OHAp. Image processing was used to establish without ambiguity that threefold symmetry dominates the images along [001]. OHAp crystals decompose in the transmission electron microscope and it is concluded that the threefold symmetry observed corresponds to an early step in the decomposition process, the dehydration of OHAp to oxyapatite (OAp). A structural model for OAp where every second O atom along the 6(3) axis in OHAp is removed has the maximal space-group symmetry P{\bar 6}. This is compatible with the p3 projection symmetry observed. Atomic shifts in this OAp model compared to the OHAp structure were estimated using the HREM images and geometric optimizations of the atomic structure. No refinements of the atomic coordinates against diffraction data were possible but the simulated HREM images of this crude model fit well with the experimental images.

Bone graft proteins influence osteoconduction: A titanium chamber study in rats

Although it is often emphasized that the matrix of bone grafts contains several growth factors, it is not known if these factors become activated and play a role in bone grafting. We therefore compared ground defatted bone which had or had not been deproteinized by heating with water to 270 degrees C at an autogenic pressure of 55 bar. This is a careful ceramic procedure which leaves the mineral unchanged. Deproteinized and non-deproteinized bone granulae derived from cortical rat bone were placed in titanium bone conduction chambers implanted bilaterally in rat tibiae. Ingrowing bone could enter the cylindrical interior of the chamber only at one end. It then penetrated the material in the chamber, but due to the length of the cylinder, it never reached the other end. The mean distance which the ingrown bone had reached in the material was then measured on histological slides. The bone formation activity was measured by TcMDP scintimetry. With the protein-containing granulae, the mean bone ingrowth distance and the scintimetric activity were increased by 41% and 31%, respectively (p < 0.01). We conclude that there is a limited favourable effect of proteins in a graft. Our grounded material had a large fracture surface area with no osteoid lining. The leakage of growth factors from such areas may explain the extraordinarily good clinical incorporation of morselized compacted allografts.

Reassessing the compound CeCd6 : the structure of Ce6Cd37

The crystal structure of the phase Ce6Cd37 was determined by single-crystal X-ray diffraction. It is closely related to, and indeed probably identical to, the previously reported compound CeCd6. The compound crystallizes in the centrosymmetric space group Pn3 with the unit cell axis 15.808(2) A. The structure may be described by successive shells centered around the origin and the position (1/2 1/2 1/2).

The η-phase field of the CuIn system

Abstract The η-phase field of the CuIn phase diagram has been investigated by means of electron and X-ray diffraction. This phase field contains three distinct phase regions: A, B, and C. All phases are based on the B8-type (NiAsNi 2 In type) structure. The A-phase is a high-temperature monoclinic phase. It is converted to the B-phase, which can be seen by the continuous rotation of satellite reflections in the diffraction pattern. Hence the B-phase has many modifications. The different diffraction patterns of the B-phase link together the patterns of the A-phase and the C-phase. The C-phase is a low-temperature phase with a large orthorhombic supercell.

(3 + 1)-Dimensional structure refinement of the fresnoite framework-structure type compound Ba2TiGe2O8

The incommensurately modulated structure of the fresnoite framework-structure type compound Ba(2)TiGe(2)O(8) has been solved using a (3 + 1)-dimensional superspace approach. The structure is orthorhombic and adopts the superspace group Cmm2(0,beta,1/2)s00 with beta approximately 0.635 at room temperature. The refinement was based on neutron powder diffraction data obtained from a powdered single crystal grown by Czochralski pulling. The modulation parameters that were obtained support the idea that frozen-in rigid-unit modes cause the modulation. The modulation is mainly manifested by positional displacements of O atoms. Barium ions are either eightfold, ninefold or tenfold coordinated in the one-dimensional modulated structure. A significant improvement of the bond-valence sum for both barium positions is achieved by the introduction of the positional modulation. This finding strongly suggests that underbonded barium positions are critically involved in provoking the incommensurate modulation in Ba(2)TiGe(2)O(8).