Synthesis and X-ray analysis of isomeric diazadithia-pentacenes.

The isomeric diazadithiapentacenes (3) and (4) were obtained in reactions of 2-chloro-3-bromoquinoline (5) with selected sulfur reagents in various conditions. The 1,4-dithiin ring opening in diazadithiapentacene (3) led to 2,3’-diquinolinyl sulfide or (via the Smiles rearrangement stage of the S→S type) to 3,3’-diquinolinyl sulfide or to diazadithiapentacene (4) depending on reaction conditions. The X-Ray study of pentacenes (3) and (4) revealed non-planar and folded structures along the central sulfur-sulfur axis. The central six-membered ring is in a boat conformation

Anharmonic Thermal Motion in the 93-K Superconductor Ba2YCu3O7 Using Multiple-Wavelength X-Ray Diffraction

We have obtained anisotropic thermal parameters for Ba2YCu3O7 from a dual-wavelength x-ray crystal structure determination on a single crystal with a small amount of twinning. Anharmonic thermal motion was found for both Ba and Cu atoms with the largest anharmonic feature attributed to a Ba-Ba breathing mode. The anisotropic and anharmonic thermal parameters are consistent with the coordination polyhedra as well as the bonding distances involved

The Crystal Structure of Ba (Bi0.977K0.023)O3 by Single-Crystal X-Ray Diffraction

We determined by X-ray diffraction the structure of a slightly K-doped BaBiO3 single-crystal. The best refinements were obtained by describing the structure in the monoclinic I2/m space group similarly to the structure of the undoped compound. In this description the Bi cations occupy two crystallographically independent sites. We found that at this low K concentration, the K atoms are not, as expected, substituting for the Ba cations but are located on one of the two Bi sites. The two octahedral Bi sites are alternatively compressed and elongated along the c-axis. The formal valences of the two Bi cations deduced from the Brown and Altermatt formula are 3.79 and 4.44, respectively. This indicates that these cations are disproportionated into Bi3+ and Bi5+; however, they are only partially ordered

A Family of Rare Earth Molybdenum Bronzes: Oxides Consisting of Periodic Arrays of Interacting Magnetic Units

The family of rare earth molybdenum bronzes, reduced ternary molybdates of composition LnMo16O44, was synthesized and a detailed structural study carried out. Bond valence sum (BVS) calculations clearly show that the molybdenum ions in tetrahedral coordination are hexavalent while the electron count in the primitive unit cell is odd. Yet, measurements show that the phases are semiconductors. The temperature dependence of the magnetic susceptibility of samples containing several different rare earth elements was measured. These measurements verified the presence of a 6.5 K magnetic phase transition not arising from the rare earth constituent, but likely associated with the unique isolated ReO3-type Mo8O36 structural subunits in this phase. To better understand the behavior of these materials, electronic structure calculations were performed within density functional theory. Results suggest a magnetic state in which these structural moieties have an internal ferromagnetic arrangement, with small ~1/8 μB moments on each Mo. We suggest that the Mo8O36 units behave like pseudoatoms with spin 1/2 derived from a single hole distributed over the eight Mo atoms that are strongly hybridized with the O atoms of the subunit. Interestingly, while the compound is antiferromagnetic, our calculations suggest that a field-stabilized ferromagnetic state, if achievable, will be a narrow band half-metal

Template-Dependent Morphogenesis of Oriented Calcite Crystals in the Presence of Magnesium Ions

This paper presents a study of the morphogenesis of oriented crystals caused by solution growth modifiers. Self-assembled monolayers of HS-(CH2)15-CO2H, HS-(CH2)10-CO2H, HS- (CH2)11-SO3H, HS-(CH2)11-OH supported on gold that induce the oriented nucleation of calcite from the (013), (113), (106)+(1.0.12) and (104) planes respectively, were used as templates for calcite crystallization from solutions containing Mg ions (Mg/Ca = 0-4). We show that when crystal growth in the presence of an additive is coupled with the control over the nucleation process, the formation of crystal arrays with extremely uniform size, shape, facets and orientation is achieved. This study for the first time demonstrates that crystal morphogenesis depends on the orientation of the crystals on the surface, such that crystals grown on different substrates exhibit uniform, but template-specific characteristics. The possible mechanisms of this new phenomenon – an additive-induced morphogenesis of differently oriented crystals – are discussed. We believe that this approach can be used as a potent crystallization strategy that would allow the synthesis of homogeneous crystals with finely-tailored morphologies.Chemistry and Chemical Biolog

Composition-dependent crystallization of alternative gate dielectrics.

We have investigated the crystallization of amorphous oxides that are considered likely candidates to replace amorphous SiO2 as the gate dielectric in advanced field-effect transistors. To avoid crystallization, the mole fraction of main-group oxide in the Zr–Si–O, Zr–Al–O, and Hf–Si–O systems must be greater than 83%, 65%, and 78%, respectively, leading to a maximum useful dielectric constant of only 6.9, 12.7, and 6.6, respectively. We conclude that the silicate systems are not likely to be useful as replacements for SiO2, while aluminates are more promising. ©2003 American Institute of Physics

Superconductivity in Rare Earth Cuprate Perovskites

Superconducting cuprate perovskites isostructural with Ba2YCu3O7 incorporating a variety of rare earth ions are reported. Resistivity and magnetic susceptibility measurements show transition temperatures in excess of 90K for several materials, including Nd, Sm, Gd, Dy, Ho, Er and Lu-containing phases. Interestingly, Ce and Tb, with a readily accessible 4+ state, form phases other than the defect perovskite which are not superconductors. However, Pr, which also has an accessible 4+ state, does give a nonsuperconducting defect perovskite isostructural with Ba2YCu3O7. The observation of high temperature superconductivity in phases containing ions carrying magnetic moments is significant because it suggests that the magnetic moment on the rare earth ion interacts only weakly with the electrons making up the band crucial to superconductivity in these materials

Influence of grain boundary characteristics on thermal stability in nanotwinned copper

High density grain boundaries provide high strength, but may introduce undesirable features, such as high Fermi levels and instability. We investigated the kinetics of recovery and recrystallization of Cu that was manufactured to include both nanotwins (NT) and high-angle columnar boundaries. We used the isothermal Johnson-Mehl-Avrami-Kolmogorov (JMAK) model to estimate activation energy values for recovery and recrystallization and compared those to values derived using the non-isothermal Kissinger equation. The JMAK model hinges on an exponent that expresses the growth mechanism of a material. The exponent for this Cu was close to 0.5, indicating low-dimensional microstructure evolution, which is associated with anisotropic twin coarsening, heterogeneous recrystallization and high stability. Since this Cu was of high purity, there was a negligible impurity-drag-effect on boundaries. The twin coarsening and heterogeneous recrystallization resulted from migration of high-angle columnar boundaries with their triple junctions in one direction, assisted by the presence of high concentration vacancies at boundaries. Analyses performed by electron energy loss spectroscopy of atomic columns at twin boundaries (TBs) and in the interior showed similar plasma peak shapes and L3 edge positions. This implies that values for conductivity and Fermi level are equal for atoms at TBs and in the interior

Subsolidus phase equilibria and crystal chemistry in the system BaO-TiO2-Ta2O5.

Subsolidus phase relations have been determined for the BaO:TiO2:Ta2O5 system by X-ray diffraction analysis of ~100 specimens prepared in air at temperatures near but below the solidus (1275 °C to 1500 °C). For the BaO:Ta2O5 subsystem, the formation of all three polymorphs of BaTa2O6 (orthorhombic, tetragonal, and hexagonal) was confirmed. The present study found no stable phases forming between Ba5Ta4O15 and BaO. In the ternary system, along the line BaTiO3–Ba5Ta4O15, the previously reported 8L and 10L hexagonal perovskite derivatives were confirmed, albeit with somewhat wider homogeneity ranges; BaTiO3 dissolves Ta5+ up to BaTi0.64Ta0.29O3, which contains 7% B-cation vacancies and exhibits a dilated cubic unit cell (a=4.0505(2) Å). The formation of Ba3Ti4Ta4O21, a member of the hexagonal A3M8O21-type ternary oxides, was confirmed as well as its solid solution, for which a different mechanism of formation is suggested. Several new compounds have been found, including four members of the orthorhombic (space group Cmcm) "rutile-slab" homologous series, BaTinTa4O11+2n, with n-values 3,5,7,9; the unit cells for these compounds are given. Three ternary phases with close-packed [Ba,O] layer structures related to that of 6L Ba4Ti13O30 were found: 13L Ba18Ti53Ta2O129, 7L Ba10Ti27Ta2O69, and 8L Ba6Ti14Ta2O39. The crystal structures of the 13L and 7L phases were determined by single-crystal X-ray diffraction (13L Ba18Ti53Ta2O129: cchcchcchcchc, C2/m, a=9.859(2) Å, b=17.067(5) Å, c=30.618(8) Å, =96.11(2)°, Z=2; 7L Ba10Ti27Ta2O69: cchcchc, C2/m, a=9.855(3) Å, b=17.081(7) Å, c=16.719(7) Å, =101.18(3)°; Z=2), and are described and compared with those of the 6L and 8L members of this family. Phases with tetragonal tungsten bronze (TTB) related structures occur over large compositional ranges in the BaO:TiO2:Ta2O5 system, both within the ternary (Ba6−xTi2−2xTa8+2xO30, x=0→0.723), and along the BaO–Ta2O5 binary (from 43 to 26 mol% BaO), which in turn dissolves up to ~12 mol% TiO2 to form extensive single-phase fields with different TTB superstructures. X-ray powder diffraction data are given for three binary BaO:Ta2O5 compounds exhibiting the TTB structure and two superstructures. The large compositional ranges of the TTB-type phases are accompanied by only small changes in specific volume. The TTB-type regions within the ternary and those emanating from the BaO–Ta2O5 binary do not co-exist in equilibrium, which likely reflects fundamental differences in the structural mechanisms that accommodate the variable stoichiometries. The similarities and differences between the BaO:TiO2:Ta2O5 and BaO:TiO2:Nb2O5 systems are described