Field and pressure response of Yb compounds close to a quantum critical point

YbCu_5−x Al _x provides the possibility to tune ground state properties by a change of the valence due to the Cu/Al substitution, by pressure as well as by the application of a magnetic field. Near to the critical concentration x _cr≈1.5 non-Fermi-liquid properties (NFL) are obvious, obeying hyperscaling. If magnetic order sets in for x>1.5, the application of moderate magnetic fields quenches order and again NFL features become evident. Hyperscaling in this case indicates strongly interacting spin fluctuations.     

Synthesis of side chain truncated 3"-aldehyde, 3"-carboxylic acid, and 1"-aldehyde from nodulisporic acid A

An efficient synthesis of the truncated 3"-aldehyde (3) from nodulisporic acid A (1) under mild conditions is described. Further oxidation of 3 to 3"-carboxylic acid (4) and its subsequent oxidative degradation produced 1"-aldehyde (5). These new derivatives are versatile intermediates for the preparation of new, side chain modified derivatives of nodulisporic acid A. [reaction: see text]     

Measurement of interchain and intrachain exciton hopping barriers in luminescent polymer

The integrated photoluminescence intensity in thin films of 'Super Yellow' copolymer has been analyzed using a Mott-like temperature dependence. This has enabled us to observe contributions from two emission channels, indicative of exciton recombination proceeding from two distinct origins. At high temperature, interchain thermally activated exciton energy transfer and migration dominates, resulting in large scale quenching of the integrated emission intensity and hence the photoluminescence quantum yield. However, at relatively low temperature, an additional increase of the integrated emission intensity occurs. This new channel of emission has been attributed to recombination from excitons where intrachain exciton energy transfer between adjacent subunits of the copolymer backbone becomes hindered. The activation energy barriers that control both of these emission channels have been obtained and are correlated with chain backbone degrees of freedom.     

Defect and dopant properties of the Aurivillius phase Bi4Ti3O12

Computer modelling techniques have been used to investigate the defect and oxygen transport properties of the Aurivillius phase Bi₄Ti₃O₁₂. A range of cation dopant substitutions has been considered including the incorporation of trivalent ions (M³⁺=Al, Ga and In). The substitution of In³⁺ onto the Bi site in the [Bi₂O₂] layer is predicted to be the most favourable. The calculations suggest that lanthanide (Ln³⁺) doping at the dilute limit preferentially occurs in the [Bi₂O₂] layer, with probable distribution over both the [Bi₂O₂] and the perovskite A-site at higher dopant levels. It is predicted that the reduction process involving Ti³⁺ and oxygen vacancy formation is energetically favourable. The energetics of oxide vacancy migration between various oxygen sites in the structure have been investigated.     

Towards more chemically robust polymer-supported chiral catalysts: alpha,alpha-diphenyl-L-prolinol based catalysts for the reduction of prochiral ketones with borane

alpha,alpha-Diphenyl-L-prolinol derivatives with para-bromo substituents in either one or both of the phenyl rings are easily bound to crosslinked polystyrene beads containing phenylboronic acid residues by Suzuki couplings. By using extended reaction periods boronic acid residues that do not take part in the couplings are simply lost by hydrolysis. The polymer-supported (PS) alpha,alpha-diphenyl-L-prolinols were used to catalyse reductions of several prochiral ketones with borane in tetrahydrofuran at 22 degrees C. The expected secondary alcohols were obtained in high chemical yields and ees were generally in the range 79-97%. One PS catalyst was recycled 14 times without loss of stereochemical performance.     

Crystal structure of the ‘mixed-layer’ Aurivillius phase Bi5TiNbWO15

The crystal structure of the Aurivillius phase Bi₅TiNbWO₁₅ has been analyzed in detail using powder X-ray and neutron diffraction. The structure can be described as a regular intergrowth of alternating single and double perovskite-like layers sandwiched between fluorite-like bismuth oxide layers, such that the layer sequence is … [WO₄]-[Bi₂O₂]-[BiTiNbO₇]-[Bi₂O₂] …. There is complete ordering of tungsten within the B sites of the single perovskite layer, so that the structure can be described as a direct intergrowth of the ‘component’ Aurivillius phases Bi₂WO₆ and Bi₃TiNbO₉. At 25 °C the structure adopts the polar orthorhombic space group I2cm, , , .     

Structural Behavior of the Four-Layer Aurivillius-Phase Ferroelectrics SrBi4Ti4O15 and Bi5Ti3FeO15

Rietveld refinement of powder neutron diffraction data has been used to study the crystal structures of the four-layer Aurivillius-phase ferroelectrics Bi₅Ti₃FeO₁₅ (at 25°C) and SrBi₄Ti₄O₁₅ (at a series of temperatures up to 800°C). At 25°C both materials adopt the polar orthorhombic space group A2₁am, in common with two-layer analogues such as SrBi₂Ta₂O₉. At temperatures well above the ferroelectric Curie temperature (i.e., at temperatures of 650°C and above, with T_c∼550°C) SrBi₄Ti₄O₁₅ transforms to the centrosymmetric tetragonal space group I4/mmm. However, there is good evidence from the raw diffraction data of a very subtle intermediate paraelectric orthorhombic phase, of Amam symmetry, in the region 550>650°C. The distortion in the ferroelectric phase can be traced to displacements of the cations in the A site of the perovskite block, with cooperative tilting of the BO₆ octahedra. The nature of the octahedral tilt system, cation disorder at the perovskite A and B sites, and the phase transition sequence in SrBi₄Ti₄O₁₅, which parallels that found in SrBi₂Ta₂O₉, are discussed.     

Structural distortions in the layered perovskites CsANb2O7 (A=Nd, Bi)

Structural distortions in the layered perovskites CsBiNb₂O₇ and CsNdNb₂O₇, belonging to the Dion-Jacobson series, have been analyzed in detail using powder neutron diffraction. Both phases adopt the polar orthorhombic space group P2₁am, with neighboring perovskite-like blocks in ‘eclipsed’ conformation analogous to the tetragonal archetype CsLaNb₂O₇. The metric relationship between these distorted derivatives and the parent structure is a_O∼b_O∼√2a_T, c_O∼c_T, with the orthorhombic distortion being due to off-center displacements of the A-site cation, together with cooperative BO₆ octahedral tilts. This mechanism of distortion is analogous to that in the Aurivillius phase ferroelectric SrBi₂Ta₂O₉, though the title compounds do not display ferroelectric behavior.