Low-temperature luminescence of silicon carbide crystals

The high-energy part of the blue fluorescence of-SiC(6H) is found to be dependent on the nitrogen (donor) concentration. The line fluorescence spectrum is found to include a new series of lines due to radiative transitions to A1 acceptor levels.We are indebted to J.G. Pichugin for providing the crystals.     

Contribution of one-electron acceptor centers to oxidative dimerization of methane

The ESR method was used to identify paramagnetic centers in the Pb–Mg oxide system and to study its catalytic properties in oxidative dimerization of methane. Reaction rate is shown to correlate well with the presence of one-electron acceptor centers in the sample.

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- Pb–Mg . , .

     

Novel Uracil-Based Inhibitors of Acetylcholinesterase with Potency for Treating Memory Impairment in an Animal Model of Alzheimer’s Disease

Novel derivatives based on 6-methyluracil and condensed uracil, 2,4-quinazoline-2,4-dione, were synthesized with terminal meta- and para-benzoate moieties in polymethylene chains at the N atoms of the pyrimidine ring. In the synthesized compounds, the polymethylene chains were varied from having tris- to hexamethylene chains and quaternary ammonium groups; varying substituents (ester, salt, acid) at benzene ring were introduced into the chains and benzoate moieties. In vivo biological experiments demonstrated the potency of these compounds in decreasing the number of β-amyloid plaques and their suitability for the treatment of memory impairment in a transgenic model of Alzheimer’s disease.     

Muon spin echo

A muon spin echo method is proposed for determining the contributions of the static and dynamic local fields in muon experiments. It is shown that if for each muon which has stopped in the sample a rf pulse of fixed duration is applied to the sample at a time τ after the muon entered the sample, then after a sufficient number of muon-positron decays have been accumulated a muon spin echo can appear at time 2τ. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 7, 500–503 (10 October 1996)     

Narrow Band Gap Compounds with β-Pyrochlore Structure in the A2O-V2O5-2TeO3 (A=Rb,Cs) System

Two new isostructural compounds with composition of Rb⁺V³⁺_0.125V⁵⁺_0.625Te⁶⁺_1.25O₆ and Cs⁺V³⁺_0.125V⁵⁺_0.625Te⁶⁺_1.25O₆ have been synthesized by solid-state reaction. The crystal structure has been determined by X-ray diffraction analysis using Rietveld refinement. Both compounds possess β-pyrochlore structure-type with cubic space group Fd-3 m (Z=8); the unit cell parameters: a=10.04120(27) Å for RbV_0.75Te_1.25O₆ Å and a=10.09119(23) Å for CsV_0.75Te_1.25O₆. The oxidation states of vanadium and tellurium have been confirmed by X-ray photoelectron spectroscopy. The compounds contain vanadium in mixed valence state (3+ and 5+). The compounds possess the unusual narrow band gap for oxide compounds – in the infrared (IR) range (∼0.6 eV). The schematic electronic structure of the compounds has been estimated using XPS, UPS and electronic conductivity data; however, the shifts of the top of the valence edge and the bottom of the conduction band in the water conditions have been calculated using theoretical approximation. The thermal behavior of obtained compounds has been studied by differential thermal analysis. RbV_0.75Te_1.25O₆ and CsV_0.75Te_1.25O₆ melts at low temperatures (∼500 °C) with decomposition and weight loss. The RbV_0.75Te_1.25O₆ powder decomposes to amorphous phase, whereas CsV_0.75Te_1.25O₆ transform into new β-pyrochlore compound with another composition.     

Application of wavelet transforms to the solution of boundary value problems for linear parabolic equations

A method based on wavelet transforms is proposed for finding weak solutions to initial-boundary value problems for linear parabolic equations with discontinuous coefficients and inexact data. In the framework of multiresolution analysis, the general scheme for finite-dimensional approximation in the regularization method is combined with the discrepancy principle. An error estimate is obtained for the stable approximate solution obtained by solving a set of linear algebraic equations for the wavelet coefficients of the desired solution.     

Experimental, Theoretical and Biological Activity Study on the Acyl-Substituted Benzo-18-crown-6, Dibenzo-18-crown-6 and Dibenzo-24-crown-8

Hexadecanoyl, dihexadecanoyl, dioctadecaneoyl, di-10-undecenoyl, and dicis-9-octadecenoyl derivatives of benzo[18]crown-6, dibenzo[18]crown-6 and dibenzo[24]-8 were synthesized by the condensation of carboxylic acids (palmitic, stearic, oleic and undecenoic acid) with benzo and dibenzo crown ethers in the presence of zinc chloride. The extraction equilibrium constants of such macrocyclic ethers with long side chains were estimated using chloroform/water and dichloromethane/water membranes transfer of Na-PAR (4-(2-pyridylazo)-resorcinol mono sodium monohydrate) with UV–Vis spectroscopy. It was found that they were in the range of 10.88–11.71 in dichloromethane and 8.04–11.77 in chloroform. These results actually show that the Na+ binding effect of macrocyclic ethers depends on the type and the length of side chains. The geometrical properties of the molecules were studied employing semi-empirical calculations by simulated annealing technique. The frontier molecular orbital energies and dipole moments were also examined. The biological activity results showed that the synthesized crown ethers have no activity against the studied microorganisms.     

Pinning enhancement in MgB2-magnetic particles composites

The temperature dependence of microwave absorption hysteresis is measured in superconducting composites, prepared from MgB₂ and powders of magnetic Ni, Co, and non-magnetic Ti. In MgB₂–Ni and MgB₂–Ti composites the increase of microwave absorption hysteresis, by 150% and 25%, respectively, is found. In both cases this is connected with pinning enhancement. In MgB₂–Co composites a decrease of the superconducting transition temperature and worsening of pinning properties are observed.