Atomic structure and transport and magnetic properties of the Sm1−x SrxMnO3 system

This is the first study of the temperature dependences of the atomic structure by neutron diffraction, as well as of the resistivity, differential magnetic susceptibility, and magnetoresistance of the ceramic system ¹⁵⁴Sm_1−x Sr_xMnO₃ (x∼0.16–0.4). Samples (x⩾0.3) having an initially orthorhombic structure transfer upon cooling from the insulating to the metallic state and exhibit giant magnetoresistance, which at liquid-helium temperature reaches as high as 90% in magnetic fields up to 30 kOe. At lower doping levels (x⩽0.25), the compound has monoclinic structure. The resistivity of such compounds in zero magnetic field displays insulating behavior upon lowering the temperature to 77 K. Fiz. Tverd. Tela (St. Petersburg) 40, 1271–1276 (July 1998)     

Cooperative behavior between oscillatory and excitable units: the peculiar role of positive coupling-frequency correlations

We study the collective dynamics of noise-driven excitable elements, so-called active rotators. Crucially here, the natural frequencies and the individual coupling strengths are drawn from some joint probability distribution. Combining a mean-field treatment with a Gaussian approximation allows us to find examples where the infinite-dimensional system is reduced to a few ordinary differential equations. Our focus lies in the cooperative behavior in a population consisting of two parts, where one is composed of excitable elements, while the other one contains only self-oscillatory units. Surprisingly, excitable behavior in the whole system sets in only if the excitable elements have a smaller coupling strength than the self-oscillating units. In this way positive local correlations between natural frequencies and couplings shape the global behavior of mixed populations of excitable and oscillatory elements.     

Selectivity in the photo‐Fenton and photocatalytic hydroxylation of biphenyl‐4‐carboxylic acid and derivatives (viz. 4‐phenylsalicylic acid and 5‐phenylsalicylic acid)

The selectivity of hydroxylation of the distal rings of 4‐phenylbenzoic acid, 4‐phenylsalicylic acid, and 5‐phenylsalicylic acid were determined using partial TiO₂‐mediated photocatalytic degradation and photo‐Fenton conditions. This separation of the binding site from the phenyl group being hydroxylated allows a less‐biased evaluation. The hydroxylation regiochemistry behaves as qualitatively expected for an electrophilic reaction, given the assumption that 4‐carboxyphenyl is a slightly electron‐withdrawing substituent. Selectivity for hydroxylation of the distal phenyl in 4‐ and 5‐phenylsalicylic acid is reversed, due to the reversal of the electronic demand, while adsorption to the TiO₂ surface is assumed to be analogous for the two structures. Copyright © 2011 John Wiley & Sons, Ltd.     

Efficient second harmonic generation of a diode laser using ring cavity with a KNbO<Subscript>3</Subscript> Crystal

The spatial mode and impedance matching of an IR diode laser beam with a ring cavity has been investigated by the example of second-harmonic generation in a KNbO₃ crystal placed in the cavity. The frequency conversion efficiency into blue light is found to be as high as 20%.     

Structural and spectroscopic features of proton hydrates in the crystalline state. Solid-state DFT study on HCl and triflic acid hydrates

Crystalline HCl and CF₃SO₃H hydrates serve as excellent model systems for protonated water and perfluorosulphonic acid membranes, respectively. They contain characteristic H₃O⁺, H₅О⁺₂, H₇О⁺₃ and H₃O⁺(H₂O)₃ (the Eigen cation) structures. The properties of these cations in the crystalline hydrates of strong monobasic acids are studied by solid-state density function theory (DFT). Simultaneous consideration of the HCl and CF₃SO₃H hydrates reveals the impact of the size of a counter ion and the crystalline environment on the structure and infrared active bands of the simplest proton hydrates. The H₇O⁺₃ structure is very sensitive to the size of the counter ion and symmetry of the local environment. This makes it virtually impossible to identify the specific features of H₇O⁺₃ in molecular crystals. The H₃O⁺ ion can be treated as the Eigen-like cation in the crystalline state. Structural, infrared and electron-density features of H₅О⁺₂ and the Eigen cation are virtually insensitive to the size of the counter ion and the symmetry of the local crystalline environment. These cations can be considered as the simplest stable proton hydrates in the condensed phase. Finally, the influence of the Grimme correction on the structure and harmonic frequencies of the molecular crystals with short (strong) intermolecular O–H···O bonds is discussed.     

An electrochemical cell with sapphire windows for operando synchrotron X‐ray powder diffraction and spectroscopy studies of high‐power and high‐voltage electrodes for metal‐ion batteries

A new multi‐purpose operando electrochemical cell was designed, constructed and tested on the Swiss–Norwegian Beamlines BM01 and BM31 at the European Synchrotron Radiation Facility. Single‐crystal sapphire X‐ray windows provide a good signal‐to‐noise ratio, excellent electrochemical contact because of the constant pressure between the electrodes, and perfect electrochemical stability at high potentials due to the inert and non‐conductive nature of sapphire. Examination of the phase transformations in the Li_1–xFe_0.5Mn_0.5PO₄ positive electrode (cathode) material at C/2 and 10C charge and discharge rates, and a study of the valence state of the Ni cations in the Li_1–xNi_0.5Mn_1.5O₄ cathode material for Li‐ion batteries, revealed the applicability of this novel cell design to diffraction and spectroscopic investigations of high‐power/high‐voltage electrodes for metal‐ion batteries.     

The Technique of Studying X-Ray Scattering over Wide Temperature Range in an Electric Field

The technique of studying the synchrotron radiation scattering on single crystals over wide temperature range using an original miniature device for applying an electric field to a sample is described. A specific feature of the device is the possibility of its application with various heating and cooling systems. The technique was used at BM01 beamline at the European Synchrotron Radiation Facility (ESRF) for the study of the influence of electric field on the processes of structural transformation in lead zirconate–titanate single crystal with a low titanium concentration. The efficiency of the technique proposed in this work is demonstrated and the results of the preliminary analysis of the results are presented.

     

Single-stage synthesis of heterocyclic alkaloid-like compounds from (+)-camphoric acid and their antiviral activity

Molecular Diversity - An effective technique for one-stage synthesis of new polycyclic nitrogen-containing compounds has been developed. The procedure involves refluxing mixtures of camphoric acid...