Structural investigation of phase transformations in an MnAl alloy

A comprehensive x-ray diffraction study is made of an MnAl alloy, including a phase analysis of the alloy. Changes in the lattice parameters of the phases during phase transformations are examined and an analysis is made of the dispersion of the metastable ferromagnetic -phase. The structural characteristics are studied with different annealing temperatures ranging from 450 to 900°C. The temperature sequence of the phase transformations of the alloy is determined, along with the optimum conditions for the formation of the magnetic phase in the alloy. The structural characteristics of the phases that are obtained provide evidence of a difference in the processes responsible for ordering of the - and -phases. It is shown that it is possible to use the lattice-parameter ratioc/a to study the directionality of phase transformations in the alloy.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 34–38, August, 1989.     

Influence of mechanoactivation on the perovskite structure

The crystal structure of the perovskite rhombohedral modification before and after mechanoactivation in air and in carbon dioxide is studied using the X-ray diffraction method. It is shown that a medium in which an as-grown perovskite crystal is milled influences the degree of its amorphization but does not influence either the size of ordered areas or the distortion of titanium and calcium coordination polyhedra. It is found that the rate of perovskite milling is smaller than the rate of pseudowollastonite milling under the same conditions.     

Two new eudesmane derivatives from Verbesina virginica

<正>Two new eudesmane derivatives were isolated from the leaves and flowers of Verbesina virginica,along with the known 6-O-β-E -p-coumaroyl-4α-hydroxyeudesmane(1).Their structures were determined as 6-O-β-Z-p-coumaroyl-4α-hydroxyeudesmane(2) and 6-O-α-E-p-coumaroyl-1β-4α-dihydroxyeudesmane(3) by spectroscopic methods.     

Experimental and calculation methods of studying the effect of second coordination sphere on standard rate constants of charge transfer for Cr(III)/Cr(II) redox couple in chloride melts

The cyclic voltammetry method was used to determine the standard rate constants of charge transfer (k _s) on a glassy carbon electrode for the Cr(III)/Cr(II) redox couple in the systems of NaCl-KCl-CrCl₃, KCl-CrCl₃, and CsCl-CrCl₃ in the temperature range of 973–1173 K. It was shown that k _s grows at an increase in the temperature and decreases as sodium cations are replaced by potassium and cesium cations in the second coordination sphere of chromium complexes. The calculations carried out using the PC GAMESS/Firefly quantum-chemical software by means of the DFT technique showed that the values of the charge transfer activation energy change monotonously in the series of Na-K-Cs in accordance with the ratio of reorganization energies. In its turn, this leads to monotonous variation of the charge transfer rate constants.     

Effect of Pr-Ca substitution on the transport and magnetic behavior of LaMnO3 perovskite

The effect of simultaneous substitution of a fluctuating cation and a divalent cation in LaMnO₃ perovskite modifies the properties of the material to exhibit large valence colossal magnetoresistance (CMR) effect. A good example of these properties is (La_1−2x Pr _x Ca _x )MnO₃ (LPCMO) type CMR material. In this communication it is reported that, with the increase in x (for x=0.1, 0.15, 0.2), the T _c varies between 100 and 120 K with improvisation in metal-insulator transition. Interestingly, resistance increases with x from few hundred ohms to few kilo ohms with corresponding decrease in the unit cell volume. The results of the studies using X-ray diffraction (XRD), electrical resistivity, magnetoresistance and ac susceptibility measurements on LPCMO samples for understanding the structural, transport and magnetic properties are discussed in detail.     

PRESSURE-VELOCITY EQUILIBRIUM HYDRODYNAMIC MODELS

This article describes mathematical models for phase separated mixtures of materials that are in pressure and velocity equilibrium but not necessarily temperature equilibrium. General conditions for constitutive models for such mixtures that exhibit a single mixture sound speed are discussed and specific examples are described.     

The influence of collective effects on the propagation of electromagnetic radiation in dense ultracold atomic ensembles

Based on the already-developed general theory (I.M. Sokolov, D.V. Kupriyanov, and M.D. Havey, JETP 112 (2), 246 (2011)), we have studied the spatial distribution of excited atoms and of the atomic polarization that a weak monochromatic field creates in a dense ultracold atomic medium. We show that, in the case of a homogeneous random spatial distribution of atoms, the amplitude of atomic polarization averaged over spatial configurations decreases outside boundary regions according to an exponential law, while its phase linearly increases. Based on this, we have numerically determined the extinction coefficient and the light wavelength in the medium, as well as its dielectric permittivity. The dispersion of the permittivity at different concentrations has been studied. We show that, for dense clouds, the real part of the dielectric permittivity acquires negative values in a certain frequency range. Based on the calculation of the spatial distribution of excited atoms, we have analyzed the character of the transfer and trapping of quasi-resonant radiation in atomic clouds of differing density.