Phase transitions of <Emphasis Type="Italic">p</Emphasis>-type ZnAs<Subscript>2</Subscript> at very high pressures

The electrical resistance and thermoelectric power of p-ZnAs₂ are measured in the pressure range 15–50 GPa. The results attest to phase transitions at 7–10 and 25–30 GPa.Translated from Neorganicheskie Materialy, Vol. 41, No. 2, 2005, pp. 135–137Original Russian Text Copyright © 2005 by Mollaev, Arslanov, Saipulaeva, Babushkin, Tatur, Marenkin, Volfkovich.     

Planar supramolecular systems based on geometrical isomers of crown-containing oligothiophenes

The properties of ultrathin films of two geometric isomers of oligothiophene derivatives containing two crowned styryl fragments in 2- (I) or 3- (II) positions of thiophene rings (Fig. 1) are studied in this work. The ability of these compounds to form stable monolayers at the air/water interface is shown. The structural organization of crown-substituted oligothiophenes in monolayers is determined by the π-π-stacking interaction of hydrophobic styrylthiophene fragments and interaction of hydrophilic macrocycles with the water subphase. Analysis of ultrathin film physicochemical characteristics has shown that the difference in the structure of oligothiophene molecules leads to the formation of distinct monolayer architectures with various electrochemical and optical characteristics. Two types of aggregates (H and J) are generated in monolayers formed from different geometrical isomers at the air/water interface. The effect of barium cation presence in the subphase on the oligomer aggregation in monolayer is discussed. The phase diagrams characterizing the behavior from two-dimensional mixtures of studied crown-substituted oligothiophenes and amphiphilic spreader are plotted basing on compression isotherms accompanied by absorbance and fluorescence spectra of monolayers of different composition. The ability to fine tune the emitted radiation parameters is demonstrated. The obtained results show the efficiency of application of geometrical isomers for investigation of the fundamental “structure-property” ratio of planar supramolecular systems, which is important for organic optics and electronics.     

High-pressure magnetic phase transition and galvanomagnetic effects in the high-temperature ferromagnet <Emphasis Type=Italic>p</Emphasis>-Cd<Subscript>0.7</Subscript>Mn<Subscript>0.3</Subscript>GeAs<Subscript>2</Subscript>

The ferromagnetic semiconductor Cd_0.7Mn_0.3GeAs₂ undergoes a high-pressure ferromagnetic-to-antiferromagnetic phase transition. We have studied this transition at different temperatures. From magnetic-field dependences of the Hall resistance measured at different temperatures, we have derived the normal and anomalous Hall coefficients of the material as functions of temperature. Its transverse magnetoresistance changes from positive to negative near the ferromagnetic-to-antiferromagnetic phase transition.     

Catalytic cracking of hydrotreated vacuum gas oil over a zeolite-containing catalyst modified with nickel and cobalt nanoparticles

The effect of the concentration of nickel and cobalt precursors on the catalytic cracking performance has been studied. A FTIR study of the catalyst surface after cracking in the presence of nickel and cobalt precursors has been conducted. It has been shown that the surface of industrial catalysts can be in situ modified with compounds of Group VIII metals in the acid-catalyzed conversion of a vacuum distillate.     

Effect of Hydrostatic Pressure on the Transport Properties of Cadmium Diarsenide Crystals

The electrical resistivity and Hall coefficient of n-type CdAs₂crystals cut along [001] and [100] were measured at hydrostatic pressures in the range 0–9 GPa. The (P) and R _H(P) curves for the [001] direction show three peaks at 1.8, 3, and 5.5 GPa. In the [100] direction, the curves show two peaks at 3 and 5.5 GPa. The peaks at 1.8 and 3 GPa are likely related to impurity levels. The peak at 5.5 GPa is due to the phase transition of CdAs₂. The parameters of the phase transition are determined.     

Pressure,temperature, and magnetic-field effects on the transport properties of Cd<Subscript>0.7</Subscript>Mn<Subscript>0.3</Subscript>GeAs<Subscript>2</Subscript>

The resistivity, Hall coefficient, and magnetoresistance ratio of the ferromagnetic semiconductor Cd_0.7Mn_0.3GeAs₂ have been measured as functions of pressure, temperature, and magnetic field. The ρ(p) and R _H(p) curves point to a structural insulator-metal phase transition under both increasing and decreasing pressure. The temperature dependences of ρ and R _H show anomalies, which are interpreted as a ferromagnetic-paramagnetic phase transition. Cd_0.7Mn_0.3GeAs₂ is shown for the first time to have a negative magnetoresistance under pressure.     

Electrical properties of n-Cd1 ? x Co x GeAs2 (x = 0.05–0.15) at high pressures

The resistivity (??) and Hall coefficient (R _H) of n-Cd_{1 ? x} Co _x GeAs₂ with x = 0.05?C0.15 were measured as functions of temperature (at atmospheric pressure) and pressure (up to p = 7 GPa). The temperature-dependent ?? data were used to determine the ionization energy of a cobalt-related impurity center. In the ??(p) and R _H(p) curves, we identified structural semiconductor-semiconductor phase transitions under both increasing and decreasing pressure. Using a mixed-phase structure-effective medium model, we assessed the dynamics of the variation of phase composition with pressure.