Synthesis and mass-spectrometric study of 2-amino- and 2-chloro-5-aryl-1,3,4-thiadiazoles

The cyclization of aldehyde thiosemicarbazones with ferric chloride yielded 2-amino-5-aryl-1,3,4-thiadiazoles, forming 2-chloro-5-aryl-1,3,4-thiadiazoles in the Sandmeyer reaction. The mass-spectrometric behavior of the 2-amino- and 2-chloro-5-aryl-1,3,4-thiadiazoles was studied; the typical routes of fragmentation characteristic of each group of compounds were found.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 10, pp. 1416–1419, October, 1986.     

Oxidation of benzene on a vanadium-molybdenum catalyst in the presence of thiophene

The catalytic oxidation of benzene by air oxygen on a vanadium-molybdenum mixed oxide (1 ? x)V₂O₅ · xMoO₃ (x = 0.25) over a temperature range of 200–320°C is studied. It is shown that the introduction of small amounts of thiophene into benzene inhibits the oxidation to maleic anhydride in this temperature range. It is established that the operation of the catalyst is accompanied by significant changes in its phase composition and morphology, with a few first operation cycles being characterized by a high conversion of benzene. A possible mechanism of the process is proposed.     

Appearance of polar optical modes in raman scattering in semiconductor nanocrystals

The Raman spectra of nine samples of glasses with different content of CdSe semiconductor nanocrystals of different dimensions obtained by annealing samples are studied. It is established that, in the spectra of nanocrystals of all samples, there appears a line of the fundamental polar vibration whose frequency is close to that of the longitudinal optical mode of the CdSe bulk crystal. In this case, the asymmetry of this line essentially depends on the sample, the semiconductor concentration, and the local excitation place. To analyze the obtained results, a factor analysis method is used to separate linearly independent components from the data set. It is established that three or four contributions with frequencies near 180, 190, 208, and, probably, 210 cm^?1 can be singled out in the contour of the line under consideration. A comparison of the obtained results with the conclusions made by using microscopic models demonstrates a significant difference between the experimental data and the results obtained by using mechanical and dielectrical continuum models. It is possible that the real picture of quantum dot vibrations is more complicated and can be described better within the microscopic model. Moreover, for a CdS nanocrystal, the experimental spectra agree satisfactorily with results of calculation of vibrations, in which 3–4 bands with a frequency difference of 10-15 cm^?1 exist in the region of the LO mode for crystals with dimensions of 5 × 5 × 5 unit cells (1000 atoms).     

Anisotropic magnetoresistance of partially relaxed SrRuO3 films

The SrRuO₃ films (50 nm thick) grown by laser evaporation on (001)(LaAlO₃)_0.3 + (Sr₂AlTaO₆)_0.7 substrates were under partially relaxed biaxial compressive mechanical stresses. The films consisted of crystallites with lateral dimensions of 40–100 nm and a relative azimuthal misorientation of about 0.9°. Ferromagnetic ordering of spins in the SrRuO₃ films was manifested by a change in the slope of the temperature dependence of their electrical resistivity ρ at T ≈ 155 K. For a magnetic field H parallel to the measuring current, the maximum values (∼7.5%) of the magnetoresistance MR = [ρ(μ₀ H = 5 T) − ρ(μ₀ H = 0)]/ρ(μ₀ H = 0) were observed at temperatures of about 100 K. At T = 95 K (μ₀ H = 5 T), the anisotropic magnetoresistance of the films was 8% and increased by a factor of approximately 1.5 with decreasing temperature to 4.2 K.     

Response of the electrical resistivity and magnetoresistance of La<Subscript>0.67</Subscript>Ca<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript> epitaxial films to biaxial compressive mechanical (001) or (110) strains

The behavior of the electrical resistivity and magnetoresistance of 40-to 120-nm-thick La_0.67Ca_0.33MnO₃ films grown on differently oriented lanthanum aluminate substrates was studied. The cell volume in thin (40 nm) La_0.67Ca_0.33MnO₃ films grown coherently on (001)LaAlO₃ was found to be substantially smaller. Mechanical stress relaxation in biaxially strained La_0.67Ca_0.33MnO₃ films is accompanied by an increase in the cell volume. The temperatures at which the electrical resistivity and magnetoresistance in biaxially strained La_0.67Ca_0.33MnO₃ films were maximum can differ by 60–70 K from those observed in bulk single crystals.     

Capacitance of the La0.67Sr0.33MnO3/(Ba,Sr)TiO3 interface induced by electric field penetration into the manganite electrode

(001)La_0.67Sr_0.33MnO₃/(001)(Ba_xSr_{1 ? x} TiO₃/(001)La_0.67Sr_0.33MnO₃(x= 0–0.25) three-layer heterostructures are grown by laser evaporation on (001)La_0.3Sr_0.7Al_0.65Ta_0.35O₃ single-crystalline substrates. In a wide temperature range (≈150 K), effective permittivity ? of (1000 nm)Ba_0.25Sr_0.75TiO₃ and (1000 nm)SrTiO₃ films grown obeys the relationship ? ～ (T ? T _CW)^?1, where T _CW is the Curie-Weiss temperature for related bulk crystals. Using experimental dependences ?(T), the capacitance of the (001)La_0.67Sr_0.33MnO₃/(001)Ba_xSr_1?x TiO₃ and (001)La_0.67Sr_0.33MnO₃/(001)SrTiO₃ interfaces, which is due to electric field penetration into the manganite electrode, is estimated (C _int≈4μF/cm²). At bias voltages of ± 2.5 V, the change in the permittivity of the STO and BSTO films in the heterostructures studied reaches 25 and 45%, respectively.