O.V. Kravchenko

Evidence for a two-band behavior of MgB2 from point-contact and tunneling spectroscopy

Ya. G. Ponomarev, S. A. Kuzmichev, M. G. Mikheev, M. V. Sudakova, S. N. Tchesnokov, N. Z. Timergaleev, A. V. Yarigin, M. V. Lomonosov Moscow State University, Department of Physics, 119899 Moscow, Russia, E. G. Maksimov, S. I. Krasnosvobodtsev and A. V. Varlashkin, P. N. Lebedev Physics Institute, RAS, Moscow, Russia, M. A. Hein, G. Müller, H. Piel, Bergische Universität Wuppertal, Fachbereich Physik, D-42097 Wuppertal, Germany, L. G. Sevastyanova, O. V. Kravchenko, K. P. Burdina, B. M. Bulychev, M. V. Lomonosov Moscow State University, Department of Chemistry, 119899 Moscow, Russia.

Binary and ternary compounds in the Mg-Sb-B and Mg-Bi-B systems as catalysts for the synthesis of cubic BN

We have studied the phase relations in the Mg-Pn and Mg-B-Pn (Pn = Sb, Bi) systems, synthesized the magnesium pnictides Mg3Sb2 and Mg3Bi2 and the new magnesium boropnictides Mg3Pn2(B2), and determined their structure and unit-cell parameters. The synthesized compounds have been investigated at high pressures (4.0–6.5 GPa) and temperatures (700–1400°C). All of them have been found to promote the hexagonal-to-cubic phase transformation of boron nitride.

Structure and properties of solid solutions in the Mg-Al-B system

Compounds with the general formula Mg1−xAlxB2 were obtained by two-step ceramic synthesis. All compounds were characterized by X-ray diffraction, NMR spectroscopy, and by four point probe resistivity measurements in various magnetic fields method. The diborides unit cell parameters were determined as a function of the Al mole fraction. With the vaues of x up to 0.40 (where x is the composition of the stock prepared for sintering), the unit cell parameters of Mg1−xAlxB2 are similar to those of pure MgB2 and the superconducting transition temperature was lowered. For stock compositions of 0:25 ≤ x ≤ 0:60, the products contain a superstructure, also superconducting phase, which becomes the only product at x = 0:50, and at x > 0:60 this phase is replaced by AlB2-based solid solutions.

Activation of metallic aluminum by tin and gallium chlorides in oxidation with water

We have studied the effect of gallium chloride and tin chloride solutions on the water oxidation of aluminum at SnCl2 concentrations of 0.68 and 6.32 wt %, GaCl3 concentrations of 0.56 and 2.67 wt %, and MCln: Al(M = Sn, Ga; n = 2, 3) molar ratios from 0.017 to 0.3. The results indicate that, when aluminum is oxidized in the presence of these salts, the reaction rate and hydrogen yield increase with reaction temperature and salt concentration and reach the highest levels when a mixture of gallium and tin chlorides is used. The reaction products are identified and the likely mechanism of the processes involved in the oxidation of aluminum is discussed.

Structure of hexaammines of aluminum borohydride and chloroborohydrides

1. It was shown by the method of IR spectroscopy that under the action of ammonia on aluminum borohydride, monochloroborohydride, and dichloroborohydride, there is a nonsymmetrical cleavage of the A1H2B groups with the formation of “salt-like” compounds, constructed from [A1(CH3)6]3+, Cl−, and BH 4 − . 2. The parameters of the rhombic cells of the compounds A1(BH4)3·6NH3, A1(BH4)2C1·6NH3, A1(BH4)C12·6NH3, and A1C13·6NH3. 3. The valence force constant of the Al-N bond was calculated in the approximation of a “diatomic” model, and the Al-N interatomic distance was determined in the octahedron [A1(NH3)6]3+.

On the existence of solid solutions based on magnesium diboride

Dense samples of pure magnesium boride and of the compositions Mg(1–x)AxB2 (A = Na, K, Ca, Sr, Ba, Sn, Ti; 0.05 < x < 0.15) were prepared by sintering and using high temperature—high pressure treatment. The lattice parameters of most of the doped MgB2 samples vary only slightly as compared to those of the pure MgB2 irrespective of the sample preparation procedure, high temperature—high pressure treatment conditions, and the amount of dopant, thus indicating the absence of extended solid solution regions. The superconducting transition temperatures of all the samples did not exceed the value characteristic of MgB2 (39±1 K). The results obtained for the dense MgB2 samples using the Andreev reflection and tunneling spectroscopies confirm the two-gap nature of superconductivity in magnesium diboride and point to analogy between the superconductivity mechanisms in this compound and in cuprates.

Crystalline carbon nitrides: thin films and bulk samples

A new method was developed for the preparation of bulk samples of crystalline carbon nitride on exposure of an amorphous nitrogen- and carbon-containing material to high temperature and ultrahigh pressure in the presence of crystallization seeds. Amorphous carbon nitride whose composition was close to C3N4was used as a starting material. Thin films of crystalline carbon nitrides prepared by the laser-electric discharge method were used as crystallization seeds.

Synthesis and investigation of superconducting properties of heterosubstituted magnesium diborides

The properties of magnesium diborides with the composition Mg1 − xMxB2, where M = Al, Ga, In, and Tl, and Mg (B1 − xEx), where E = N, P, and Si (0.05 ≤ x ≤ 0.3), made by sintering of calculated amounts of boron, magnesium, and a substituent element or by a solid-phase exchange reaction of a corresponding halogenide of a metal with MgB2 at 1070–1170 K are studied. Using RFA technique, it is shown that, from all employed heterosubstituents, only the atoms of aluminum, gallium, and silicon in the amount x ≤ 0.2 are incorporated into magnesium and boron sublatticies. Here, the temperature of the superconducting transition Tc = 39 ± 1K determined for pure magnesium diboride hardly changes.