V. N. Gurin

The crystal structure of defective YAlB14 and ErAlB14

Abstract The crystal structures of Y 0.62 Al 0.71 B 14 and Er 0.62 Al 0.73 B 14 belong to the MgAlB 14 type structure. The refinements are based on the space group Imma and converged at the conventional R values 5.0% (2908 reflections) and 3.4% (2394 reflections) reflections) respectively. The cell dimensions of the single crystals are a=5.8212(3) A , b=10.4130(8) A , c=8.1947(6) A and a=5.8200(1) A , b=10.3950(4) A , c=8.1825(3) A respectively. Important structural characteristics are partial occupancy of both metal positions and a splitting of the rare earth atomic position.

Single-crystal X-ray diffraction study of NdB6, EuB6 and YbB6

Abstract The structure of three rare earth hexaborides, NdB 6 , EuB 6 and YbB 6 , was studied by single-crystal X-ray diffraction. The refinements indicated boron vacancies for EuB 6 and YbB 6 , but the site occupancies of NdB 6 were strongly model dependent, showing either boron or metal vacancies. No impurity atoms could be detected in the structures. Refinements of a model allowing anharmonic thermal motion of atoms gave small but significant fourth-order anharmonic thermal parameters for NdB 6 . Static displacements of both the rare earth and boron atoms were considered and tested as an alternative representation. It was concluded that the observed features in the difference density maps were due to charge asphericity.

A new scandium boride: ScB19

Abstract A new scandium boride has been found to exist between ScB 12 and β-boron. A single phase was established at a nominal composition of [B]/[Y]=19.0–19.5. X-ray powder diffraction and electron diffraction analyses showed that the new ScB 19 phase has a tetragonal system with a = b =1.02915(4) nm and c =1.42463(9) nm and space group P 4 1 2 1 2 (No.92) or P 4 3 2 1 2 (No.96).

Crystal structural refinement of the new compound TmAlB14

Abstract Single crystals of the new phase TmAlB14 were grown using the high-temperature solution method. The crystal structure of TmAlB14 was refined from X-ray powder diffraction data using the Rietveld method. The structure is of MgAlB14-type with the space group Imma and unit cell parameters a = 5.8212(3) A , b = 10.3837(2) A and c = 8.1762(3) A. The final, conventional R-valve and profile R-value are 0.031 and 0.064, respectively. The structure is characterized by a partial occupancy of both metal positions and a splitting of the thulium atomic position.

Specific features of oxygen electroreduction on platinized carbon black-functionalized carbon nanotube composite

The electrochemical behavior of multiwalled carbon nanotubes (MWCNTs) containing functional surface groups has been studied by the methods of cyclic voltammetry and direct-current polarography with stationary and rotating disk electrodes. The surface catalytic activity of supported platinum in the reaction of molecular oxygen electroreduction and the influence of functionalized MWCNTs on this process were determined.

Crystallization in the Al-Si, Al-Ge, and Al-Si-Ge Systems at Centrifugation

Crystallization in the Al-Si, Al-Ge, and Al-Si-Ge systems at centrifugation is studied. Of them, the Al-Si system is the least prone to sedimentation. In the others, sedimentation considerably changes the structure of the alloys at the bottom of the ingots compared with their top. At certain concentrations of the constituents, the number of crystallites in the lower part of the ingot is larger than in the upper part and the crystallites at the bottom are coarser than at the top. The Si: Ge atomic ratio in the Al-Si-Ge system changes by a factor of 2–12 against the initial ratio (1: 1) when the (Si + Ge) concentration changes as a result of centrifugation. Also, this ratio changes over the crystal surface (in the samples not subjected to centrifugation, this ratio remains unchanged over the surface). Crystallites in the Al-Si-Ge system are covered by Ge.

Influence of Gravity on the Habit of Single Crystals of Compounds Grown from Solutions

The effect of different gravity levels on the growth habit of single crystals of some compounds has been determined. Isometric crystals were obtained both at microgravity and at 10–11g. Accelerations of 2–6g promoted the formation of elongated and flattened crystals. The microhardness of KCl and KBr crystals increased with increasing acceleration at which they were obtained).

Structure and mechanical properties of Al-Si(Ge) alloys upon melt centrifugation and quenching

The structures of eutectic binary Al-12.7 at. % Si and Al-29.7 at. % Ge alloys and a ternary Al-10 at. % Si-10 at. % Ge alloy produced by quenching levitated melts or through solidification either in the presence or in the absence of a centrifugal acceleration of 7g are studied. Centrifugation is found to cause an increase in the silicon content in the Al-Si alloy in the direction opposite to the direction of centrifugal acceleration and an increase in the germanium content in the Al-Ge alloy in the direction of centrifugal acceleration. These differences are explained by the fact that the densities of silicon and germanium clusters and solidification centers differ from the liquid-phase density at temperatures of solidification. The related changes in the values of the Young’s modulus and in the stress-strain curves can be due to sedimentation-induced changes in the composition of samples cut from the middle part of an ingot. The processes of decomposition and recovery are shown to have a substantial effect on the elastic moduli of these alloys.

Effect of centrifugation on the crystal structure of barium nitrate

The structure of barium nitrate crystals grown from supersaturated aqueous solutions under normal conditions and during centrifugation at 11.8×103g is studied for the first time by a high-precision powder neutron diffraction method. It is established for the first time that centrifuging leads to displacement of the nitrate oxygen atoms in the xy plane (resulting in rotation of the NO3 trigonal prism about the third-order axis) and to variation of the bond lengths and angles in the coordination polyhedron of Ba atoms. The latter changes result in the appearance of a torsional mode in the phonon spectrum of barium nitrate and in increasing stiffness of the crystal lattice, as evidenced by a significant increase in microhardness of the samples grown in the centrifuge.

Obtaining of crystals of polyelemental solid solutions of rare earth hexaborides

Single crystals of polyelemental rare earth hexaborides with the preset formula La0.5(Ce0.1Pr0.1Nd0.1Sm0.1Eu0.1)B6 were obtained for the first time. Synthesis and crystallization were performed by the solution–melt method in an immiscible Al/Pb system. Step-by-step chemical analysis was made with the aid of a CAMEBAX microprobe. The inclusion of all rare earth metals (REMs) in the hexaboride lattice was proven, and differences in the composition of obtained crystals caused by nonstationarity of the bulk crystallization process were found. The lattice periods of the polyelemental REM hexaborides were found to be smaller than that of hexaboride of lanthanum, the main element of the metal sublattice. The measured microhardness of the new material lies within the range of the microhardness values of hexaborides of all its constituent REMs. Speculations are provided on the peculiarities of the growth mechanism, crystallization, and composition of the obtained crystals.