M. M. Korsukova

Raman effect in icosahedral boron-rich solids

Abstract We present Raman spectra of numerous icosahedral boron-rich solids having the structure of α-rhombohedral, β-rhombohedral, α-tetragonal, β-tetragonal, YB66, orthorhombic or amorphous boron. The spectra were newly measured and, in some cases, compared with reported data and discussed. We emphasize the importance of a high signal-to-noise ratio in the Raman spectra for detecting weak effects evoked by the modification of compounds, accommodation of interstitial atoms and other structural defects. Vibrations of the icosahedra, occurring in all the spectra, are interpreted using the description of modes in α-rhombohedral boron by Beckel et al. The Raman spectrum of boron carbide is largely clarified. Relative intra- and inter-icosahedral bonding forces are estimated for the different structural groups and for vanadium-doped β-rhombohedral boron. The validity of Badgers rule is demonstrated for the force constants of inter-icosahedral B–B bonds, whereas the agreement is less satisfactory for the intra-icosahedral B–B bonds.

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.

Preparation of HfB2 and ZrB2 single crystals by the floating-zone method

High-quality HfB2 crystals were prepared from a hafnium-excessive molten zone by the RF-heated floating-zone method. ZrB2 crystals were grown from self-fluxes of zirconium and boron. Helium atmosphere, which caused convection in the molten zone, was more suited for increasing the growth rate than argon. Further, the growth rate was found to increase by choosing a self-flux with low atomic number, i.e., boron, and with increasing the growth temperature.

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.

Floating zone growth and high-temperature hardness of YB4 and YB6 single crystals

Abstract Single crystals of NbB2 and TaB2, the Va group diborides, were prepared by the floating zone method. The crystals, free of subgrain boundaries, were obtained by controlling the molten zone composition. The crystals did not have preferential growth directions, though the IVa and VIa group diboride crystals preferentially grow normal and parallel to the c-axis of the hexagonal lattice, respectively. Their growth directions were consistent with anisotropy in their high-temperature hardness.

Temperature dependences of the parameters of atoms in the crystal structure of the intermediate-valence semiconductor SmB6: investigation by high-resolution powder neutron diffraction

The crystal structure of intermediate-valence (IV) samarium hexaboride has been studied on single-crystal double-isotope samples 154Sm11B6 by X-ray diffractometry at room temperature and by high-resolution powder neutron diffraction in the temperature range 23 K<or=T<or=300 K. The X-ray experiment revealed the occurrence of vacancies at the boron site, larger thermal vibrations of the Sm atom than of La in the isostructural non-IV material LaB6 and an aspherical charge distribution around the Sm nucleus. The neutron diffraction experiment confirmed the anomalous temperature dependence of the lattice parameter and revealed both a peculiar temperature dependence of the anisotropic thermal vibrations of the boron atom and a temperature-dependent change in the ratio of the isotropic thermal parameters of the Sm and B atoms. Thermal vibrations of the Sm ion can be satisfactorily described by the Einstein model with characteristic temperature Theta E approximately=120 K within the whole temperature range. The data obtained are discussed in terms of the influence of the fluctuating valence of the Sm ion on the structural parameters of atoms.

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.

Formation of power-law size distributions of defects during fracture of materials

The experimental data on the surface relief of loaded ribbons of an amorphous alloy have been obtained. The distributions of surface defects formed under loading have been analyzed using the wavelet transform and box counting method. Moreover, the data on the time accumulation of microcracks in the volume of a loaded granite specimen have been examined. It has been shown that power-law size distributions of defects (scaling) appear on the surface and in the bulk before fracture. It has been revealed that the appearance of the power-law distributions is one of the indications of the formation of the self-organized critical state. The formation of the self-organized critical state in the bulk and on the surface of the material has been considered. It has been established that the formation of the self-organized critical state precedes the fracture of a solid.

Dependence of the atomic structure and surface relief of platinum foils on the annealing and rolling conditions

The effect of cold rolling, polishing, and thermal annealing conditions on the atomic structure and surface geometry of platinum foils has been studied. The surface morphology has been analyzed using low-energy electron diffraction, atomic force microscopy, and scanning tunneling microscopy. The chemical composition of the surface has been evaluated by Auger electron spectroscopy. It has been demonstrated that a variation in the conditions used for the preparation of the samples makes it possible to produce surfaces with different degrees of perfection from atomically smooth to rippled, fractal, and diffraction-disordered surfaces.

Manifestation of flat and rippled surface reliefs of platinum foils in LEED patterns

The atomic structure and surface relief of a polycrystalline platinum foil upon rolling and subsequent recrystallization in ultrahigh vacuum have been studied by low energy electron diffraction (LEED) and atomic force microscopy (AFM). The symmetry of LEED patterns shows that the surface structure of recrystallized platinum foil corresponds to (111) face, but different shapes of diffraction maxima are indicative of a diversity of the geometric relief. AFM data reveal various types of the surface relief, including flat, rippled in one direction, and differently oriented, which are considered in comparison to the corresponding LEED patterns. Using the established laws, it is possible to trace the formation of various reliefs by diffraction techniques without using probe methods.