Nickelocene as precursor of microporous organometallic-derived carbon and nickel oxide-carbon nanocomposite.

Microporous flower-like and spherical carbon particles, made of graphene-like layers, have been obtained via chlorination of nickelocene (Ni(C5H5)2). Their mechanism of formation, in terms of morphology and micro-nanostructure, has been followed from 200 to 900°C. Conventional transmission electron microscopy and high-resolution-TEM observations allow determining that their structure is made of highly disordered graphene-like layers. The Raman spectrum of the high temperature sample exhibits the characteristics D and G bands. The peak positions, the ratio of their intensities (ID/IG) and full width at half maximum suggest a high degree of disorder in the nanostructures. The calculated in-plane correlation length of these graphene-like layers is 1.15nm. In all the carbon particles, electron energy-loss spectroscopy shows sp2 carbon bonding content higher than 95% and mass density in the range of 1.0-1.6g/cm3. Textural studies show Type I adsorption isotherms with surface area of 922m2/g for the sample produced at 900°C. In addition, the basic hydrothermal treatment of the sample chlorinated at 600°C yields a composite material with NiO nanoparticles well dispersed within the carbon matrix.

Electronic structure of MgS and MgYb2S4: Electron Energy-Loss Spectroscopy and self-consistent multiple scattering calculations.

The electronic structure of MgS and MgYb2S4 have been studied using the fine structure of the Mg-K, S-K, Mg-L2,3, S-L2,3 and Yb-N5 edges measured by electron energy-loss spectroscopy (EELS). Our experimental results are compared with real-space full multiple scattering calculations as incorporated in the FEFF9.6 code. All edges are very well reproduced. Total and partial densities of states have been calculated. The calculated densities of states of Mg and S are similar in both compounds. The energy distribution of these states suggests a covalent nature for both materials. For MgYb2S4 a band gap smaller than for MgS is predicted. In this compound the top of the valence band and the bottom of the conduction band are dominated by Yb states.

Chlorination and solvothermal treatment of Zr(C5H5)2Cl2: a synthetic combination to produce nanometric tetragonal ZrO2.

A novel synthetic strategy based on the combination of the chlorination of an organometallic precursor followed by solvothermal treatment is found to be successful in the synthesis of tetragonal nano-ZrO(2) or nano-ZrO(2), embedded in an amorphous carbon matrix, depending on the solvent employed in the solvothermal step. The chemical and structural features (chemical composition, size and surface defects) of the intermediate and final materials have been determined experimentally mainly by high resolution transmission electron microscopy, electron energy loss spectroscopy, and Z-contrast images. These local techniques reveal that the nanoparticles consist of tetragonal ZrO(2) with an average size of 1.7 ± 0.4 and 6.2 ± 0.9 nm for the embedded in carbon and the free nano-ZrO(2), respectively.

Microscopic structure and dynamics of molten Se50Te50 alloys.

In this work we investigate the microscopic structure and dynamics of the molten equimolar alloy, Se(50)Te(50) using a combination of neutron and x-ray diffraction experiments, reverse Monte Carlo analysis, and first principles molecular dynamics. The range of temperatures studied covers the semiconductor/metal transition. From our results it can be seen that the latter is associated with an increase in coordination numbers and a reduced tendency to heterocoordination. In agreement with previous inelastic neutron scattering experiments, our molecular dynamics calculation predict a certain widening of the stretching vibrational modes band in connection with the increase of coordination and the presence of longer bonds in the metallic phase.

Electron microscopy study of incommensurate modulated structures in misfit ternary chalcogenides.

The study of misfit structures by means of transmission electron microscopy and associated techniques is reviewed. It is complementary to X-ray crystallography and provides a local and direct view of the beautiful complexity of these systems. Three different types are presented in this work. Misfit columnar structures can be contemplated as a case of one-dimensional misfit structures while misfit layer structures are considered as two-dimensional misfit structures. Some extra dimensionality is added when these misfit layer structures wrap to give rise to tubular crystals. Electron microscopy/diffraction shows clearly in many examples the presence of structural modulations as weak satellite reflections that are very difficult to detect by X-ray diffraction methods. Besides, high-resolution images show the presence of stacking defects in some of these misfit layer structures.