Electronic Structure and Lattice Relaxation Related to Fe in Mgo

The electronic structure of Fe impurity in MgO was calculated by the linear muffin-tin orbital--full-potential method within the conventional local-density approximation (LDA) and making use of the LDA+$U$ formalism. The importance of introducing different potentials, depending on the screened Coulomb integral $U$, is emphasized for obtaining a physically reasonable ground state of the Fe$^{2+}$ ion configuration. The symmetry lowering of the ion electrostatic field leads to the observed Jahn--Teller effect; related ligand relaxation confined to tetragonal symmetry has been optimized based on the full-potential total energy results. The electronic structure of the Fe$^{3+}$ ion is also calculated and compared with that of Fe$^{2+}$.Comment: 13 pages + 4 PostScript figures, Revtex 3.0, SISSA-CM-94-00

Amorphization during mechanical grinding of Bi-Pb-Sr-Ca-Cu-O ceramic superconductors

The report describes the amorphization occurring during mechanical grinding of a mixture of Bi-HTSC 2223 and 2212 phases. It is shown that the visually determined grain size and the actual crystal size diminish upon grinding. Noticeable amorphization occurs even after only 30 min of grinding. Furthermore, different crystal planes exhibit variable sensitivity towards the amorphization process and the high T c-phase is more easily affected by this phenomenon than the low T c-phase in the Bi-system. Identical results were obtained whether the starting composition of the spray dried powder was 2223 or 223

Generation of singlet molecular oxygen from H2O2 with molybdate-exchanged layered double hydroxides: effects of catalyst composition and reactions conditions

(Mg, Al)-layered double hydroxides (LDHs) with different Mg/Al ratios in the octahedral layer were prepared via the coprecipitation method and were exchanged with varying amounts of molybdate. The composition of the LDH supports and the state of the molybdate in these materials were studied with X-ray diffractometry, X-ray photoelectron spectroscopy and Raman. As was proven by NIR luminescence, aqueous H2O2 is converted at the surface of these catalysts into excited, singlet molecular dioxygen (1O2). The singlet dioxygen diffuses away from the 1O2 generating centers and can perform selective oxygenations, such as endoperoxidations of dienes, or hydroperoxidations of olefins. Catalyst composition and reaction conditions (temperature, solvent) have major effects on the catalyzed 1O2 generation and the subsequent substrate oxygenation. In order to maximize the efficiency of the H2O2 use and to limit the epoxidation side reaction, it is advisable to limit the amount of exchanged molybdate to 0.2 mmol per g and to use an LDH with Mg0.9Al0.1 composition in the octahedral layer. The latter material provides the optimum basicity for the catalytic activity of the exchanged molybdate.SCOPUS: ar.jinfo:eu-repo/semantics/publishe