Valence Reduction Research Articles

Ba2-xLaxYCu3O7+/- delta perovskite compounds: Crystal chemistry.

We report the crystal chemical properties of solid solutions obtained by substitution of lanthanum for barium in the high-${T}_{c}$ superconductor ${\mathrm{Ba}}_{2}$${\mathrm{YCu}}_{3}$${\mathrm{O}}_{7}$. An orthorhombic-to-tetragonal symmetry change occurs at x=0.4 in ${\mathrm{Ba}}_{2\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{La}}_{\mathrm{x}}$${\mathrm{YCu}}_{3}$${\mathrm{O}}_{7\ifmmode\pm\else\textpm\fi{}\mathrm{\ensuremath{\delta}}}$ with the tripled perovskite supercell maintained. Charge compensation is achieved through reduction of the formal copper valence (\ensuremath{\delta}=0) in the orthorhombic phase and by a mixture of excess oxygen accommodation (\ensuremath{\delta}>0) and formal valence reduction in the tetragonal phase. The resistivity increases by several orders of magnitude, even though the average formal copper valence changes very little.

IR-spectroscopy of impurity complexes in germanium

A large number of impurity complexes which create shallow levels, have been discovered in pure and doped germanium single crystals. The structure and the composition of several of these complexes have been determined using far-infrared spectroscopy and information about crystal growth related residual impurities. In general, the complexes consist of a heavy substitutional impurity binding one or more light. interstitial impurities such as hydrogen or lithium. The coupling between the tunneling motion of the interstitial component and the electronic structure of the complexes can create complicated ground state manifolds which contain stress insensitive components. Spectroscopy further shows that multivariate centers, such as double or triple acceptors, become hydrogenic complexes upon binding one or two hydrogen atoms, respectively. Neutralization of shallow acceptors in silicon, which has recently attracted attention, appears to be the same phenomenon as the valency reduction of multiple acceptors in germanium.

Polymerization of ethylene with the system (C 5H 5) 2TiEtCl-AlEtCl 2

The catalytic system (C 5H 5) 2TiEtCl-AlEtCl 2 in benzene and heptane was investigated. Only two species are formed at an equimolar ratio Al: Ti, viz. active (C 5H 5) 2TiEtCl.AlEtCl 2 (I) and inactive (C 5H 5) 2TiCl.AlEtCl 2 formed from (I). The rate constant of propagation is k p 20° = 6.4 l/mole sec and is independent of the medium. The rate of polymerization decreases with time because of valence reduction. The bimolecular law is obeyed during a run but the apparent termination constant is inversely proportional to the initial catalyst concentration. The kinetic data with different ratios Al:Ti and the dependence of the number of polymer molecules/Ti atom show that AlEtCl 2 is a termination agent and a chain transfer agent.

Study of (π‐C5H5)2Ti(C2H5)Cl and its higher homologs in soluble Ziegler catalysts

AbstractSoluble ethylene polymerization catalysts derived from (π‐C5H5)2Ti(R)Cl and R′AlCl2 where R is ethyl or higher alkyl and R′ may be methyl or ethyl, were studied both by polymerization kinetics at 0°C and by diagnostic experiments. An early acceleration in rate occurred at this temperature in toluene solvent which was due to a solvent dependent increase in catalyst activity, not to a gradual formation of catalyst. No such solvent effect was found in nonaromatic solvents. The subsequent decay in rate, at least at low temperatures, did not depend upon valence reduction. The effect of Al/Ti ratio was studied, and certain discrepancies in the literature were shown to be due to the method of making kinetic measurements. Oxygen, which has previously been reported to affect the polymerization rate with these catalysts, was also found to eliminate the acceleration period in toluene when present in the amount of 1% of the catalyst. These catalysts gave polymers at low temperature for which the active site had long life and which did not undergo chain transfer. Therefore, they approximate many of the characteristics of living polymers.

Cerium-Stabilized UO_2^+2:Nd^+3 Laser Glass

The ground-state absorption at 1.06 μ in UO2+2:Nd+3 laser glass is directly related to the uranium content of the glass. Valence reduction of UO2+2 to U+5 (and possibly U+4) is responsible; however, the oxidized state can be stabilized with cerium. The percentage of the total uranium present as the UO2+2 species was increased from 63% to 91% in a particular soda-lime-silica base.