Reactivity of Solids最新文献

It is shown that along with the oxygen loss in yttrium, gadolinum and erbium compounds, (RE)Ba₂Cu₃O₇, the orthorhombic distortion of the crystal lattice decreases smoothly and disappears at oxygen index values between 6.3 and 6.4. The unit cell volume expansion that accompanies the oxygen loss is due to a rise in the lattice parameter c, which in the range of 6.3 to 7.0 can be used to estimate the oxygen content of the sample.

A study on the influence of high hydrostatic pressure on the electrical conductivity of silver azide single crystals at the ranges of temperature 263–373 K and pressure 10⁵−4 × 10⁸ Pa is described. Motional and formation activation volumes for intrinsic defects and activation energies of the processes have been determined. The theoretical estimates of the activation volumes in terms of Zener strain-energy model have been made, and were found to be in reasonable agreement with the values obtained from the experiment. The results support the view that silver azide has Frenkel disorder, transport in single cyrstals is generally ionic at low voltage, extrinsic under normal conditions, and occurs by the motion of interstitial ions. Conduction becomes intrinsic at temperatures above ~ 330 K. There are some differences between the data obtained for single crystals and those published for powder samples, which is probably related to the peculiarities of the powders.

The kinetics of the isothermal dehydration of single crystals of vermiculite which have a layered structure, have been studied in vacuo of ~ 6.7 × 10⁻⁵ Pa by quartz crystal microbalance. A kinetic model is proposed for the process, which corresponds to gas diffusion from a semi-infinite medium with boundary conditions, and takes into consideration the mass transfer through the “solid-gas” interface. The kinetics can be described in terms of a quasilinear expression in the initial stages and as a first-order equation for greater times. From the experimental data the diffusion coefficients of the interlayer region have been estimated. The values obtained are in agreement with those measured by a quasi-elastic neutron-scattering method.

Mixed Fe,Co oxides are prepared by thermal or mechanochemical decomposition of mixed carbonate precursors. The thermal decomposition of carbonates at 300°C in air gives single-phase cation-deficient spinels encompassing the whole range of compositions. These samples show high dispersity and have lattice distortions particularly in the range 0.523–0.646 (Co/(Co+Fe)). The oxides prepared at 600°C are mixtures of Co₃O₄/CoFe₃O₄ for that composition interval. A decrease in cation vacancy content and an increase in the unit cell parameters are observed at this temperature. The mechanochemical decomposition of the carbonate precursors takes place after longer grinding times for Co-rich samples, which give Fe,Co-mixed monoxides. The Fe-rich samples give spinel oxides.

Very active catalysts can be prepared from FeCl₃-GIC precursors through a two-step reduction and washing with HCl. Mössbauer spectrometry during the first (H₂) reduction step indicates that Fe⁰ in the form of α-Fe is obtained together with two non-equivalent Fe^II sites. This α-Fe is catalytically inactive and can be readily removed from the system treatment with HCl. The second reduction with potassium vapour transforms the remaining Fe^II into catalytically active Fe⁰ which is strongly resistant to the HCl treatment.