The dielectric permittivity, optical transmission spectrum and temperature dependence of resistance were investigated in single crystals of Bi4Ti3O12 and BaBi4Ti4O15. It is found that the dielectric anomalies previously reported below the Curie temperature are due electron space charges of inhomogeneous distribution in the material and near the non-ohmic electrodes.
Images of pale yellow crystals of 4Nb205. 9WO 3, obtained with a 1 MV high-resolution electron microscope revealed twinned domains of a tetragonal tungsten bronze structure with a superlattice of 3 x 1 subcells. Comparison with computer calculations suggests that the cations filling the pentagonal tunnels include both Nb and W. Crystals darkened due to reduction on longer heating included no domains and were sensitive to electron irradiation; cations were knocked on from the filled to the vacant pentagonal tunnels. This suggests that some oxygens are released from the -M-O-M-O-Mstrings in the tunnels on reduction to weaken the chemical bonding. The number of deficient oxygens is known from the weight gain on oxidizing the crystal. Some additional experiments reveal that there is no '6Nb205. 1 IWO 3' phase. The resolving power of the present microscope is discussed on the basis of the analysis of the chromatic aberration.
The thermal stability of epitaxial SrTiO3 thin films grown by molecular-beam epitaxy on Si (001) has been studied using x-ray diffraction (XRD), optical microscopy (OM), scanning electron microscopy (SEM), and thermodynamic calculations. Our studies focus on the stability of the SrTiO3/Si structures under the conditions typically employed in the pulse laser deposition (PLD) growth of complex metal oxide heteroepitaxy on Si. We observe additional Bragg peaks in thermally treated SrTiO3 buffered Si structures, corresponding to possibly TiSi2 and/or SrSiO3, reaction products which are consistent with the reaction schemes we propose. In addition, OM and SEM reveal microstructures that are not readily accounted for solely by the solid state reactions as put forth by previous workers but can be reasonably explained by our proposed reaction schemes. Using our observations and thermodynamic analysis, we argue that reactions involving the gaseous species SiO(g), the reactivity of which has not been previously considered in this system, are important. We attribute the onset of degradation of the SrTiO3 film at high temperatures, to the circular void forming reaction Si(s)+SiO2(s)→2SiO(g) at the interface and suggest that the reactions considered by previous workers involving all solid state reactants occurs only at the conclusion of degradation. Our results points to the need for keeping the PLD temperature as low as possible to minimize the production of reactive SiO(g) in avoiding the deleterious reactions.
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