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Semiconductors are key materials in modern electronics and are widely used to build, for instance, transistors in integrated circuits as well as thermoelectric materials for energy conversion, and there is a tremendous interest in the development and improvement of novel materials and technologies to increase the performance of electronic devices and thermoelectrics. Tetramorphic Ag(10)Te(4)Br(3) is a semiconductor capable of switching its electrical properties by a simple change of temperature. The combination of high silver mobility, a small non-stoichiometry range and an internal redox process in the tellurium substructure causes a thermopower drop of 1,400 microV K(-1), in addition to a thermal diffusivity in the range of organic polymers. The capability to reversibly switch semiconducting properties from ionic to electronic conduction in one single compound simply by virtue of temperature enables novel electronic devices such as semiconductor switches.

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The intermetallic compounds Li(x)Si(y) have attracted considerable interest because of their potential use as anode materials in Li ion batteries. In addition, the crystalline phases in the Li-Si phase diagram turn out to be outstanding model systems for the measurement of fast Li ion diffusion in solids with complex structures. In the present work, the Li self-diffusivity in crystalline Li(12)Si(7) was thoroughly probed by (7)Li NMR spin-lattice relaxation (SLR) measurements. Variable-temperature and -frequency NMR measurements performed in both the laboratory and rotating frames of reference revealed three distinct diffusion processes in Li(12)Si(7). The diffusion process characterized by the highest Li diffusivity seems to be confined to one dimension. It is one of the fastest motions of Li ions in a solid at low temperatures reported to date. The Li jump rates of this hopping process followed Arrhenius behavior; the jump rate was ~10(5) s(-1) at 150 K and reached 10(9) s(-1) at 425 K, indicating an activation energy as low as 0.18 eV.

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Active reaction sites for 02 reduction in La0.~Sr01MnO3 electrode have been characterized by addressing the origin of the cathodic polarization effects on this electrode material. Cathodic polarization (up to -1.2 V vs. Pt reference electrode} had several effects on O2 reduction kinetics. First, the O2 reduction rate was favorably increased when the perovskite electrode was cathodically polarized. Second, in situ x-ray photoelectron spectroscopy results indicated that the Mn ions are electrochemically reduced and concomitantly the oxygen stoichiometry decreases. Reduction of Mn ions was further demonstrated in the cyclic voltammogram traced under nitrogen atmosphere. Third, hysteresis in cathodic currents was observed in the cyclic voltammograms of the perovskite/YSZ/Pt system, and the hysteresis phenomena were more prominent at higher O~ pressure. We interpreted these findings to mean that the internal and/or external surface oxide vacancies participate in the O2 reduction reaction. However, it has been explained from the Po2-dependent hysteresis phenomena that, even though those surface sites are active in the O2 reduction~ their activity is less than that of the three-phase boundary sites since additional diffusional processes are required for the former sites. Consequently, the three-phase boundary sites are the major reaction sites at lower O2 pressure, which leads to a small hysteresis. However, at higher 02 pressure, the surface sites also participate in the reaction, resulting in a larger hysteresis.Strontium-doped lanthanum manganites (Lal xSr= MnO3) are widely studied for their possible applications as cathode materials in solid oxide fuel cell systems. 1-3 Even though many results have been reported on aspects of the oxygen reduction kinetics and on the nature of the active reaction sites, there still remain controversies on this issue in the literatureYThe manganite perovskites are electronic conductors, 9 but under cathodic potential they are supposed to be partially reduced, creating oxide vacancies. 6' 1~ Rate enhancement found in cathodically polarized electrodes has been interpreted with an assumption that surface oxide vacancies are active for oxygen reduction. 5' Another set of the previous results that oxygen reduction kinetics can be improved by Sr doping to LaMnO~ has also been explained with the same assumption. ''6On the contrary, other reports suggested that the surface sites cannot participate in O2 reduction since the O 2-bulk diffusion in the manganites is negligible compared to the cobalt analogs or YSZ electrolytes. 12 The favorable effect of the cathodic polarization was also attributed to the enlargement of the three-phase boundary lines. 7' 13 It is likely, however, that the surface or grain boundary diffusion in porous electrodes cannot totally be discarded for their possible contribution to the overall kinetics since in general surface or grain boundary diffusion is much faster than bulk diffusion.In this study, we tried to elucidate the nature of the active sites for O2 reducti...