Interfaces (grain boundaries and surfaces) are studied in acceptor (Fe) doped SrTiO3 bicrystals, single crystals, and ceramics as a function of temperature (423 K T 1023 K), oxygen partial pressure (1 Pa Po io Pa), and Fe doping content (1.9 1018 cm3 c 9.5 1O' cm3) using electrochemical methods. In particular, impedance spectroscopy and dc polarization techniques have been applied. The electrochemical investigation of tilt and twist grain boundaries in bicrystals combined with structural and chemical grain boundary charactizationby transmission electron microscopy, electron diffraction x-ray analysis, and electron energy loss spectroscopy allowed us to clarify grain boundary effects in SrTiO3. The use of reversible (nonblocking) YBa2Cu3O6+ electrodes proves to be a convenient technique to measure conductivities without electrode effects, since blocking effects at the sample surface were minimized. These results have been compared with those obtained for grain boundaries in polycrystalline samples as well as with the interfaces between metallic electrodes and SrTiO3 single crystals. Besides individual features, all findings at the investigated boundaries and interfaces could be consistently explained by the appearance of pronounced Schottky barriers which were composed of depletion layers of mobile majority charge carriers (h, V).

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Effective surface rate constants for oxygen incorporation have been determined for single-crystal Fe-doped SrTiO 3 from chemical relaxation experiments, by in situ, spatially resolved optical absorption spectroscopy, and from tracer exchange experiments, by subsequent secondary ion mass spectrometry analysis. Experiments were carried out as a function of temperature, oxygen partial pressure, and dopant concentration. The effect of various metal films ͑Pt, Ag, Cr͒ and metal oxide films ͑YBa 2 Cu 3 O 7Ϫ␦ and La 0.8 Sr 0.2 CoO 3Ϫ␦ ͒ on the surface reaction kinetics was also investigated. The implications of the results with regard to the mechanisms of chemical and tracer incorporation into SrTiO 3 are discussed.

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Strontium titanate (SrTiO,) is known as a good high-temperature resistive oxygen sensor material; its response time depends on oxygen bulk diffusion and surface exchange processes. In the present work, "0 diffusion has been investigated in lanthanum-doped SrTiO, single crystals in the temperature range 700" to 900°C by secondary ion mass spectrometry (SIMS). Oxygen tracer diffusivities between 2 X lo-'' and 1 X cmZ/s have been calculated from the SIMS results. Low surface enrichment of "0 compared to the "0 concentration in the gas atmosphere gives clear evidence for a surface exchange reaction.