Condensed-Matter Physics & Materials Science Seminar
"Epitaxial oxygen sponges: Reversible redox reactions in strontium cobaltite thin films"
Presented by Ho Nyung Lee, Oak Ridge National Lab.
Monday, April 15, 2013, 2 pm
Bldg. 480 conference room
Hosted by: MG Han
Perovskite-typed complex oxides with multivalent transition metals exhibit a wide spectrum of physical properties, including ferroelectricity, superconductivity, ferromagnetism, ion conductivity, and catalytic activity. Owing to the high ionic conductivity and, sometimes, electronic conductivity offered from several multivalent transition metal oxides, perovskite oxides have attracted lots of attention for solid oxide fuel cell and electrochemical sensor applications. However, high ionic conduction in perovskite oxides has been achieved only at elevated temperatures. Such a high temperature operation imposes serious drawbacks, including the thermomechanical degradation, interdiffusion at the interface, and destruction of structural integrity. They eventually degrade the device performance, since its redox activity cannot be fully reversible. Therefore, there have been a lot of efforts to reduce the working temperature for prolonged, reliable operation. Strontium cobaltites, SrCoO3-x (SCO), in that sense, are good candidates due to the fact that mixed ionic and electronic conductivity and good catalytic activity are offered from the multivalent nature of Co ions. Here, I will present epitaxial growth of two SrCoO3-x phases, i.e. the 'brownmillerite' SrCoO2.5 and the 'perovskite' SrCoO3, by pulsed laser epitaxy. Results from a systematic study with x-ray diffraction, x-ray absorption spectroscopy, x-ray magnetic circular dichroism, SQUID, and PPMS will be presented, comparing the structural and physical properties between two phases. In particular, based on in situ XRD and spectroscopic ellipsometry measurements under various ambient, I will show that those two phases can be reversible via reduction and oxidation processes at drastically reduced temperatures (<300 oC). The work was supported by the U.S. Department of Energy, Basic Energy Sciences, Materials Sciences and Engineering Division.