OXYGEN IN OXIDES: an inert packing atom, or does it have a life of its own? Seminar

We are accustomed to regarding oxygen in oxide structures as being present as the O²- ion, or at least, in the (-II) oxidation state. There are a few examples known of peroxides and superoxides, with oxidation states of (-I) and (-0.5), respectively, but usually the oxidation state is fixed and, for instance, oxygen is not regarded as redox-active. This talk will consider why oxides are built of O(-II) ions, since the O(-II) ion is spontaneously unstable in the gas phase, and therefore, the circumstances under which ionisation may occur. Experimental results will be presented showing how (i) hole conductivity in certain perovskite ceramics may be associated with ionisation of O(-II) ions, leading to (ii) insulator – semiconductor transitions in certain cases, (iii) a novel mechanism for resistive switching and possible memristive applications and (iv) the onset of electronic conductivity in materials such as yttria-stabilised zirconia, the archetypal oxide ion conductor that is used as the solid electrolyte in solid oxide fuel cells, in part due to the absence of any electronic conductivity. Finally, suggestions will be made as to (v) the possible occurrence and significance of redox-active oxygen as a component of lithium battery cathodes and (vi) the source of metallic conductivity associated with 2D electron gases in interlayer structures formed between insulators such as LaAlO₃ and SrTiO₃