Effect of oxygen stoichiometry on the electrical properties of $La0.5Sr0.5CoO3$ electrodes

We report on the metal-insulator transition of $La0.5Sr0.5CoO3$ thin films deposited by pulsed laser ablation on $LaAlO3$ substrates. The films were cooled in oxygen partial pressures between 760 and $10−5$ Torr and electrical resistivity of the films was measured as a function of cooling oxygen pressure. $La0.5Sr0.5CoO3$ films changed from metallic to insulating behavior depending on their oxygen content. A defect model has been proposed to explain this transition and the change in conductivity is related to the change in the oxidation state of the cobalt ions. The model explains the relationship between oxygen partial pressure and electrical conductivity in $La0.5Sr0.5CoO3$⁠, which describes the experimental dependence reasonably well. Positron annihilation studies were also done on the same set of samples and the S parameter was seen to increase by 8% from a fully oxygenated sample to a sample cooled in $10−5$ Torr.