Interstitial O and O vacancies in La2CuO4 during high temperature treatments

We have investigated the absorption of interstitial O and the formation of O vacancies in La2CuO4+? in equilibrium with O2 pressures between 10-4 and 103 Torr or in vacuum at high temperature. Below 600°C, the concentration of vacancies is negligible, and the equilibrium between La2CuO4+? and gaseous O2 only involves interstitial O. The pressure-composition equilibrium curve has been determined at 550°C by repeated vacuum extraction in a fixed volume. The analysis of the partial desorption stages indicates the formation of interstitial O2- ions (?~p1/6) with corrections due to the formation of peroxide species, like pairs of O ions with ionization number -2 (?~p1/3) or -3 instead of -4. An equilibrium fraction of O vacancies up to 8% can be created above 720°C in a pressure lower than 0.1 Torr, and reversibly filled by reoxygenation at high temperature, without decomposition of the sample. The vacancies are much less mobile than interstitial O and therefore they must be in the CuO2 plane; they can be frozen in the sample and coexist with interstitial O.