Excess Oxygen in LaMnO3+δ

Using a volumetric method, the relationship among the oxygen desorption and absorption equilibrium pressure P, excess oxygen δ, and temperature T (P–δ–T) has been investigated for polycrystalline LaMnO3+δ. To interpret the measured P–δ–T relation, we have proposed a defect model in which La and Mn site vacancies exclude the formation of vacancies at their first and second nearest neighbors. The proposed model has been compared with other defect models derived on the basis of the law of mass action. The calculated P–δ–T relation has been favorably compared with the observed P–δ desorption isotherms. Significant deviations from the calculated relation, however, have been observed when the P–δ isotherm is acquired during the absorption process. This hysteretic behavior is attributed to the formation and annihilation of La and Mn site vacancies accompanied by the redox reaction. Using the present volumetric method, we are able to control the excess oxygen δ accurately by quenching the sample heat treated under appropriate oxygen equilibrium pressure. The X-ray diffraction and SQUID magnetization measurements on these oxygen-controlled samples are in good agreement with previous results.