EFFECT OFNONSTOICHIOMETRY ON THE VOLUME OF LaMnO_(3+δ)-BASED PEROVSKITE OXIDES

摘要

On the dense specimen of lanthanum manganite, isothermal dilatometry was performed at fixed temperatures and in stepwise changes of oxygen partial pressure, P_(O_2), for both the regions of oxygen-deficient and oxygen-excess nonstoichiometry. In the oxygen-deficit region, the isothermal expansion upon reduction had an almost linear relationship with the oxygen content, and the slope was constant irrespective of temperature. This behavior is well reproduced by the estimation of the lattice expansion caused by increase in the average ionic radius of Mn, which accompanies the formation of oxide ion vacancies. In the oxygen-excess region, the volume of the dense specimen expanded with P_(O_2) increasing, while the unit cell volume was known to decrease with P_(O_2) increasing. This difference strongly suggests that the predominant ionic defect in the oxygen-excess lanthanum manganite is cation vacancy: when the oxide incorporates oxygen, further crystal lattices are created from the excess oxygen and the cations which are provided by formation of cation vacancies, resulting in the expansion of the specimen.