Lattice expansion upon reduction of perovskite-type LaMnO3 with oxygen-deficit nonstoichiometry

Volume variation of dense LaMnO3+δ specimens was measured at elevated temperatures and in stepwise changes of oxygen partial pressure. The specimen expanded upon reduction at fixed temperatures, and its dependence on temperature and oxygen partial pressure conformed to that of oxygen-deficit nonstoichiometry, which was formulated on the basis of a defect model by Mizusaki et al. [Solid State Ionics 129 (2000) 163]. The measured expansion was in almost linear relationship with the oxygen nonstoichiometry. In consideration of reported behavior that the unit cell shrinks upon Sr-doping and upon formation of cation vacancy, the origin of the expansion upon reduction is regarded as the increase in the size of B-site cation, which accompanies the charge compensation for the formation of oxide ion vacancies. The expansion rate that was estimated from the variation in averaged ionic radius of Mn was higher than the experimentally observed one; it is supposed that the effect of the increase in B-site radius is partly offset by the increase in the crystallographic distortion.