NONSTOICHIOMETRY AND THERMOCHEMICAL STABILITY OF THE PEROVSKITE-TYPE LA1-XSRXMNO3-SIGMA

In order to make clear the extent of nonstoichiometry and decomposition P(O2) of the perovskite-type La1-xSrxMnO3-delta, measurements are made by high temperature gravimetry, coulometric titration and iodometry on the oxides with x = 0.2 and 0.4. The decomposition P(O2) was found to increase with increasing x. The decomposition products for x = 0.4 were (La0.6Sr0.4)2MnO4 and MnO, and mixtures of (La1-xSrx)2MnO4, MnO, and La2O3 for x = 0.2. For x = 0.4, the plot of 3-delta versus log P(O2) showed plateaus at delta = 0 and 0.0225, while no plateau was observed corresponding to the Mn3+ state at delta = x/2 (= 0.2). By calculation for oxygen partial molar enthalpy and entropy, it was found that oxygen vacancies distribute randomly on the oxygen sublattice and electronic state is essentially metallic for La0.6Sr0.4MnO3-delta with delta > 0.0225. In the region of delta < 0.0225, oxygen vacancies seemed to distribute on limited sites. However, details of the defect equilibrium for this region was not clarified.