Abstract
Samples of the La1-xCaxMnOy system have been prepared under different atmospheres. Compositional analysis by electron probe microanalysis, atomic absorption, and thermogravimetric analysis reveals that the samples prepared in air show, in all cases, cationic and anionic vacancies which lead to a general composition (La1-xCax)zMnzOy, where z < 1 and y < 3. When these samples are oxidized, z remains constant, and the oxygen content increases up to y ∼ 3, i.e., until the anionic perovskite sublattice is practically fully occupied. The precise knowledge of both the Mn3+/Mn4+ ratio and the oxygen content allows one to assess its influence on the properties. The oxidation of these materials is accompanied by a strong decrease of their resistance; also, an increase in the magnetoresistance takes place, which is especially important at room temperature.
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