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Oxygen-excessive LaMnO 3+y samples with 0.02 Յ y Յ 0.26 have been prepared by thermal decomposition in air at 450°C of a precursor obtained from an aqueous solution of lanthanum nitrate and manganese acetate (atomic ratio La/ Mn = 1). The obtained solid is annealed at different temperatures from 700 to 1000°C and under three oxygen pressures: 1.00, 0.21, and 10 −4 atm. Samples with 0.14 Յ y Յ 0.26 are simple-phased rhombohedral perovskites, the distortion of which decreases as the oxygen content (and Mn Mn4+ cation. The volume (V f ) per LaMnO 3+y formula unit decreases regularly with increasing oxygen content (i.e., oxidation). Study has shown that the oxygen content is best described in terms of La and Mn vacancies, rather than of interstitial anions. Density measurements confirm the cation vacancy model involving equal amounts in La and Mn sublattices. The infrared spectra of LaMnO 3+y samples have

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