NONSTOICHIOMETRY AND PROPERTIES OF MIXED-VALENCE MANGANITES

Two families of perovskites, LaMnO3+delta and R1-xMnO3-delta (R =La, P r, Nd) have been prepared in polycrystalline form by a soft-chemistry route. The precursors were annealed at moderate temperatures either in air or under high oxygen pressure. In the strongly oxygenated LaMnO3delta perovskites (with nominal Mn4+ contents up to 58%), a detailed n eutron powder diffraction study shows the presence of both La and Mn v acancies, the Mn being in a substantially higher proportion. The prese nce of Mn vacancies perturbs the conduction paths for the transport of holes across Mn-O-Mn, weakening the double-exchange interaction. The unwanted presence of Mn vacancies can be minimized in the R1-xMnO 3-de lta family: for x > 0.07 the insulator-metal transition at temperature s close to T-C is recovered, as well as the colossal magnetoresistance behaviour.