Charge-carrier localization on Mn surface sites in granular LaMnO3+δ samples

Mn K-edge X-ray absorption experiments have been carried out on the self-doped LaMnO3+delta system and compared to a (La1-xCax)MnO3 reference series. For both systems, the absorption edge and the unit cell volume vary linearly with the formal Mn(IV) content, indicating a direct correlation between the hole count in Mn 3d states and the concentration of substituents or cationic vacancies in the perovskite structure. However for the LaMnO3+delta system, a deviation from linearity occurs in the volume change at high delta values. This discontinuity is ascribed to a limit of solubility for the cationic vacancy insertion, evidenced for 30% of Mn(IV). Larger hole contents can still be measured in samples synthesized at low temperature and showing a small grain size, but these excess holes are then localized on, Mn surface sites. Based on resistivity and thermopower measurements, the differences in hole doping through aliovalent substitution or vacancy insertion are discussed, highlighting the existence of a large ferromagnetic insulating composition range in the LaMnO3+delta system for 0.1 less than or equal to delta less than or equal to 0.15. (C) 2001 Academic Press.