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

Mn K-edge X-ray absorption experiments have been carried out on the self-do ped LaMnO3+delta system and compared to a (La1-xCax)MnO3 reference series. For both systems, the absorption edge and the unit cell volume vary linearl y with the formal Mn(IV) content, indicating a direct correlation between t he hole count in Mn 3d states and the concentration of substituents or cati onic vacancies in the perovskite structure. However for the LaMnO3+delta sy stem, a deviation from linearity occurs in the volume change at high delta values. This discontinuity is ascribed to a limit of solubility for the cat ionic 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 surfa ce sites. Based on resistivity and thermopower measurements, the difference s in hole doping through aliovalent substitution or vacancy insertion are d iscussed, highlighting the existence of a large ferromagnetic insulating co mposition range in the LaMnO3+delta system for 0.1 less than or equal to de lta less than or equal to 0.15. (C) 2001 Academic Press.