Charge-carrier localization in the self-doped La1-yMn1-yO3 system

Solution techniques allow the preparation of La1-yMn1-yO3 at low temperatures. However, the as-prepared compounds show thermodynamically induced vacancies on both cationic sites. The transport and magnetic properties strongly depend on both bulk and surface defects. In order to separate these effects, we have studied La1-yMn1-yO3 compositions by varying the vacancy content y and the grain size. The electronic structure of these phases has been investigated by means of the X-ray absorption spectroscopy at the Mn K-edge. XAS experiments have been carried out on the La1-yMn1-yO3 system as compared with the La1-xCaxMnO3 reference series. For both series, the absorption edge and the unit cell volume vary linearly with the formal Mn(IV) content, resulting from a direct correlation between the hole count in Mn 3d states and the concentration of doping or of cationic vacancies in the perovskite phase. However, in the La1-yMn1-yO3 system, a deviation from this linearity occurs for vacancy contents above 30% of Mn(IV). This corresponds to a limit of solubility of the cationic vacancy in the bulk. Larger hole contents (up to 40%) may still be measured, but XANES spectra indicate that the excess holes are then trapped onto Mn sites, probably located at the surface. Despite this localization, transport measurements indicate a transition from an insulating to a metallic behavior in the low-temperature ferromagnetic regime beyond the critical concentration of 30% of Mn(IV). (C) 2000 Elsevier Science B.V. All rights reserved.