PROPERTIES OF LACO1-TCRTO3 .2. REDUCTION AND REOXIDATION

The reduction and reoxidation properties of the LaCo1-tCrtO3-delta sol id-solution series have been studied by TPD, TPR, PXD, TGA, and XPS. A t moderate temperatures, reduction by hydrogen removes oxygen from LaC o1-tCrtO3 in a topotactic process which converts Co-III to Co-II. Bulk Cr-III is not affected; hence LaCo1-tCrtO3-(1-t/2) is the borderline composition of the reduced state. TPR shows that the reduction occurs in several steps, indicating that different reaction mechanisms are in volved. Surface reduction, confirmed by XPS for the Cr-rich samples, i s one such process. The as-prepared samples contain higher-valent (>II I) chromium surface species, the concentration of which is significant ly decreased upon the reductive treatment. The reduction and subsequen t reoxidation processes are reversible, and reduced samples with t < 0 .60 are unstable with respect to reoxidation even at ambient. The unit -cell volume per formula unit of LaCo1-tCrtO3-delta increases upon red uction, and indications of vacancy ordering are found for the Co-rich samples, where La2Co2O5 is the reduced end state. (C) 1995 Academic Pr ess, Inc.