Problems with the thermogravimetric determination of oxygen stoichiometries in pure and rare-earth substituted La_2RuO_5

摘要

The oxygen stoichiometries of pure and rare-earth substituted La_2RuO_5 have been investigated by thermogravimetry (TG) in reducing atmosphere. Assuming that the observed total weight loss is caused by the reduction of Ru~(4+) to Ru-metal, remarkable oxygen deficiencies were calculated. These would correspond to ruthenium oxidation states significantly lower than the ones experimentally observed by XANES. To explain this discrepancy we investigated the reduction products by X-ray absorption spectroscopy (XAS). EXAFS measurements at the Ru-K edge revealed the presence of an X-ray amorphous ruthenium oxide, indicating an incomplete reduction. The apparent oxygen deficiencies obtained for pure and rare-earth substituted samples correlate with the amount of remaining ruthenium oxide. The presence of a ruthenium oxide species was furthermore verified by Ru-Lm XANES investigations. Our results show that the determination of oxygen contents by thermogravimetry might fail even for the easily reducable nobel metal oxides and therefore has to be applied with caution if the reaction products cannot be identified unambiguously.