Local Charge Disproportion in a High-Performance Perovskite

The temperature-dependent local charge disproportion in the (Ba 0.5 Sr 0.5 )(Co 0.8 Fe 0.2 )O 3−δ perovskite-type oxide (denoted as BSCF) was monitored by in situ electron energy-loss spectroscopy applying monochromized electrons in a transmission electron microscope. At elevated temperatures, oxygen is removed from the crystal lattice in these perovskite-type oxides that is due to a reduction of the transition metal sites. To determine the site-specific valence in the BSCF perovskite, we measured the cobalt and iron L-edges and the oxygen K-edge on increasing the temperature from 298 to 1223 K. It was found that the loss of oxygen at elevated temperatures is mainly due to a reduction of the cobalt site compared to the iron site. The understanding of the site specific redox behavior in this material is important in explaining its long-term instability at intermediate temperatures, which has not been elucidated priory this study.