Simultaneous Probing of the Fe and Co Sites in the CO₂-Absorber Perovskite Sr_0.95Ca_0.05Co_0.5Fe_0.5O_3-δ:  a Mössbauer Study

The Co and Fe sites have been probed in pristine and CO2-treated Sr0.95Ca0.05Fe0.5CO0.5O3-delta using emission (EMS) and transmission (TMS) Mossbauer spectroscopy and found to have different local oxygen coordination environments and, most probably, a layered arrangement in the lattice. The EMS and TMS spectra of the pristine material could be evaluated with a three-doublet model, with only one of the doublets being different in the two cases. The doublets were assigned to Fe species with localized and delocalized electronic states. Absorption of CO2 at 650 degreesC resulted in magnetic ordering, which appeared to be static in EMS and rather dynamic in TMS spectra. This difference was attributed to the lower average oxygen coordination number of Co, and therefore ready absorption of CO2 at O vacancies. CO2 treatment at 950 degreesC triggered partial phase separation, confirmed also by scanning electron microscopy and X-ray microanalysis measurements. It was concluded that absorption of CO2 occurs preferentially in Co-rich regions, where the oxygen vacancy concentration is higher. The Mossbauer data indicated a lower average valence state of Fe/Co atoms in the Co-rich region.