Defect-chemical role of Mn in Gd-doped CeO2

It has been known that addition of a small amount of Mn facilitates sintering of CeO2-based electrolytes, that is, rendering the sintering possible at much lower temperature. This phenomenon is generally understood as being due to Mn acting as acceptors to increase the concentration of charge compensating oxygen vacancies and, hence, to enhance the mass transport. In this case, one would even expect that the electrolytic domain of CeO2 may be enlarged particularly towards the reducing atmosphere because Mn tends to become more effective as acceptors due to its ever reducing valence down to +2 (or Mn″Ce) with decreasing oxygen partial pressure. In order to elucidate the defect-chemical role of Mn, we have examined the equilibrium electrical conductivity of 10 mol% Gd-doped and each 5 mol% Mn and Gd-codoped ceria in association with XRD and EPMA. It has been found that Mn addition neither enhances the ionic conductivity nor extends the electrolytic domain, indicating that Mn is, by no means, so effective an acceptor as expected. Furthermore, despite that the specimens appear phase-pure via XRD, EPMA results reveal at grain boundaries the precipitates enriched with Mn. It is, thus, suggested that the solubility limit of Mn in Gd-doped ceria is highly likely less than 1 mol%.