It was recalled that the addition of a small amount of Mn facilitated the sintering of CeO2-based electrolytes at much lower temperatures. This was generally explained as being due to Mn acting as an acceptor to increase the concentration of charge-compensating O vacancies and hence enhanced mass transport. It was thus expected that the electrolytic domain of CeO2 should be enlarged towards the reducing atmosphere because Mn tended to become more effective as an acceptor due to its ever-reducing valence down to +2 (or Mn Ce″) with decreasing O partial pressure. In order to clarify the defect-chemical role of Mn, the equilibrium electrical conductivity of 10mol%Gd-doped and 5mol%Mn plus Gd co-doped ceria was examined in association with X-ray diffraction and electron probe micro-analysis. It was found that Mn additions neither enhanced the ionic conductivity nor extended the electrolytic domain; indicating that Mn was not as effective an acceptor as expected. In spite of the fact that the specimens appeared to be phase-pure, via X-ray diffraction, the electron probe micro-analysis results revealed precipitates - enriched with Mn - at grain boundaries. It was suggested that the solubility limit of Mn in Gd-doped ceria was very likely to be less than 1mol%.

Defect-Chemical Role of Mn in Gd-Doped CeO2. S.H.Park, H.I.Yoo: Solid State Ionics, 2005, 176[15-16], 1485-90