Proton and oxide ion conductivity in grain boundaries and grain interior of Ca-doped Er2Ti2O7 with Si-impurities

Dense samples of nominal composition Er1.96Ca0.04Ti2O6.98 were prepared by conventional ceramic synthesis to evaluate the ionic conductivity. The electrical properties were studied by impedance spectroscopy under oxidizing conditions as a function of pO2, pH2O and pD2O in the temperature range 360–600°C. The microstructure was investigated by analytical transmission electron microscopy before and after the impedance study. In both cases the grain boundaries contained an amorphous phase, enriched in Si and Ca. Three different contributions to the sample impedance, with different time constants, were identified, one assigned to the grain interior and two to grain boundaries. For the grain interior, oxygen vacancies were found to be the major charge carrier. In contrast, protons were found to play an important role as charge carriers in the grain boundary related processes. The ionic transport over grain boundaries in Er1.96Ca0.04Ti2O6.98 is discussed in terms of effects of additional phases in the grain boundaries and space charge layers.