Oxygen and proton conduction in the yttrium tantalate pyrochlore phase

The pyrochlore solid solution Y3+xTa1–xO7–x in the Y2O3-Ta2O5 system exists from 17.5 to 25 mol% Ta2O5. The conductivity of Y3+xTa1–xO7–x increases with x to a maximum near x=0.2, as a result of the increased content of disordered oxygen vacancies. The activation energy and pre-exponential factor vary in a similar manner. The transference numbers measured in air indicate that these materials are predominantly proton conductors at temperatures below 400 °C, oxygen electrolytes at temperatures between 400 and 750 °C and predominantly hole conductors at higher temperatures.