OXIDE-ION CONDUCTIVITY IN THE SOLID-SOLUTIONS - CA1-XNAXBI2NB2O9-X 2 AND CABI2NB2-YTIYO9-Y/2 - THE RELATIVE IMPORTANCE OF A-SITE VERSUS B-SITE SUBSTITUTION/

Oxide ion conductivity has been measured in the complete solid solutio n series Ca1-xNaxBi2Nb2O9-x/2, 0 less than or equal to x less than or equal to 1, and for comparison, in CaBi2Nb2-yTiyO9-y/2 0 less than or equal to y less than or equal to 0.15. At x = 0.1, the conductivities of Ca1-xNaxBi2Nb2O9-x/2 and CaBi2Nb2O9 are similar. In contrast, subst itution of titanium for niobium to give the same oxygen vacancy concen tration (y = 0.1) lowers the activation energy to 1.0 eV from the valu e of 1.7 eV measured for the parent compound. For compositions with 0. 2 less than or equal to x less than or equal to 1, all samples display a discontinuity in the Arrhenius plots of the ionic conductivity at 8 50 degrees C. Differential thermal analyses confirm the presence of a phase transition, attributed to an order-disorder transition of the ox ygen vacancies. A change in the activation energy also is observed at lower temperatures. Measurements of the electronic conductivity and of the influence of the oxygen partial pressure on the conductivity sugg ests that this transition is due to a change from an ionic extrinsic r egime to an ionic intrinsic regime.