Oxide Ion Conductivity in the Solid Solutions: Ca1 − x Na x Bi2Nb2 O 9 − x / 2 and CaBi2Nb2 − y Ti y O 9 − y / 2 : The Relative Importance of A Versus B Site Substitution

Oxide ion conductivity has been measured in the complete solid solution series , , and for comparison, in , . At , the conductivities of and are similar. In contrast, substitution of titanium for niobium to give the same oxygen vacancy concentration lowers the activation energy to 1.0 eV from the value of 1.7 eV measured for the parent compound. For compositions with , all samples display a discontinuity in the Arrhenius plots of the ionic conductivity at 850°C. Differential thermal analyses confirm the presence of a phase transition, attributed to an order‐disorder transition of the oxygen 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 suggests that this transition is due to a change from an ionic extrinsic regime to an ionic intrinsic regime.