Abstract
The electrical conduction in the sintered oxides of the systems 
has been investigated by measuring the conductivity and the ion transference number. The face‐centered cubic (fcc) phase in these systems was found to show a high oxide ion conduction, and this highly conductive phase could be stabilized by adding a certain amount of 
over a wide range of temperatures. The oxide ion conductivities of this phase decreased with increasing content of 
, and a maximum value was obtained at the lower limit of the fcc solid‐solution formation range. Of these systems, Nb‐containing specimens had the highest conductivity over a wide range of compositions and temperatures. 
was the best conductor, with conductivities of 
at 700°C and 
at 500°C; these values were more than ten times higher than the conventional oxide ion conductors such as the stabilized zirconias. X‐ray diffraction and density studies of fcc phase in the 
system have confirmed that there is an appreciable amount of oxide ion vacancies which is prescribed by the content of pentavalent oxide. The relation of activation energy for conduction to the composition in this phase is discussed by comparing that of the trivalent oxide‐doped system 
.