By using various types of trivalent dopant, perovskite-type oxide solid solutions of the form, BaCe1-xMxO3, were synthesized and their conduction properties at high temperatures were studied electrochemically. It was found that the oxides exhibited mixed conduction, via protons and oxide ions, under fuel-cell conditions. The transference number of protons decreased, and that of oxide ions increased, as the radius of the dopant cation, M, became large. The reason for this was explained in terms of the lattice parameters of the orthorhombic crystals and the ease of migration of oxide ions through it.

H.Iwahara, T.Yajima, H.Ushida: Solid State Ionics, 1994, 70-71, 267-71