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The electrical conduction in the sintered oxides of the systems Bi2Os-M205 (M = V, Nb, or Ta) 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 M~O5 over a wide range of temperatures. The oxide ion conductivities of this phase decreased with increasing content of M205, 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. (Bi203)0.s5 (Nb205)0.15 was the best conductor, with conductivities of 1.9 • 10 -1 12 -1 cm -1 at 700~ and 1.1 • 10 -2 FL -1 cm -1 at 500~ 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 Bi20~-Nb205 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 Bi203-M2O3.The high temperature phase of bismuth sesquioxide (8-phase) is known to have a defect fluorite-type structure including a large amount of vacant sites in the oxide ion sublattice (1, 2). This material exhibits extremely high oxide ion conduction accompanied by little electronic conduction under ordinary atmosphere (3-6); the conduction mechanism is probably analogous to that of the stabilized zirconias. This phase is, however, stable only in the narrow temperature range from 730 ~ to 825~ (mp) (1), and the conductivity falls remarkably below 730~ where the monoclinic structure (s-phase) is a stable phase (5, 7) and the conduction is electronic (p-type). Furthermore, this material is apt to collapse because of the large volume change due to the phase transition (8), and, therefore, it is not suitable for practical use as a solid electrolyte.However, the present authors have found that the high oxide ion conductivity can be held far below 730~ by adding a certain amount of WO~ (6), Y203 (9), or Gd203 (10) to the bismuth sesquioxide. The highly conductive phases in these systems were facecentered cubic (fcc) phases which were closely related to that of the high temperature form of pure bismuth sesquioxide (5-Bi20~) in their crystal structures. The conductivities of these phases were much higher than those of the conventional stabilized zirconias at corresponding temperatures. Outwardly, this means that the a-form can be stabilized even at lower temperature than 730~ by suitable additives.According to Roth et al. (11), a stable fcc phase exists in the system Bi2O~-Nb205 at temperatures lower * Electrochemical Society Active Member. Key words: solid electrolyte, bismuth oxide, oxide ion conduction. ) unless C...

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