The sample of La₂Mo_1.8Ga_0.2O₉ ceramics was prepared for the first time by high-temperature solid-state reaction. X-ray powder diffraction pattern shows that the sample is of a single cubic phase of La₂Mo₂O₉. Using the ceramics as solid electrolyte and porous platinum as electrodes, the ion conduction properties of La₂Mo_1.8Ga_0.2O₉ ceramics were investigated by means of ac impedance spectroscopy, oxygen concentration cell and oxygen pump (electrochemical oxygen permeation) at 600～1000 ℃ in various atmospheres. The observed values of oxygen concentration cell coincided with theoretical ones perfectly, indicating that the conduction in the ceramics was purely ox-ide-ionic with oxygen ion transference number of 1 under oxygen atmosphere over the whole temperature range of 600～1000 ℃. The electrochemical oxygen permeation rate determined was in accordance with the theoretical one calculated from Faraday’s law, verifying direct experimentally the oxide-ionic nature of the conduction under oxygen atmosphere. Under high oxygen partial pressure [p(O₂)＝10^－5～10⁵ Pa], La₂Mo_1.8Ga_0.2O₉ was almost a pure ionic conductor, which was consistent with the result by means of oxygen concentration cell. Whereas at low oxygen partial pressure [p(O₂)＝10^－5～10^－15 Pa], the sample exhibited a mixed ionic and electronic conduction. The ionic conductivities of this ceramics are higher than 10^－2 S•cm^－1, which is obviously superior to the matrix La₂Mo₂O₉ over the temperature range of 600～1000 ℃. The ce-ramics is shown to have the oxide ionic conductivity of 0.07 S•cm^－1 at 1000 ℃.

Key words: La₂Mo₂O₉, La₂Mo_1.8Ga_0.2O₉, oxide ionic conductor, oxygen concentration cell, oxygen pump