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Abstract: La2−x Ba x Mo2O9−x/2 (x ≤0.18) have been prepared by solid state reaction method. The lattice parameter of La2−x Ba x Mo2O9−x/2 (x ≤0.18) determined by XRD data refinement shows a linear dependence on the dopant Ba content x. For the specimen with a La/Ba molar ratio of 0.18–0.2, additional reflection of secondary phase exists in the XRD pattern, so the value of solubility limit for Ba in La2Mo2O9 is defined in range of 0.18< x <0.2. As the replacement degree of La3+ by Ba2+ increases, the bulk conductivity of La2−x Ba x Mo2O9−x/2 (x ≤0.18) decreases initially and then increases, a minimum value at La1.9Ba0.1Mo2O8.95 exists. Hebb–Wagner studies in argon atmosphere, which use an oxide-ion blocking electrode, show that La2−x Ba x Mo2O9−x/2 (x ≤0.18) are predominantly oxide-ion conducting in the temperature ranging from 773 to 1173K. The average thermal expansion coefficient of La1.84Ba0.16Mo2O8.92 determined by high-temperature XRD was deduced as great as 17.5×10−6 K−1 between 298 and 1173K. [Copyright &y& Elsevier]