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Abstract: Oxide ion conduction and phase transition phenomena have been investigated on La2−xPbxMo2O9−x/2 solid solutions (x≤0.6). X-ray and neutron diffraction experiments as well as thermal analyses indicate that a single phase of high-temperature polymorph (β-phase) is preserved down to room temperature for the composition of x≥0.06. In the solid solution range, ionic conductivity gradually decreases with Pb-substitution. Such phase transition and ionic conduction properties of La2−xPbxMo2O9−x/2 are quite similar to the Bi-substituted system, La2−xBixMo2O9, although ionic conductivity at lower temperatures is slightly higher in the former system than the latter with the same-amount of substitution. Room temperature neutron diffraction experiment suggests that oxide-ion deficiency is introduced mainly into O2 site by the charge compensation with Pb-substitution, which would contribute to a little enhancement in conductivity compared with Bi-substituted samples along with the variation of nearest O2 O3 bond length. [Copyright &y& Elsevier]