Partial substitution of Mo6+ by S6+ in the fast oxide ion conductor La2Mo2O9: Synthesis, structure and sulfur depletion

► Isovalent substitution of molybdenum by sulfur in La2Mo2O9 up to 40 mol%. ► Stabilization of the β-form for a sulfur content greater than or equal to 5 mol%. ► Decomposition by sulfur exsolution induced by thermal treatment or ball-milling. ► Reduction or even cancellation of the distortive thermal expansion above 400 °C. ► Decrease of conductance at high T involved by the low thermal expansion above 400 °C.