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

Powder-solid state reaction route using La2(SO4)3 as sulfur source was used to prepare compositions of the solid solution La2Mo2−ySyO9. Single phases were only obtained in the substitution range extending up to y=0.8 (40 mol% S) at the annealing temperature of 850 °C with regard to the limit of stability of the lanthanum sulphate reactant. Within the synthesis conditions, a stabilization of the high temperature β-form is observed from and above y=0.1 (5 mol% S). Temperature-controlled X-ray diffraction and thermogravimetric analyses have shown that La2Mo2−ySyO9 raw powders undergo thermal decompositions in two steps. Heating above 900 °C, a sulfur depletion to the benefit of molybdenum in La2Mo2−ySyO9 raw powders leads to the formation of La2SO6. At higher temperature, the exsolved La2SO6 phase then decomposes into La2O3, which in turn reacts with the sulfur-depleted La2Mo2O9 phase to form La2MoO6. The present study also reveals that depending on the substitution rate y, the sulfur depletion can be induced by ball-milling of raw powders. Along the La2Mo2−ySyO9 series, the isovalent substitution of molybdenum by sulfur tends to restrict in magnitude, or even to suppress above 400 °C, the distortive thermal expansion of the cubic β-type structure, thus strongly decreasing the conductance at high temperature.