Effect of humidified atmosphere on the physicochemical properties of S-doped La2Mo2O9 oxide-ion conductors

LAMOX are new materials deriving from La2Mo2O9 extensively studied due to their recently discovered high ionic conductivity (better than best stabilized zirconia above 600 degrees C). So, applications as fuel cell electrolytes can be predicted. Other applications such as electrode materials and oxidation catalysts are also possible to the control of the molybdenum oxidation states. The lanthanum molybdate La2Mo2O9 is taken as a reference in this paper. The transition from a weakly conducting phase (alpha-La2Mo2O9, monoclinic, ordered) to the highly conducting phase (beta-La2Mo2O9, cubic, disordered) occurs at 580 degrees C. The aim of this work was to generate protonic and anionic conductors. To prepare the composition of the solid solution La2-2x(Mo, S)(2)O(9- 3)x center dot xH(2)O, a powder solid-state reaction pathway was used as a sulfur source. Pure cubic phase (b -form, the space group P2(1)3) was obtained when the S6+ contents had reached 50 mol%. The structure and lattice parameters of La2MoSO9 are obtained from Rietveld refinement. The simultaneous DTA and TGA measurements demonstrate thermal stability. The sintered specimens are annealed at 850 degrees C to test their electrical properties. The temperature dependence of electrical conductivities for La2Mo2O9 and La2MoSO9 obeyed the Arrhenius law below 580 degrees C, whereas the Vogel-Tammann-Fulcher (VTF) regime could satisfactorily represent the conduction behaviors beyond 580 degrees C. The infrared and Raman spectra were performed at ambient temperature. All theoretically predicted vibrations have been observed, sulfate ions' environment influences their internal vibration modes. Investigating the vibrational spectra of these two phases (alpha and beta) can help us to acquire more definitive information on the internal vibrations of XO4 (X=Mo, S).