Thermodynamic stability of La2Mo2−yWyO9, La2Mo2−yWyO8.96+0.02y and La7Mo7(2−y)/2W7y/2O30 (y = 0, 0.5 and 1.0)

Abstract

The role of W content on the limit oxygen partial pressure (pO₂) for stability of fast oxygen-ion conductors La₂Mo_2−yW_yO₉ with y = 0, 0.5 and 1.0 has been studied by means of thermogravimetric analysis (TGA) under controlled atmospheres. At 718 °C, below the pO₂ stability limit of La₂Mo_2−yW_yO₉, the perovskite related compounds La₇Mo_7(2−y)/2W_7y/2O₃₀ were stabilized even for y = 1.0. At 608 °C, the first stage of reduction of β-La₂Mo_2−yW_yO₉ leads to the formation of the crystallized oxygen deficient La₂Mo_2−yW_yO_8.6+0.02y phase. X-ray powder diffraction shows that the stabilization of the high temperature β-form through tungsten substitution observed in fully oxidized La₂Mo_2−yW_yO₉ samples is preserved upon slight reduction. The n-type conductivity arising from the mixed valence state of molybdenum becomes less and less predominant as the W content increases. Further reduction causes amorphization. At both temperatures, W substitution does not enhance the thermodynamic stability of the La₂Mo_2−yW_yO₉ ion conductor under a reducing atmosphere but only slows down the kinetics of reduction.