RESUMO
The role of W content on the limit oxygen partial pressure (pO2) for stability of fast oxygen-ion conductors La2Mo2-yWyO9 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 La2Mo2-yWyO9, the perovskite related compounds La7Mo7(2-y)/2W7y/2O30 were stabilized even for y = 1.0. At 608 °C, the first stage of reduction of ß-La2Mo2-yWyO9 leads to the formation of the crystallized oxygen deficient La2Mo2-yWyO8.6+0.02y phase. X-ray powder diffraction shows that the stabilization of the high temperature ß-form through tungsten substitution observed in fully oxidized La2Mo2-yWyO9 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 La2Mo2-yWyO9 ion conductor under a reducing atmosphere but only slows down the kinetics of reduction.
RESUMO
Thermogravimetric analysis (TGA) technique in controlled oxygen partial pressure (pO(2)) atmospheres has been used to obtain equilibrium oxygen content data as a function of pO(2) on the La(2)Mo(2)O(9-δ) system resulting from the partial reduction of fast oxide-ion conductor La(2)Mo(2)O(9) (LM). Thermodynamic conditions for stabilization of crystalline La(7)Mo(7)O(30) and amorphous La(2)Mo(2)O(7-y) at 718 °C have been determined and discussed. At 608 °C, the compound reported for the first time La(2)Mo(2)O(8.96) (LM896) has been found. The crystalline form and transition temperature in LM896 have been identified by X-ray diffraction at room temperature (XRD) and at controlled temperature. Conductivity curves obtained by electrochemical impedance spectroscopy (EIS) as a function of temperature for both LM and LM896 have been compared. The results indicate that LM896 is a mixed ionic and electronic conductor (MIEC).