Effect of oxygen partial pressure on the electrical conductivity of La2Mo2−xWxO9−δ (x = 0.0, 0.5, 0.7) materials

Abstract

The effect of oxygen partial pressure (pO2) on the electrical conductivity of La2Mo2−xWxO9−δ (x = 0.0, 0.5, 0.7) was investigated from 873 to 1073 K using AC impedance spectroscopy in oxygen, air, argon, and CO–CO2 mixtures with CO volume percentages, 0.5%, 9%, 33% and 50% used for producing lower oxygen partial pressures. The results have shown that the electrical conductivity increases slowly with the decreasing pO2 when the CO content is not greater than 0.5 vol.%. pO2 has more significant influence on the electrical conductivity of W-doped La2Mo2O9 than on that of undoped La2Mo2O9. For La2Mo1.3W0.7O9, the logσ attained in all atmospheres except oxygen and CO–CO2 mixture with 50 vol.% CO, exhibits a good linear relationship with log pO2. The corresponding slopes attained are as large as 0.019–0.021, much lower than 1/4, suggesting that the enhancement of total electrical conductivity could be mainly due to the increase of ionic conductivity when pO2 decreases. XRD and XPS analyses for La2Mo2−xWxO9−δ have confirmed that a small amount of Mo6+ has been reduced to Mo4+ during the electrical conductivity measurements in CO–CO2 mixture with 9 vol.% CO, and further decreasing pO2 causes the formation of La7Mo7O30. This may lead to the remarkable changes in their electrical conductivities. A critical pO2 value for each compound at a specified temperature between 873 and 1073 K has been found, and below this pO2 value the compound would become unstable. The critical pO2 values attained at 973 K are as follows: 10−12, 10−15 and 10−17 Pa for La2Mo2O9, La2Mo1.5W0.5O9 and La2Mo1.3W0.7O9 respectively.