On the local order of amorphous La2Mo2O6.7

Abstract

The amorphous reduction product of the oxide ion conductor La2Mo2O9 was previously shown to be a good, sulphur-tolerant, anode material for solid oxide fuel cell devices (X. C. Lu, J. H. Zhu, J. Electrochem. Soc., 2008, 155(10), B1053). In this paper, we study the local order of amorphous La2Mo2O6.7 using X-ray absorption spectroscopy analyses and electronic paramagnetic resonance. The extended X-ray absorption fine structure analysis of local arrangements around Mo in La2Mo2O6.7 is first carried out on the basis of strong distortions from three crystalline models of La2Mo2O9, La7Mo7O30 and La2Mo2O7. The extended X-ray absorption fits obtained from both La2Mo2O9 and La7Mo7O30 yield similar atomic arrangements in the amorphous phase, upon large atomic displacements. However it is also possible to fit the spectrum using the paths of La2Mo2O7, in better agreement with EPR results suggesting the presence of Mo-Mo pairs. Simpler arrangements, built from theoretical single scattering paths, are considered. All models are discussed and compared. A most probable short range structure around Mo in this amorphous phase is proposed. It both fulfills EPR results and appears coherent with the presence of ionic conductivity.