On the local order of amorphous La₂Mo₂O_6.7

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 La₂Mo₂O_6.7 using X-ray absorption spectroscopy analyses and electronic paramagnetic resonance. The extended X-ray absorption fine structure analysis of local arrangements around Mo in La₂Mo₂O_6.7 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.