Structural studies on W ⁶⁺ and Nd ³⁺ substituted La₂Mo₂O₉ materials

The structure of a series of new ionic conductors based in lanthanum molybdate (La2Mo2O9) has been investigated using transmission electron microscopy (TEM), high-resolution X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The superstructure 2a(c) x 3a(c) x 4a(c) of the low temperature alpha-polymorph relative to the beta-polymorph was confirmed by HRTEM imaging and electron diffraction. Furthermore, the effects of partial cation substitution in the La2-xNdxMo2O9 and La(2)Mo(2-y)WyO(9) series have been also evaluated in the search of new clues to understand the structure and stabilisation of the high temperature and better conductor beta-polymorph. The thermal analysis studies show that Nd-substitution does not stabilise completely the beta-polymorph at room temperature, although no superstructure ordering was observed by both XRD and HRTEM. On the other hand, W-substitution stabilises the cubic flpolymorph for y > 0.25, although, electron diffraction indicates a slight distortion from the cubic symmetry for low W-content. This distortion disappears as the W content increases and the Rietveld refinements gradually render better results. (c) 2005 Elsevier Inc. All rights reserved.