Structural studies on W 6+ and Nd 3+ substituted La 2Mo 2O 9 materials

Abstract

The structure of a series of new ionic conductors based in lanthanum molybdate (La 2Mo 2O 9) has been investigated using transmission electron microscopy (TEM), high-resolution X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The superstructure 2 a c×3 a c×4 a c of the low temperature α-polymorph relative to the β-polymorph was confirmed by HRTEM imaging and electron diffraction. Furthermore, the effects of partial cation substitution in the La 2− x Nd x Mo 2O 9 and La 2Mo 2− y W y O 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 β-polymorph. The thermal analysis studies show that Nd-substitution does not stabilise completely the β-polymorph at room temperature, although no superstructure ordering was observed by both XRD and HRTEM. On the other hand, W-substitution stabilises the cubic β-polymorph 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.