Structural studies on W6+ and Nd3+ 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 2ac×3ac×4ac 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 La2−xNdxMo2O9 and La2Mo2−yWyO9 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>0.25y>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.

Graphical abstractNd and W substituted La2Mo2O9 series were studied by XRD, TEM and thermal analysis to study the effects of substitution on the structure. The low temperature polymorph is a 2×3×4 superstructure of the high temperature polymorph. HRTEM images in the [0 0 1] and [0 0 1] zone axes for La2Mo2O9, showing very clearly the 2ac×3ac and 2ac×4ac superstructure, respectively.Figure optionsDownload full-size imageDownload as PowerPoint slide