Unravelling Crystal Superstructures and Transformations in the La6–xMoO12−δ (0.6 ≤ x ≤ 3.0) Series: A System with Tailored Ionic/Electronic Conductivity

Abstract

Crystalline La6-xMoO12-δ materials with different lanthanum/molybdenum ratios (0.6 ≤ x ≤ 3.0) have been prepared via a freeze-drying precursor route. The influence of the lanthanum content, sintering temperature and cooling rate on the phase existence range and polymorphism was evaluated. Lanthanum rich compounds present three different polymorphs: a cubic (disordered) fluorite and two complex rhombohedral superstructures related to the fluorite. For the first time, the structural resolution of these rhombohedral superstructures, 7×7×1 and 5×5×1, has been successfully accomplished by neutron pow-der diffraction and transmission electron microscopy studies. As the La/Mo ratio decreases, the cubic symmetry is stabilized, although a phase transformation from cubic to monoclinic occurs at a low cooling rate. Impedance spectroscopy measure-ments under different atmospheres (dry and wet N2 and 5% H2-Ar) show that all materials exhibit mixed proton-electronic conductivity. The n-type electronic conductivity is attributed to Mo6+ reduction and increases for those phases with lower lanthanum content, i.e. for quenched samples, from 5 mS cm-1 for La5.4MoO11.1 to 9.5 mS cm-1 for La4MoO9 at 700 oC in very reducing and wet conditions, which are significantly better than the values published to date for mixed lanthanum tungstates/molybdates. This make these materials potential candidates for hydrogen separation membranes.