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
powder 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 measurements 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 °C in very
reducing and wet conditions, which are significantly better than the values published to date for mixed lanthanum tungstates/
molybdates. This makes these materials potential candidates for hydrogen separation membranes.
