In this work, La6–x MoO12−δ (0 ≤ x ≤ 0.8) materials are investigated as a new family of ceramic proton conductors. Different polymorphic phases with rhombohedral and cubic structure are obtained, depending on the cooling rate applied during the synthesis process. The materials have been thoroughly characterized by different techniques, including X-ray powder diffraction, transmission electron microscopy, and X-ray photospectroscopy in order to fully understand the structural features of the samples. Thermogravimetric analysis and impedance spectroscopy in dry/wet gases (N2, O2, and 5% H2–Ar) confirmed the existence of proton conductivity at low temperature. Under a reducing atmosphere, the materials are mixed ionic-electronic conductors. The sample prepared at the fastest cooling rate exhibits cubic structure and higher n-type electronic conductivity compared to those prepared at a slower cooling rate with rhombohedral structure.

La5.4MoO11.1 proton conductors with different metal doping (Ca2+, Sr2+, Ba2+, Ti4+, Zr4+, and Nb5+ ) have been prepared and structurally and electrically characterized. Different polymorphs are stabilized depending on the doping and cooling rate used during the synthesis process. The most interesting results are obtained for Nb-doping, La5.4Mo1–x Nb x O11.1–x/2, where single compounds are obtained in the compositional range 0 ≤ x ≤ 0.2. These materials are fully characterized by structural techniques such as X-ray and neutron powder diffraction and transmission electron microscopy, which independently confirm the changes of polymorphism. Scanning electron microscopy and impedance spectroscopy measurements in dry/wet gases (N2, O2, and 5% H2–Ar) showed an enhancement of the sinterability and electrical properties of the materials after Nb-doping. Conductivity measurements under very reducing conditions revealed that these materials are mixed ionic-electronic conductors, making them potential candidates for hydrogen separation membranes.

Crystalline La 6−x MoO 12−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 N 2 and 5% H 2 −Ar) show that all materials exhibit mixed proton−electronic conductivity. The n-type electronic conductivity is attributed to Mo 6+ reduction and increases for those phases with lower lanthanum content, i.e., for quenched samples, from 5 mS cm −1 for La 5.4 MoO 11.1 to 9.5 mS cm −1 for La 4 MoO 9 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.

Cationic and anionic frameworks of La 5.4 MoO 11.1 proton conductors have been modified by means of metal (Ti 4+ , Zr 4+ , and Nb 5+ ) and fluorine (F − ) doping. This synergic effect leads to the stabilization of highsymmetry and single-phase polymorphs. The materials have been fully characterized by structural techniques, such as X-ray and neutron powder diffraction and transmission electron microscopy. The fluorine content was determined by ion chromatography. Impedance spectroscopy analysis under different atmospheres (dry and wet N 2 and O 2 and wet 5% H 2 −Ar) showed an improvement in the electronic conductivity under reducing conditions, making these materials potential candidates for hydrogen separation membranes.