Unravelling crystal superstructures and transformations in the La 6-x MoO 12-δ (0.6≤ x ≤ 3.0) series. A system with tailored ionic/electronic conductivity. - Archive ouverte HAL Accéder directement au contenu
Article Dans Une Revue Chemistry of Materials Année : 2020

Unravelling crystal superstructures and transformations in the La 6-x MoO 12-δ (0.6≤ x ≤ 3.0) series. A system with tailored ionic/electronic conductivity.

Résumé

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.
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Dates et versions

hal-02912024 , version 1 (05-08-2020)

Identifiants

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Adrián López-Vergara, Lucía Vizcaíno-Anaya, Jose Porras-Vázquez, Gianguido Baldinozzi, Lucia dos Santos-Gómez, et al.. Unravelling crystal superstructures and transformations in the La 6-x MoO 12-δ (0.6≤ x ≤ 3.0) series. A system with tailored ionic/electronic conductivity.. Chemistry of Materials, In press, ⟨10.1021/acs.chemmater.0c02673⟩. ⟨hal-02912024⟩
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