Topotactic\nReduction-Induced Stabilization of β‑La<sub>2</sub>Mo<sub>2</sub>O<sub>8.68</sub> Phase: Structure, Static Oxygen\nDisorder, and Electrical Properties

La<sub>2</sub>Mo<sub>2</sub>O<sub>9</sub> is acknowledged\nas an\nexceptional oxide ion conductor. It undergoes a reversible phase transition\naround 580 °C from the nonconductive low-temperature monoclinic\nα-La<sub>2</sub>Mo<sub>2</sub>O<sub>9</sub> phase to the highly\nconductive high-temperature cubic β-La<sub>2</sub>Mo<sub>2</sub>O<sub>9</sub> phase. In addition, La<sub>2</sub>Mo<sub>2</sub>O<sub>9</sub> demonstrates complex chemistry under reducing conditions.\nThis study reports, for the first time, the stabilization at ambient\ntemperature of a novel cubic phase through a topotactic reduction\nof α-La<sub>2</sub>Mo<sub>2</sub>O<sub>9</sub> employing CaH<sub>2</sub>. This phase contains approximately ∼3 atom % oxygen\nvacancies relative to the nominal composition (La<sub>2</sub>Mo<sub>2</sub>O<sub>8.68(1)</sub>). The cubic symmetry is associated with\na static distribution of these vacancies, in contrast to the dynamic\ndistribution observed in the high-temperature cubic β-La<sub>2</sub>Mo<sub>2</sub>O<sub>9</sub> phase reported previously. Additionally,\nthe material exhibits mixed-ion-electronic conduction, which expands\nits potential use in applications requiring both ionic and electronic\ntransport.