Topotactic Reduction-Induced Stabilization of β-La₂Mo₂O_8.68 Phase: Structure, Static Oxygen Disorder, and Electrical Properties

La₂Mo₂O₉ is acknowledged as an exceptional oxide ion conductor. It undergoes a reversible phase transition around 580 °C from the nonconductive low-temperature monoclinic α-La₂Mo₂O₉ phase to the highly conductive high-temperature cubic β-La₂Mo₂O₉ phase. In addition, La₂Mo₂O₉ demonstrates complex chemistry under reducing conditions. This study reports, for the first time, the stabilization at ambient temperature of a novel cubic phase through a topotactic reduction of α-La₂Mo₂O₉ employing CaH₂. This phase contains approximately ∼3 atom % oxygen vacancies relative to the nominal composition (La₂Mo₂O_8.68(1)). The cubic symmetry is associated with a static distribution of these vacancies, in contrast to the dynamic distribution observed in the high-temperature cubic β-La₂Mo₂O₉ phase reported previously. Additionally, the material exhibits mixed-ion-electronic conduction, which expands its potential use in applications requiring both ionic and electronic transport.