Unexpected Two-Dimensional Polarons Induced by Oxygen Vacancies in Layered Structure MoO 3- x .

Unveiling the effects of oxygen vacancies on the structural stability of layered α-MoO 3 is critical for optimizing its physical and chemical properties. Herein, we present experimental evidence regarding the phase stability of α-MoO 3 with ∼2% oxygen vacancy concentrations. Interestingly, we report a previously ignored oxygen-deficient orthorhombic MoO 3- x phase in space group Cmcm . Further density functional theory calculations reveal a detailed phase transition mechanism from α-MoO 3 to MoO 3- x . More importantly, we demonstrate that two-dimensional (2D) large polarons must exist to stabilize the MoO 3- x crystal structure. 2D large polarons are suspected to exist in numerous quasi-2D systems but have never been found in layered α-MoO 3 or other molybdenum oxides. Our work contributes to a basic understanding of the polaronic behavior in MoO 3- x and may broaden the application realm of molybdenum oxides.