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 alpha-MoO3 is critical for optimizing its physical and chemical properties. Herein, we present experimental evidence regarding the phase stability of alpha-MoO3 with similar to 2% oxygen vacancy concentrations. Interestingly, we report a previously ignored oxygen-deficient orthorhombic MoO3-x phase in space group Cmcm. Further density functional theory calculations reveal a detailed phase transition mechanism from alpha-MoO3 to MoO3-x. More importantly, we demonstrate that two-dimensional (2D) large polarons must exist to stabilize the MoO3-x crystal structure. 2D large polarons are suspected to exist in numerous quasi-2D systems but have never been found in layered alpha-MoO3 or other molybdenum oxides. Our work contributes to a basic understanding of the polaronic behavior in MoO3-x and may broaden the application realm of molybdenum oxides.