A Polar large polaron in WO3.

WO3 shows marked oxygen deficiencies that are responsible for its exceptional electrochromic properties. Polaron, therefore, represents a crucial approach to explore the intriguing effects of defects, but how it is formed and whether a strongly distorted perovskite structure is essential remain open questions. Here we provide a First-principles investigation of a large polaron formation in a supercell of WO3. We show how the W--O bond length variations lead to trigger a local phase transition and how the formed large polaron induces a collective polarization that makes it a polar.