Interstitial Tungsten Atoms as a Primary Defect in Nonstoichiometric WO _3‐x

ABSTRACT Nonstoichiometric tungsten oxides (WO _3‐x ) are foundational to technologies from electrochromic devices to photocatalysis. Conventionally, their functional properties are ascribed almost exclusively to oxygen vacancies. Here, using high‐quality single‐crystalline WO _2.72 and by combining atomic‐resolution time‐resolved STEM with meV‐resolution electron energy‐loss spectroscopy and density functional theory, we directly reveal interstitial tungsten atoms (W _i ) as a previously overlooked but ubiquitous class of point defects, with a quantified concentration of ∼0.11 W _i per unit cell. These W _i atoms reside within the structural tunnels and exhibit dynamic hopping among three metastable sites. In contrast to the shallow states induced by oxygen vacancies, W _i defects create deep, localized electronic states that pin small polarons and induce a pronounced, quantifiable blue‐shift of ∼0.3 eV in the absorption edge, as directly measured by atomically resolved EELS. This discovery establishes interstitial transition‐metal atoms as a new paradigm of intrinsic defects in nonstoichiometric oxides, offering powerful levers to engineer their electronic and optical functionalities.