Interstitial Tungsten Atoms as a Primary Defect in Nonstoichiometric WO 3‐x

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.