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.