Interstitial Tungsten Atoms as a Primary Defect in Nonstoichiometric WO_3-x

Area of Science:

• Materials Science
• Solid-State Physics
• Nanotechnology

Background:

• Nonstoichiometric tungsten oxides (WO_3-x) are crucial for electrochromic devices and photocatalysis.
• Their properties are traditionally attributed solely to oxygen vacancies.

Purpose of the Study:

• To identify and characterize previously overlooked point defects in WO_2.72.
• To investigate the role of these defects in the electronic and optical properties of tungsten oxides.

Main Methods:

• Atomic-resolution, time-resolved scanning transmission electron microscopy (STEM).
• Millielectronvolt-resolution electron energy-loss spectroscopy (EELS).
• Density functional theory (DFT) calculations.

Main Results:

• Directly observed interstitial tungsten atoms (W_i) as ubiquitous defects in WO_2.72.
• Quantified W_i concentration at ~0.11 per unit cell, with dynamic hopping observed.
• Identified W_i defects creating deep electronic states, causing a ~0.3 eV blue-shift in the absorption edge.

Conclusions:

• Interstitial transition-metal atoms represent a new paradigm of intrinsic defects in nonstoichiometric oxides.
• W_i defects significantly influence the electronic and optical properties, offering new avenues for material engineering.
• This finding challenges the conventional understanding of defect physics in oxides.