Imaging, counting, and positioning single interstitial atoms in solids

Interstitial atoms are ubiquitous in solids and they are widely incorporated into materials to tune their lattice structure, electronic transportation, and mechanical properties. Because the distribution of interstitial atoms in matrix materials is usually disordered and most of them are light atoms with weak scattering ability, it remains a challenge to directly image single interstitial atoms and measure their geometrical positions. In this work, direct imaging and measuring of single interstitial atoms have been realized with adaptive-propagator ptychography. The measurement of their three-dimensional coordinates enables quantitative analysis of the pair distribution function of the interstitial atoms and reveals the anisotropic occupation of oxygen in the interstitial sites in titanium. The current work paves the way for the determination of interstitial atoms in materials, and for the correlation between the atomic-scale behavior of interstitial atoms and the physical properties of materials.