  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.
