  We study experimentally and numerically the motion of a self-phoretic active
particle in two-dimensional (2D) loosely-packed colloidal crystals at fluid
interfaces. Two scenarios emerge depending on the interaction between the
active particle and the lattice: the active particle either navigates
throughout the crystal as an interstitial or is part of the lattice and behaves
as an active atom. Active interstitials undergo a run-and-tumble motion, with
the passive colloids of the crystal acting as tumbling sites. Instead, active
atoms exhibit an intermittent motion, which stems from the interplay between
the periodic potential landscape of the passive crystal and the particle's
self-propulsion. Our results shed new light on the behaviour of dense active
phases and constitute the first step towards the realization of
non-close-packed crystalline phases with internal activity.
