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