an interstitial
Between the neatly stacked cannonballs of a close-packed crystal there are always small gaps — the interstices. An interstitial defect is an atom or ion that has crammed itself into one of those gaps, a site that in the perfect crystal is empty. It is the mirror image of a vacancy: a vacancy is a missing atom, an interstitial is an extra one wedged in where it does not belong.
Squeezing an ion into a gap that is smaller than itself strains the surrounding cage, so interstitials usually cost MORE energy to form than vacancies — which is why in many close-packed oxides vacancies dominate. But when the interstitial site is roomy (as in the open fluorite structure of ZrO2 or UO2, with its large empty cube-centre holes) interstitials become cheap and common. In Kroger-Vink notation an interstitial sits on the site labelled i: an interstitial oxygen carrying its two negative charges is O_i'', an interstitial silver ion is Ag_i•. The two interstice geometries that matter are the small tetrahedral hole (surrounded by 4 atoms) and the larger octahedral hole (surrounded by 6).
Interstitials matter because they open a second, sometimes faster, route for atoms to move: the interstitial and interstitialcy diffusion mechanisms, in which an ion hops from gap to gap rather than from empty seat to empty seat. Small, highly mobile species — hydrogen, lithium, and in fluorite oxides even oxygen — often travel this way. And an interstitial is one half of a Frenkel defect: an ion that has jumped off its own site into a nearby gap, leaving a vacancy behind.
In fluorite-structured ZrO2 the anion sublattice has large empty holes, so an oxide ion can slip off its site into one, becoming an interstitial O_i'' and leaving an oxygen vacancy V_O•• behind — an anti-Frenkel pair. That easy anion Frenkel disorder is one reason zirconia and ceria are such good oxygen-ion conductors.
Interstitials are cheap where the structure is open and expensive where it is close-packed, so which defect dominates is set by the crystal geometry.
Interstitial does not mean impurity. A self-interstitial is a host atom of the crystal itself sitting in a gap; a foreign atom in a gap is specifically an interstitial impurity.