Endohedral fullerene
Fullerene molecule with additional atoms, ions, or clusters enclosed within itself
Endohedral fullerenes, also called endofullerenes, are fullerenes that have additional atoms, ions, or clusters enclosed within their inner spheres. The first lanthanum C60 complex called La@C60 was synthesized in 1985. The @ (at sign) in the name reflects the notion of a small molecule trapped inside a shell.
From Wikipedia, under CC BY-SA. More on occurri.