Calcium intercalation into single-crystal ${\mathrm{C}}_{60}$ thin films has been studied with scanning tunneling microscopy. In low concentrations, Ca induces features that exhibit variations in apparent height that are not present for pristine ${\mathrm{C}}_{60}$, consistent with differences in local state densities in a solid solution. At higher Ca concentrations, structure is evident due to bonding with Ca in octahedral and tetrahedral sites. A model of the ${\mathrm{Ca}}_5$${\mathrm{C}}_{60}$ structure is deduced in which octahedral sites of the fcc lattice are occupied by four Ca ions while one of the tetrahedral sites is filled. The other tetrahedral site is filled at higher concentration, but these later-filled sites are arranged in rectangular or triangular lattices so that their distances are maximized.

• Received 8 September 1992

DOI:https://doi.org/10.1103/PhysRevB.46.12914