  The fcc-based structure of Yb2.75C60 is unique among metal-doped fullerene
compounds, exhibiting long-range-ordered vacancies, significantly off-centered
divalent Yb cations, and distorted, crystallographically inequivalent,
orientationally ordered C60 anions. A simple electrostatic-energy analysis,
which models the constituents using point charges, is shown to provide insight
into how each of these features stabilizes this unusual crystal structure. The
results have general implications for a variety of other intercalated metal
fullerides.
