New model for icosahedral carbon clusters and the structure of collapsed fullerite

We propose a new class of structures consisting of compact carbon clusters with surface threefold coordination and bulk fourfold coordination. These clusters are the diamond analogs of fullerenes, maintaining icosahedral symmetry and almost perfectly tetrahedral bonding. Two potentially stable forms are investigated: molecules with hydrogenated surfaces and solid close-packed arrangements of the carbon clusters. The latter form may account for several recent experimental observations of collapsed fullerite, a new form of carbon produced by compression of ${\mathrm{C}}_{60}$ crystals.