The bulk modulus of C₆₀ molecules and crystals: A molecular mechanics approach

In this letter, the bulk modulus of an individual C60 molecule is calculated in terms of the C,C bond force constant. A range of values for the bulk modulus is obtained with literature values for the force constant. The values obtained all exceed the bulk modulus (441 GPa) of diamond. With a C,C bond force constant equal to that between adjacent carbon atoms in graphite, 7.08 mdyn/Å, a bulk modulus of 903 GPa is obtained. On the basis of a simple composite model it is calculated that single closest-packed C 60 crystals of C60 will have a bulk modulus of roughly 668 GPa under hydrostatic pressures. The calculated bulk modulus for a single C60 "buckyball" therefore suggests the possibility that a C60 crystal could be the most incompressible material known, at a pressure above about 50 GPa.