The melting of fullerites from small or large fullerenes

The physical limits are studied theoretically for a mass of fullerenes which may be used to produce a stable crystal, i.e., fullerite. The dependence of the parameters of interfullerene interaction on the mass of fullerene $C_{nc}$ is used to study the evolution of the properties of fullerite as a result of variation of the number of carbon atoms $nc$ in fullerene $C_{nc}$. The dependences of the energy of activation process and surface energy are calculated for different values of temperature and pressure in the $15 \le nc \le 147$ range. It is demonstrated that fullerite becomes unstable at $nc < 20$, because small-sized light fullerenes cannot be localized due to weak Van der Waals forces. At the same time, fullerites with $nc \ge 110$ exhibit abnormally low values of surface energy; this must bring about the fragmentation of nanoclusters of hollow spherical $C_{nc}$ molecules of such a large size. Four empirical equations are used to estimate the dependence of the melting temperature of fullerite on $nc$. It is demonstrated that the melting temperature in the case of fullerites from small or large fullerenes is lower than that in the case of fullerites at $50 < nc < 90$. It is inferred that the $30 < nc < 100$ range is optimal for the formation of stable fullerite.