The paper was published in Carbon 2022, 189, 37-45
Even though fullerenes have been discovered more than 30 years ago studies of physicochemical, electrophysical and optical properties of fullerites under the high compressive pressures are still of significant interest for both experiment and theory. It is known that under the high pressure and high-temperature conditions, buckminsterfullerene C60 undergoes a series of phase transitions and transform into the polymer]. Such polymers have wide range of applications (from solar cells to ultra-hard materials). At the same time, polymerization itself is a complex process whose description at the atomic level is a challenging task. A large number of fullerene isomers, the possibility of variable surface functionalization as well as the implantation of different atoms inside fullerenes, lead to a great variety of possible structures
Indeed, shortly after the discovery of the buckminsterfullerene, it has been shown that a cavity within the carbon frame of fullerene molecules can accommodate a variety of atoms, both non-metals (He, N, P) and metals, especially from a series of rare earth, with the formation of endohedral metallofullerene (EMF) molecules. In most endohedral metallofullerenes, the introduction of metal atoms into carbon cages leads to a change of the electron affinity relative to the corresponding empty cages
The transfer of electrons leads to an increase in fullerenes chemical activity. This activity should potentially lead to easier polymerization of EMF-based fullerites at high temperatures and pressures compared to the case of C60 or other fullerites. Polymerized EMF can be a new material with promising applications in a wide range of scientific and technological fields. However, despite the promise, the extreme rarity of EMF makes it impossible to carry out a systematic large-scale study
Our colleagues from Kirensky Institute of Physics (Krasnoyarsk, Russia) developed a unique technology for producing EMFs in macroscopic quantities, which allowed to study the structural behavior of fullerenes containing Sc under extreme conditions. Our colleagues from TISNCM (Moscow, Russia) used diamond anvils and studied the polymerization process itself. It was obtained that the introduction of Sc2C2 completely changes the whole picture of the phase transformation of the material.
Difference between enthalpies of molecular H and fully polymerized fullerene H0 for C82 (top) and Sc2C2@C82 (bottom) vs. pressure. In the insets, the dependence of relation between the amount of sp3- and sp2-hybridized bonds of both initially molecular (colored line) and fully polymerized fullerene (black line) on the pressure are presented. The atomic structure of polymerized C82 and Sc2C2@C82 are presented on the right
The polymerization of fullerenes can be represented as a phase transformation from an unbound molecular form to a chemically bonded polymerized state. It is convenient to consider this process through the enthalpy H dependences of the corresponding phases on the pressure. They were obtained by structural optimization of molecular and polymerized fullerites with a gradual increase of the isotropically applied external pressure. It is obtained that both C82 and Sc2C2@C82 are not able to reach maximal polymerization but their behavior under the pressure has a fundamentally different character.
Thus, in the case of pristine fullerenes, we found that the enthalpies of fully polymerized structure and molecular fullerite (red line in the top figure) intersect at 23 GPa which allows us to propose the stepwise increase in sp3-hybridized bonds at this pressure. Before that, at 15 GPa, only rare sp3 bonds begin to form in the molecular phase (inset in the top figure). With further increase in pressure the percentage of these bonds increases, however, it does not exceed 6% until 23 GPa. The minimization algorithm of DFT does allow spontaneous transformation from partial to full polymerization at 23 GPa and, thus, it is possible to study the intermolecular bonding in the partially polymerized state also at higher pressures.
In the case of the EMF, we got a completely different behavior of the structure under pressure. It is obtained that pressure application leads to a gradual increase of sp3 bonds in the structure without the appearance of a phase transition into a fully polymerized structure, which is clear from the asymptotic behavior of the enthalpy difference with pressure increasing (blue line in bottom figure). The increasing of pressure leads to a smooth incremental formation of the polymerized structure from 12.5 GPa with a sp3-hybridization degree increasing until 23 % at 27.5 GPa (inset in the bottom figure). This result allows us to conclude that Sc encapsulation smooth and relieve the polymerization process. The Bader charge analysis shows that in molecular Sc2C2@C82 each scandium gives 1.54e in total, 1.21e is taken by the inner C2 (0.48e and 0.73e), and the remaining 1.87 electrons transfer to fullerene framework, causing its slight polarization, the region near the scandium is more negatively charged compared to the remaining fullerene atoms. In the case of polymer formation, the polarization becomes slightly less, the charge on Sc2 decreases by 0.2e (1.45e per scandium) with transferring of 1.66e to fullerene framework. This allows us to conclude that the presence of scandium inside the fullerene polarizes the carbon bonds, which leads to an increase in their chemical activity and facilitates polymerization. The presence of scandium within the fullerene makes the (4+4) cycloaddition reaction more uniform. Almost all fullerenes bind through (4+4) bonds, although individual ones are also present. Unlike pristine fullerite, not all fullerenes bond with each other.
The predicted behavior was confirmed experimentally. In addition, it was found there that the bulk modulus of elasticity of the high-pressure phase of Sc2C2@C82 is 509 GPa, while in the case of C82 a value of 330 GPa was obtained.
