High Pressure Study Of Metallofullerene Co-crystals

Plenty of solvent molecules can be confined in the large octahedral voids between spherical fullerenes,allowing the formation of various solvated fullerene co-crystals.Under high pressure,solvated fullerene co-crystals exhibit unique structural and property transformations due to various interactions between fullerenes and different solvent molecules.For example,high-pressure studies on C₆₀*m-xylene showed that the fullerene cages undergo a pressure-induced amorphization transition while the m-xylene molecules keep intact to maintain the long-range periodicity.This novel ordered amorphous carbon cluster（OACC）structure is such a superhard phase to create ring crack indentations on the diamonds.Then in the high-pressure studies of C₆₀-cubane co-crystal,a similar OACC structure has also been observed.Particularly,as holding time increases under high pressure,further energy release of the high-energy cubane leads to a transition towards a new long-range disorder phase.All these results indicate that the high-pressure transformations of solvent molecules are essential to obtain new materials.However,it remains a huge challenge to regulate the high-pressure behavior of solvent molecules.Therefore,we introduced charge transfer interaction in solvated fullerene co-crystals and investigated the effect of metallofullerenes on conformational transformations of solvent molecules.Metallofullerene is a special kind of fullerene,in which the encapsulated metals or metal clusters transfer electrons to the carbon cage,such as La³⁺@C₈₂^3-,(Sc³⁺)₂C₂^2-@C₈₂^4-,and(Sc³⁺)₃N^3-@C₈₀^6-.The real question is interactions between metallofullerenes and solvent molecules under high pressure are hard to find out,meanwhile,the strategies to select solvent molecules and embedded metals are quite unclear.For all these reasons,synthesis of solvated metallofullerenes and design of high-pressure experiments are impeded.High-pressure studies on Lu₃N@C₈₀*m-xylene co-crystal,Lu₂@C₈₂*m-xylene co-crystal,and Sc₃N@C₈₀*cubane co-crystal with different embedded metals and solvent molecules were carried out.Here are the results:1.High-pressure behaviors of pure m-xylene and Lu₃N@C₈₀*m-xylene co-crystal were studied by infrared spectroscopy to discuss the effect of metallofullerene on the high-pressure transformation of m-xylene.Unlike in pure m-xylene and C₆₀/C₇₀*m-xylene co-crystals,the m-xylene molecules in Lu₃N@C₈₀*m-xylene co-crystal began to deform at 5.0 GPa and irreversibly reacted between 15.1 GPa-20 GPa.This is owing to that the Lu₃N@C₈₀s act as"electron donors",transferring electrons to m-xylene and these electrons will automatically fill into the antibonding orbital of benzene rings,at this time the benzene rings will lose stability.Accompanied by the collapse and amorphous of carbon cages,the unsaturated hydrocarbons from reacted m-xylene further polymerize with the collapsed carbon cages and form new saturated hydrocarbons.These results reveal the interactions between metallofullerenes and solvent molecules under high pressure.Meanwhile,it is found that introducing charge transfer to solvated fullerene co-crystal promotes the high-pressure ring-open reaction of m-xylene.This is quite different from the high stability of m-xylene in bulk m-xylene and ordinary solvated fullerene co-crystals.2.The structural transformations of C₆₀*m-xylene co-crystal,C₇₀*m-xylene co-crystal,Sm@C₉₀*m-xylene co-crystal,Lu₃N@C₈₀*m-xylene co-crystal,and Lu₂@C₈₂*m-xylene co-crystal were studied under high pressure.By embedding different metal atoms or clusters,the carbon cages of metallofullerene are negatively charged while cages of C₆₀and C₇₀ remain uncharged.In Sm@C₉₀,Sm atom transfers two electrons to the carbon cage,so the carbon cage is[C₉₀]^2-;In Lu₂@C₈₂,two Lu atoms contribute four electrons totally,so the carbon cage is[C₈₂]^4-;In Lu₃N@C₈₀,three Lu atoms transfer nine electrons totally,three of which are transferred to the N atom,and the other six electrons are transferred to the carbon cage,so the carbon cage is[C₈₀]^6-.By comparing infrared spectra,HRTEM images and X-ray diffractions,we found“the more electrons on the carbon cage,the lower pressure for m-xylene transformation and the higher degree of disorder in the released sample”.This indicates that the high-pressure behaviors of solvent molecules can be well regulated by changing embedded metals,through which fullerene-based materials with new structures and properties can be obtained.3.The ability of charge transfer to regulate the high-pressure reaction of solvent molecules was applied to Sc₃N@C₈₀-cubane co-crystal to achieve pressure-driven,abnormal lattice volume expansion.Cubane is a high-energy-density molecule with unique dice-shaped cubic structure,which is very sensitive to charge transfer.Cubane is reported to decompose and expand under charge transfer.However,the laws of thermodynamics rule that materials should shrink along the directions of compression.Crystalline materials that expand in volume under pressure have never been achieved.Cubane showed surprising stability under high pressure,and the molecular characteristics and crystal structure are still intact up to 60 GPa.Therefore,a co-crystal built from[Sc₃N]⁶⁺@[C₈₀]^6-and high-energy-density cubane has been studied,hoping to promote the ring-expansion reaction of cubane under high pressure to achieve negative volume compressibility.High-pressure XRD of the sample and corresponding pressure dependence of the relative unit cell volumes suggest that Sc₃N@C₈₀s remain stable and thus maintain the crystal structure,while the cubane molecules release energy and undergo a progressive configurational transformation into lower-density configurations,resulting in a lattice expansion in volume（about 1.8%）.A crystalline material with negative volume compressibility under high pressure was designed based on the high-energy-density cubane and the charged metallofullerene.