Synthesis And High Pressure Phase Transitions Study Of New Intercalated Fulierenes

Due to the unique structures and novel properties, fullerenes have attracted muchattention recently. Fullerene has been regarded as good building block forconstructing various new function materials, for which the structures and propertiesare closely related to the interaction and bonding types of the constructed units.Intercalation is an effective may to synthesize new fullerene based materials. Theintroduction of dopants and the yielded space confinement by the dopants can changethe interaction between molecules and affect the structures and properties of fullerides.On the other hand, high pressure can change the distances between molecules oratoms, and then affect their interactions and bonding to obtain new high pressurephases. Carrying out high pressure experiments on intercalated fullerenes is a verypromising method to obtain many new materials with different structures andproperties. Nowadays, the studies on the effect of pressure-tuned interaction betweenmolecules and confinement on the structure and properties of intercalated fullerenesare very few and the mechanism of phase transition and property change underpressure are still unclear. Based on these questions, we carried out high pressurestudies on several typical fullerene intercalated materials with different types ofinteraction between the constructed units, i.e. C60(Fc)2, C70(Fc)2, solvated C60/C70andalkali metals doped C60nanotubes (AxC60), to research their structures and propertiesat ambient conditions and under high pressure.1. High pressure studies have been carried out on single crystal C60(Fc)2nanosheets up to25.4GPa. Our results show that above5GPa C60molecules start to gradually form polymer chains with increasing pressure, which is found to bereversible upon decompression.Spectroscopy studies and theoretical calculation are performed to investigate thephase transitions of single crystalline C70(Fc)2under high pressure up to35GPa. Adimer phase and1D zigzag chain-like polymer of C70molecules are found to beformed from about3and8GPa, respectively. The polymerization is reversible up to atleast20GPa while became partially reversible from above35GPa.The formation of reversible polymerization in C60(Fc)2and C70(Fc)2is induced bythe space confinement effect, increased charge transfer interaction and overriddensteric repulsion of counterions at high pressure. Meanwhile, only certain C60/C70molecules can take part in this reaction, which is related to the resulting spatialrestriction, the possible insufficient reducing of the volume, and competingorientational ordering of fullerenes. This study provides us an effective method toobtain fullerene polymers and help us to better understand the effect of charge transferand confinement effect on the polymerization. Such high-pressure studies onintercalated fullerene compounds may also be expected to produce completely newmaterials.2. An exceptional hard crystalline structure has been synthesized by compressingm-xylene intercalated C70crystals up to42GPa. This is another example of orderedamorphous carbon cluster structure (OACC) formed by compressing solvatedfullerenes with an exceptionally high hardness. Unlike C60*m-xylene, we find thatC70*m-xylene undergoes two phase transitions upon compression. The first transitionis from orthorhombic to rhombohedral phase, which is induced by the orientationalordering transition of C70molecules. Another striking phase transition fromrhombohedral to possible tetragonal phase is observed above30GPa, which is absentin compressing C60*m-xylene, indicating the formation of a new OACC structureunder pressure. The phase transition pathway, different from that in solvated C60should be related to the anisotropic deformation of C70molecules under pressure.In order to investigate the effect of solvent on the high pressure behavior offullerene, solvated C60and desolvated C60are studied for comparison. We find thatthe pressure-induced bonding change and structural transformation of C60s are similarin the two samples, both undergoing deformation and amorphization. Nevertheless,the high pressure phases of solvated C60can indent diamond anvils while that of desolvated C60s cannot. Meanwhile more ordered phase can be preserved in thereleased solvated sample.In both cases of C70*m-xylene and C60*m-xylene, solvent plays an important roleas a spacer and bridge to preserve the stability of deformed or amorphized C60molecules and allow bonding between neighboring molecules at pressure to form orsp3covalently bonded network. Such3D network thus exhibit excellent mechanicalproperty. This study first extends the OACC structure to the solvated fullerenescontaining larger cages, controls the formation of OACC structure by changing theinitial fullerene molecules, suggests a universal rule for the high pressure behaviors ofof solvated fullerenes with lower symmetric fullerene molecule and gives us a betterunderstanding on the formation mechanism of the superhard phases.3. Alkali metals (Li, Na and K) doped C60nanotube is synthesized by means ofvapor evaporation method at different evaporating temperatures. It is found that allthree alkali metals used can be efficiently doped into C60nanotubes, forming thedoped AxC60nanotubes. The melt points, ionic sizes and vapor pressures of Li, Na andK are thought to be strongly related to the doping concentration. Due to the highermelt points and vapor pressures of Li and Na, the efficient doping can be occurred athigher temperature and the doping concentration changed from low to high level,depending on the experiment temperatures. In contrast, K doping is always saturated(K6C60) in our studied temperature range due to the larger ionic sizes and higher meltpoints and vapor pressure. This study can help us to obtain1D/quasi-1Dnanomaterials with different electronic properties in fullerene doped materals, whichhave potential application in the field of nanometer-scale devices andnano-electronics.