Electrical Transport Properties Of C₇₀ Under High Pressure

Following graphite and diamond, it was found that fullerene is the third allotropeof carbon. The first discovered fullerenes molecular are C60and C70, which have theunique molecular structures and physical properties. A large number of studies haveshown that fullerenes have important application values and broad applicationprospects in the optical, magnetic,superconducting, and functional materials. In recentyears, research has been carried out on the nature of the fullerene material varies withthe changes of the external environment. The main objective has been placed onC60.Due to the sample C70of the high purity is not easy to get, and the C70moleculehas a complex vibration mode, the study has relatively seldom reported on thephysical properties of C70under high pressure, and there are still controversy. So, inthis thesis, we have put our main focus on the research of electrical transportproperties of C70under high pressure, including the measurements of the alternatecurrent impedance spectra, direct current resistivity and Raman spectra. A diamond anvil cell andin-situ electrical transport measurement were employed in our experiments. Relatedmeasurement results are listed as follows:Impedance spectroscopy of C70was studied under high pressure show that theresistance rapidly decreases, with the increasing pressure from7.3GPa to12.3GPa.After12.3GPa, the increasing of the electrical resistance reveals the occurring ofstructural phase transition. The result is consistent with that of experimental reportsby Jeremy et al. We think that t this change is caused by the formation of an insulatingamorphous carbon phase with the collapse of the C70cage when the pressure reached12.3GPa.High pressure in-situ resistivity measurement indicates: during the compression,the electrical resistivity of C70decreases from8.4GPa to11.4GPa is attributed to thedistance decrease between the molecules under pressure. The band gap become narrow and the carrier concentration increase. So with the pressure increasing, theresistivity decrease. When the pressure increase from11.4to12.2GPa, the resistiviryfell nearly five orders of magnitude. The molecular structure of C70was damaged, andan amorphous phase thansition is occurrencing. After12.2GPa, the increasing of theelectrical resistance reveals the formation of an insulating amorphous carbon phasewith the collapse of the C70cage and transition process is completed by35.7GPa. Theresult is consistent with the conclusion of high pressure in-situ impedancespectroscopy measurement. During the decompression, the value of electricalresistivity does not return to the initial status after releasing pressure from35.7GPa.This result indicates that the amorphization of C70was irreversible.It is shown that the electrical resistivity of C70decreases with increasingtemperature by the temperature dependent resistivity measurement that in the wholeobserved pressure region(from7.8GPa and35.0GPa). This indicates that C70remainsthe semiconductor behavior and the phenomenon of metallization does not appear.High pressure Raman scattering spectra is recorded from1.3GPa to2.4GPawhich shows all peaks shift toward lower frequency and many new peaks appear. Wethink that the anomaly was attributed to a phase transition from fcc to rh structure. Anamorphization phenomenon stars when pressure is between8.2-13.5GPa. The Ramanpeak released from36.2GPa is different from the peak for original C70. Therefore, theresult further indicates that the amorphization of C70was irreversible above36.2GPa.