The Electrical Properties Of Graphite, C₆₀, And Cu₂O Under High Pressure

In this paper, the author has devised a specially designed diamond anvil with diamond film electrodes heavily doped with boron for high pressure in situ electrical measurements. Using this setup, the properties of allotropes of carbon and Cu₂O with different morphology have been studied, and a series of new physical phenomenon and laws are observed. The new developed setup has potential application in measuring corrosive and superhard materials.Using the newly designed setup, the author measured the electrical resistivity of compressed graphite in two different pressure cycles in one single experiment and the experiments identified the phase transition of the graphite at 15.1 and 17.9 GPa for the first and second pressure cycles, respectively. SEM is used for measuring the grain size of graphite powders before and after pressure annealing, the size reduction phenomenon can be clearly observed. These results indicate that the phase transitions are sensitive to the grain size of the initial graphite sample.Using the newly devised W-Ta thin-film thermocouple on diamond anvil cell for in situ temperature measurement under high pressure, diamond electrodes, and high resistance measurement instruments, the author measure the resistance and estimate the band gap of the direct band gap semiconductor C₆₀ under high pressure. It is observed that with the applied pressure up to 8GPa, the resistance decrease gradually which is not accordance with previous electrical measurement results. The experiment results indicate inexistence of band gap variation region between ambient pressure and 8GPa. The band gap is derived as follows: 0.49 eV, 0.43 eV, and 0.36 eV at 13 GPa, 15 GPa, and 19 GPa, respectively.An accurate in situ electrical resistivity measurement of the cuprous oxide cubes has been conducted in a diamond anvil cell at room temperature with the pressure up to 25 GPa. The abnormal electrical resistivity variation found at 0.7-2.2 GPa is attributed to the phase transformation from cubic to tetragonal. Three other discontinuous changes in electrical resistivity are observed around 8.5, 10.3, and 21.6 GPa, corresponding to the phase transitions from tetragonal to pseudocubic to hexagonal to another hexagonal phase, respectively.The first-principles calculations illustrate that the electrical resistivity decrease of tetragonal phase has no relations with bandgap shrinkage but the quantity increase of defects with increasing pressure. The current results indicate that the cuprous oxide cubes begin to crush at about 15 GPa and completely transform into the nanocrystalline state at 25 GPa. From ambient pressure to 15 GPa, the resistivity of Cu₂O cubes with six {100} rectangle faces keep a increasing trend with pressure increasing, while the resistivity of Cu₂O octahedra with eight {111} triangle faces keep a decreasing trend with pressure increasing.