Yabo Xu

Optimization on the crystal growth of fullerenes

Abstract Inspecting critically the former vapour growth technique of C 60 single crystals from the viewpoint of crystal growth dynamics, we conclude that the conditions should be improved. Both the temperature difference distribution throughout the growth process and the temperature distribution in the furnace are carefully selected. With this optimization, large C 60 single crystals of good quality with size more than 6 mm can be grown reproducibly. The Laues analysis of the crystal show that the exposed surfaces of C 60 solids are of two types of crystalline faces, namely 1 1 1} and 1 0 0}.

Investigation of High Pressure Induced Orientational Phase Transition in C60 Single Crystal

By using a piston-cylinder type hydrostatic high pressure unit, the electrical dc conductance of bulk C60 single crystal was measured in the pressure range of 0 to 0.8 GPa at different temperatures. With increasing pressure, a discontinuous increase of the conductance was observed, which can be ascribed to the orientational ordering phase transition. The slope of the phase line corresponding to the orientational ordering transition in the P-T phase diagram is determined to be dT/dP c = 150 KJGPa. At elevated temperatures, the slopes of the conductance versus pressure give an anomalous discontinuous change before the orientational ordering transition.

Preparation and vibrational modes of C60 · 2CHBr3

C60· 2CHBr3 polycrystalline powder was prepared by the solution method. The vibrational modes of the sample were studied by Raman scattering and infrared adsorption techniques. The Raman spectra showed that the Ag modes were downshifted by 4–5 cm−1, while the Hg(l) mode was unshifted. The analyses of the downshifts of the Ag modes revealed that a small amount of electrons were transferred from the hydrogen atoms to the C60 molecules. In the infrared spectra, the F1u modes of the pristine C60 were unshifted after the intercalation with CHBr3. However, the vibrational modes of CHBr3 changed remarkably. The C-Br stretching mode was downshifted by ∼4 cm−1. The adsorption of the C-H twisting mode was weakened obviously, and the adsorption of the C-H stretching mode was not observed. These results exhibited that there were nonnegligible interactions between CHBr3 and C60 molecules, and that the strongest interaction took place between the hydrogen atoms and the C60 molecules. The interaction should induce not only the changes of the vibrational modes reported here, but also those of the electronic states. So this work should offer important glues to the mechanism of the superconductivity at 117 K of the field-doped C60· 2CHBr3.

Orientational phase transition in C60 single crystal under high pressure

By using a piston-cylinder type hydrostatic high-pressure unit, the electrical dc conductance of bulk C60 single crystal was measured in the pressure range of 0 to 0.6 GPa at 290 K. Wlth increasing pressure, a discontinuous increase of the conductance was observed at the pressure of 0.176 GPa, which can be ascribed to. the orientational ordering phase transition. In the simple cubic phase, the increase of conductance with increasing pressure is faster than that in the face-centered-cubic phase.