In situ x-ray diffraction study of C-60 polymerization at high pressure and temperature

• The C-60 polymerization was studied by x-ray diffraction in situ in the pressure range 13-18 GPa and at temperatures up to 830 K and nonhydrostatic conditions. Quenched samples were studied ex situ by Raman spectroscopy. The results of the high-pressure and high-temperature treatment are strongly dependent on the history of the sample and stress. An inaccurate increase of the pressure leads to the formation of internal stresses in the sample which gives elliptical Debye-Scherrer diffraction rings already at room temperature. During the subsequent heat treatment a soft amorphous phase is formed. At certain conditions no elliptical diffraction patterns were observed at 13 GPa and 830 K. Samples with a relatively low internal stress showed a transformation to a phase with decreased c-cell parameter compared to the known two-dimensional rhombohedral phase. The observed phase can be described as distorted cubic, but a better fit is achieved using a rhombohedral cell. It is suggested that this phase is a three-dimensional polymer with each C-60 molecule bonded to eight neighbors. This phase showed an increased hardness (about 37 GPa) and a Raman spectrum distinctly different from previously known polymeric phases.

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