DEBYE-SCHERRER "ELLIPSES" FROM 3D-C60 POLYMERS

L. Marques¹, M. Mezouar², J.L. Hodeau³, M. Nunez Regueiro⁴, N. Serebryanaya⁵, V.A. Ivdenko⁵, V.D. Blank⁵

¹Dept. Fisica, Universidade de Aveiro, Portugal,
²ESRF, BP220, 38043 Grenoble, France,
³CNRS, Laboratoire de Cristalographie, BP166, 38042 Grenoble, France,
⁴EPM-Maformag CNRS, Grenoble, France,
⁵Res. Center of Superhard Materials, Troisk, Moscow region, 142092 Russia

Keywords : C60 under pressure, Crystallography under extreme conditions, fullerenes

The pressure and temperature C₆₀ phase diagram has been largely studied using quenched samples obtained in a high pressure/high temperature belt apparatus. We will present X-ray synchrotron radiation studies of such samples and 'in-situ' high temperature /high pressure diffraction experiments, performed in a large volume cell under hard X-ray beam at the European Synchrotron Radiation Facility.

Both on quenched samples and 'in-situ' experiments, we have obtained 1D and 2D polymerisation of fullerenes at high pressure (30-80kBar) and high temperature (600-1000K) . The origin of this polymerisation are covalent bonds formed between molecules. As temperature is increased, we follow the continuous transformation from fcc fullerite to an orthorhombic phase of polymerised C₆₀ chains that, at higher temperature, polymerise to 2D-C₆₀ polymers. Above 1100-1200K the C₆₀ cage collapses with formation of an amorphous phase. We interpret the transformation from the 1D-polymerized phase towards the 2D-rhombohedral one, by means of an homogenous polymerisation.

Furthermore, at pressures higher than 80kBar, up to 130kBar, other highly compressed 3D-C₆₀ phases are synthesized. These later quenched samples from the region 130kBar/830K show the most striking crystallographic properties ; they give rise to Debye-Scherrer ellipses instead of circles, in two of the three axes. Though elliptic pattern are possible in samples measured under high pressure, this eliptical deformation is very large (9%) and this is the first time that such pattern is observed in a crystalline solid at ambient pressure. This implies that the deviatoric stress introduced by the non-hydrostatic compression has been frozen in these samples after release of pressure. Our crystallographic analysis shows that C₆₀ cages are still present but distorted. Within the observed average fcc structure, the inter-molecular distance has been strongly reduced, in the 3D directions, to a separation compatible with strong inter-molecule chemical bonding, i.e. polymerization into a zeolite-like C₆₀ structure. Consequently, the multiple, almost degenerated, bonding possibilities permited by the highly symmetrical C₆₀ molecule within the polymerized 3-D structure allow the fixing down to ambient pressure of the deviatoric stress present under non-hydrostatic compression.