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.