L. Marques

Microstructural mapping of C60 phase transformation into disordered graphite at high pressure, using X-ray diffraction microtomography

An extended use of synchrotron-based X-ray diffraction microtomography (XRD-µCT) to study simultaneously the phase distribution and microstructure in phase-transformation processes is proposed. This three-dimensional non-invasive imaging approach has been applied to understand the phase transformation of C60 rhombohedral polymer (C60R) into disordered graphite (DG) at high pressure and high temperature. The heterogeneous sample was synthesized (5 GPa, 1100 K) using a Paris–Edinburgh cell and selective image reconstructions were achieved for all different phases present in this sample. The XRD-µCT analysis evidences elongated DG domains with a fiber texture where nested (002)DG planes show ±70° preferential orientation relative to the compression axis. In contrast C60R domains are found to be small and spotty, preferentially in the middle of the sample. The parent and product phases are mutually interpenetrative and exhibit a crystallographic relationship. This study evidences that formation of (002)DG planes occurs parallel to {111}C60C pseudo-cubic planes. Among these four possible alignments, uniaxial pressure favors one [111]C60C direction. Transmission electron microscopy observations validate these nondestructive XRD-µCT results.

Pressure Studies on Fullerenes

Carbon is the element with the richest behaviour of all elements of the periodic table. The chemistry of organic compounds is the key factor for life, but also pure carbon presents a large variety of comportments as a result of the different hybridisations and the number of chemical bonds that it can develop. Consequently, a large number of distinct disordered carbon phases, besides the crystalline allotropes (graphite and diamond and its stacking varieties) were already known.

Polymerized Fullerite Structures: Experiment and Theory

The C60 molecule discovered by Kroto et al. [1] is such an extremely stable closed-cage cluster that it can be viewed in some respects as a super-atom or new chemical species [2]. Considerable research involving chemical and physical properties has been done since the discovery of a method allowing its production on a sufficiently large scale [3].

In situdiffraction study of high-pressure transformation of C60to disordered sp2-carbon

24 European Crystallographic Meeting, ECM24, Marrakech, 2007 Page s216 Acta Cryst. (2007). A63, s216 shares the same two stage densification mechanism as quartz. The first one from berlinite to a CrVO4 type structure where Al is six-fold coordinated and P still fourfold occurs around 14 GPa. In the final compression step, phosphorous becomes sixfold coordinated by oxygen. This previously unknown phosphorous coordination is of great importance from a fundamental point of view and opens new possibilities on the high pressure chemistry of phosphorus

Anisotropic stress imprinting on 3D C60 polymers

We present synchrotron X-ray diffraction measurements performed on C60 samples quenched from 13 GPa and 820 K. The analysis of all the reciprocal space of a quenched sample shows that it consists of a 3-D arrangement of polymerised fullerene molecules. Moreover, it is shown that the intensity distribution in reciprocal space occurs in ellipsoidal surfaces instead of the normal spherical surfaces characteristic of a powder sample. This implies that the applied anisotropic stress is imprinted in the quenched transformed samples.