New high-pressure fullerene structures

Abstract

New polymerized phases of fullerenes were prepared under high pressure and high temperature. Their crystal structures were determined through a combined experimentaltheoretical effort; x-ray diffraction methods, laboratory and synchrotron, complemented by density functional theory (DFT) modeling, were employed. Two novel C70 phases were prepared at 10GPa-270ºC and 7GPa-600ºC [1-3]. The first of these structures consists of one-dimensional (1D) zig-zag polymer, while the second structure consists of buckled-hexagon two-dimensional (2D) polymerized planes (see fig.1). A new C60 face centered cubic (fcc) phase, with a short lattice constant, was prepared at 9.5GPa and 550ºC [4]. DFT calculations showed that the short interfullerene distances corresponds to polymeric 56/56 2+2 cycloaddition bonds. This bonding type forms between molecules having different standard orientations, although no covalent bond forms between similarly oriented molecules. Several ordered 3D polymerized C60 structures, based on the wellknown ordered binary-alloy structures, were constructed. They display lower symmetry than the experimentally observed fcc structure suggesting that this should be a disordered/frustrated structure. The electronic properties of these novel fullerene structures were investigated by DFT. While the 1D and 2D C70 structures are semiconductors the 3D C60 polymers show metallic behaviour [4].