C 60 Transformations at High Pressures
- PDF / 1,346,393 Bytes
- 6 Pages / 420.48 x 639 pts Page_size
- 47 Downloads / 232 Views
C60 TRANSFORMATIONS AT HIGH PRESSURES C. S. YOO, W. J. NELLIS, M. L. SATTLER, R. G. MUSKET, N. HINSEY AND W. BROCIOUS Lawrence Livermore National Laboratory, Livermore, CA 94550, USA ABSTRACT C60 molecules have been studied at both shock and static high pressures. Under shock compressions C 60 fullerenes are stable into the 13-17 GPa pressure range. The onset of a fast (-0.5 pis) reconstructive transformation to graphite occurs near 17 GPa. The graphite recovered from 27 GPa and about 900 K is relatively well ordered with La = 100 A. Above 50 GPa a continuous transformation to an amorphous state is observed in recovered specimens. A transparent, metastable carbon phase was recovered from thin films of C60, shocked to 69 GPa and 2200 K and then rapidly quenched to 1000 K. The selected area diffraction patterns indicate that the metastable carbon contains an amorphous diamond and n-diamond. Under hydrostatic compressions C60 molecules transform reversibly to a semi-transparent phase in the pressure range of 15-25 GPa with a large pressure hysteresis. The high pressure phase consists of interconnected strongly interacting C6o agglomerates, or networks of fullerenes, whose stability continuously increases with increase of pressure. Above 27 GPa the transition becomes irreversible, and the material recovered from high pressure is metastable and diamond-like at ambient conditions. These pressure-induced transitions are explained in terms of nr-electron rehybridization between C60 molecules, which occurs at substantially decreased intermolecular distances.
INTRODUCTION A closed spheroidal structure of fullerenes [1] is a new form of carbon, in addition to a 2dimensional layer of graphite and a 3-dimensional network of diamond. The transformations among these carbon forms are of ongoing scientific and technological interest to understand the physics and chemistry of carbon and for novel materials applications. The graphite to diamond transi"on has been studied extensively [2]. New high pressure phases, including n-diamond [3] and transparent pressurized graphite [4], have recently been observed. Generally speaking, graphite is stable at high temperatures and low pressures, and diamond is stable at high pressures and low temperatures. Because of the unusual open structure of fullerenes, the nature of phase transformations of fullerenes is of great current interest over a wide range of pressures and temperatures. C 60 molecules are extremely rigid balls, containing a large open space inside; while, they form a relatively soft solid at moderate pressures [5]. Thus, unlike diamond and graphite, the nearest neighbor C-C distances of C60 molecules rapidly collapse with increasing pressures. This can cause a nt-electron rehybrization between neighboring C60 molecules and may induce a transition from the cluster structure of C60 to a network structure or to a layer structure. In this paper we will show the C60 transforms to graphiteand diamondrapidly, < -0.5 ±s,at high shockpressures and to strongly interacting C60 agglomerates o
Data Loading...
