Gap reduction and the collapse of solidC60to a new phase of carbon under pressure

Abstract

We have studied solid Ceo, measuring the shift of the optical absorption edge with pressure to 35 GPa. In the range of 0-17 GPa, extrapolation of the absorption edge indicates that metallization should occur by 33 GPa. However, from 17 to 25 GPa an irreversible transition to a "transparent phase" occurs. Raman scattering of the depressurized sample shows no trace of CM, diamond, or graphite, indicating that the transition involves the collapse of the C6o molecules into a new structure of carbon. PACS numbers: 64.70.Kb, 78.20.Dj, 78.30.HvRecently there have been a number of fascinating results on the properties of fullerite (solid C6o) at ambient conditions and at high pressure. These include observations of high-7V superconductivity [1] in doped fullerite and speculations that compressed fullerite might be harder than diamond [2]. In this work we intended to study the insulator-metal (IM) transition in pure fullerite and its ultrahigh pressure compressibility, as a material which hardens with increasing pressure might have important technological implications. We pressurized fullerite to 35 GPa in a diamond-anvil cell (DAC) and studied its optical absorption spectrum and Raman scattering cross section. Initially, with increasing pressure, the optical absorption threshold energy (ATE) shifts to lower energy, implying that fullerite should metallize in the 30-40 GPa pressure range. However, in the range 17-25 GPa the fullerite undergoes a sluggish irreversible transformation to a phase with reduced optical density; the transition is accelerated by raising the pressure to 35 GPa. Upon reducing the pressure to zero the sample does not revert to the low-pressure fullerite phase. Raman scattering measurements showed no trace of the C60 molecules so we conclude that the C6o molecules have collapsed. Furthermore, the Raman spectra of the collapsed fullerite (CF) phase cannot be identified with that of diamond or graphite, so we conclude that the CF phase is a new phase of carbon existing at zero pressure.High-purity C 6 o (less than 1% C70 content) was vacuum evaporated onto a diamond culet to form a rather uniform homogeneous film with a thickness of 2.5 //m, as measured by optical interference techniques. A few grains of ruby were scattered on the sample for pressure determination. A T301 stainless-steel gasket with a 100-//m-diam hole was placed over the sample and mounted in the DAC. We used xenon as a quasihydrostatic pressure medium; this was loaded cryogenically [3]. Measurements presented in this Letter were made at room temperature (295 K). Raman scattering was carried out using the 5145-A argon ion laser line and a Spex Triplemate monochromator with a Tracor Northern diode array. Laser power fluence was restricted to below 300 W/cm 2 to prevent overheating and burning or thermal deformation of the samples. Absorption spectroscopy was carried out in the visible; spectra were normalized to the transmission of a suitable pinhole. Pressure was measured using the ruby fluorescence scale [4] and was quasihydrostatic:...

Long-Range Ordered Carbon Clusters: A Crystalline Material with Amorphous Building Blocks

“…Nature offers countless examples of camouflage and display (3,11,12). Although specific demonstrations of camouflage vary among species, the strategies used have common themes: background matching, disruptive coloration, and disguise (3,11,12).…”

“…Although specific demonstrations of camouflage vary among species, the strategies used have common themes: background matching, disruptive coloration, and disguise (3,11,12). In background matching, animals use colors and patterns similar to their habitats (3,(11)(12)(13), such as counter/obliterative shading (for example, rabbits with white underbellies and brown backs) (11,12).…”

“…Although most strategies of camouflage and display (3,12) emphasize the visible spectrum that most animals (especially mammals) can see (19)(20)(21), there are animals that can see (or otherwise sense) ultraviolet (UV) or IR light and use these spectral regions for signaling or hunting. For example, most birds, many arthropods, and some fish have UV vision (22), and many snakes, such as pit vipers, can sense IR light using specialized organs (23).…”

“…Polymeric fullerenes have been synthesized at different pressures and temperatures when pure C 60 or C 70 were used as starting materials (8)(9)(10)(11). Under cold (ambient temperature) compression, the C 60 cages collapse, and the crystalline C 60 phase transforms into amorphous carbon above 30 GPa (12,13). Under high-temperature compression, pure C 60 forms different polymerized structures, with the C 60 cages transforming into graphitic carbon or a mixture of sp 2 and sp 3 amorphous phases.…”

Pressure-induced phase in tetragonal two-dimensional polymericC60

“…A variety of structural transformations of fullerite to amorphous graphite, diamond and ''collapsed'' fullerite, as well as to other crystalline carbon networks, were reported at the early stages of fullerene studies. [1][2][3][4] Later new phases of polymerized fullerites have been synthesized starting from the pristine C 60 material by the use of highpressure and high-temperature treatment. [5][6][7] The various crystal structures of the polymers have been identified clearly as one-dimensional orthorhombic, 6 two-dimensional tetragonal, 6 and two-dimensional rhombohedral phases.…”