Abstract
Carbon is an element with extremely versatile bonding properties and theoretical calculations have suggested the possible existence of several hundred structural allotropes. Many, or even most, of these are predicted to be formed under conditions of high pressure and temperature. On the other hand, experimental high pressure studies have identified surprisingly few structural allotropes. In this paper, physical properties and structural transformations observed in high pressure experiments, at and above room temperature, are reviewed for a large number of solid carbon allotropes. The materials discussed include bulk carbon such as graphite, diamond, glass-like and amorphous carbon, two-dimensional graphene, and molecular carbon in the form of one-dimensional carbon nanotubes and zero-dimensional fullerenes. Results from recent studies on twisted graphene, graphdiyne, graphyne, carbon dots and other interesting all-carbon allotropes are also briefly described. Observed similarities and differences between the high pressure behavior and evolution of carbon materials are discussed. In spite of the enormous volume of experimental work carried out on these materials, few new structural allotropes have been identified and most carbon materials studied convert into diamond at sufficiently high temperature and pressure. Further theoretical work thus seems to be needed to elucidate possible transformation processes and transition paths for the many undiscovered allotropes proposed from calculations. In particular, it is recommended that, for every new allotrope predicted by theory, suitable precursors and transformation conditions should also be investigated. Efficient creation of new structural allotropes or functional materials based on pure carbon by high pressure methods should ideally start from designed, preassembled precursor structures or composites for which transition paths can be theoretically predicted.
Highlights
Ever since the first successful synthesis of diamond from graphite by groups at ASEA AB [1,2] and General Electric Company [3], high pressure science and technology have been closely associated with the element carbon, in the eye of the general public
Most experiments show that the reaction thresholds for transformations into graphite and diamond are lower than for transformations into other phases, but it is possible that the low reaction probabilities are caused by the strongly disordered basic structure of most carbon nanotube samples and the resulting need for major atomic rearrangement
Many experimental reports about probable or possible new structural phases have been given, the actual structures have only been identified with reasonable certainty in a small number of cases
Summary
Ever since the first successful synthesis of diamond from graphite by groups at ASEA AB [1,2] and General Electric Company [3], high pressure science and technology have been closely associated with the element carbon, in the eye of the general public. This review will explore the properties, evolution, and structural transformations of all major types of carbon allotropes under high pressure, in a wide range of temperatures, and discuss similarities and differences in their behaviors. Many of these are molecular solids based on all-carbon molecules Some related materials, such as simple intercalation compounds, will be mentioned, but the high pressure properties of hydrocarbons, carbides, graphite oxide and other compounds with covalent bonding between carbon and other elements will be excluded because of space and time limits. For large-scale synthesis of carbon materials or for studies of carbon by, e.g., neutron scattering, electrical transport measurements or magnetic measurements, other types of high pressure equipment [1,2,3,28,31] with larger volume are used Such devices include large anvil [31], piston-cylinder and ‘‘belt’’ (see Fig. 2) devices and several types of multi-anvil devices [28]. While such devices provide a large working volume their higher cost and more complicated design limit their use to specialized laboratories
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