Researchers from the United States and China have created a new form of superhard carbon material under extreme pressures – very hard carbon clusters with a hybrid of crystalline and amorphous structures, and comparable to diamond in its inability to be compressed. Thematerial can exist under normal conditions, and could bring a wide range of mechanical, electronic and electrochemical applications.
Although carbon has previously been viewed as either crystalline or amorphous, it was thought that hybrid forms of carbon could combine these elements. To show this, a teamled by Lin Wang from the Carnegie Institution of Washington, whose work was reported in Science [Wang et al., Science (2012) doi: 10.1126/science.1220522], examined the stability of carbon-60 at high pressure. They chose C60 cages, comprising highly organized spheres of carbon constructed of pentagon and hexagon rings that form a round, hollow shape, as it likes to crystallize with solventsto form a new series called solvated C60, with its structure being solvent dependent.
The organic m-xylene, seen as the most interesting solvent because of its shape control function on C60, was introduced into gaps in the balls, forming a new structure, before the scientists applied pressure to the combination of carbon cages and solvent to observe any changes under different stresses. Although at low pressures the cage structure remained intact, when the pressure was increased they were found to collapse into more amorphous carbon clusters, while still remaining in their original sites, forming a lattice structure.
When the solvent involved in preparing the new carbon is removed using heat treatment, the material loses its lattice periodicity, demonstrating that the solvent is crucial for maintaining the chemical transition that underpins the new structure. As there are many similar types of solvent, it could be also possible that an array of such carbon lattices could be produced from this same pressure approach.
The fact that there was a small range of pressure – around 320 000 times normal atmosphere – at which this new structured carbon was created without returning to the cage structure when pressure was removed, could be key to potential applications for the new material. Also, with the new material offering superior mechanical and ultrahard properties at high pressure, it could be used, for instance, as an anvil in the large volume compressors used to synthesize diamond and other industrial materials, as well as in deep well drilling.
