A MYSTERY form of carbon has finally been identified as a diamond-graphite hybrid.
Carbon is well-known for morphing into many different forms such as graphite and diamond. Graphite is made of parallel sheets of carbon atoms held together by weak chemical bonds. In diamond, the bonds between the carbon atoms are much shorter and stronger.
For years, scientists have been puzzled by a form that appears when graphite is squeezed under the pressure of 170,000 atmospheres. The soft, grey carbon becomes transparent and as hard as diamond. But release the pressure and it turns back into ordinary pencil 鈥渓ead鈥�.
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Wendy Mao at the University of Chicago in Illinois and her colleagues have now discovered why this happens by using X-rays to study the material鈥檚 bonding. They squeezed graphite between two flat-faced diamonds until the material revealed became so hard that the diamonds cracked.
The X-rays showed that the graphite layers were pushed together so tightly that half the weak bonds between the layers had turned into diamond-like links, creating a diamond-graphite hybrid (Science, vol 302, p 425). The material reverted to graphite once the pressure was removed because the strong bonds formed under high pressure are reversible.