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Glassy diamond-a new form of superhard carbon observed

Wendy Mao and Yu Lin, Stanford UniversityCarbon has long been a rich and active research area which offers exciting discoveries of new allotropes including both crystalline and disordered structures such as buckyballs, carbon nanotubes, graphene and diamond-like amorphous carbon with numerous and exciting potential in technological applications. However, the amorphous allotrope of diamond, i.e., the amorphous carbon based on complete sp3-bonding, has never been reported.

A team that includes researchers at Stanford University, Geophysical Laboratory, HPCAT, and HPSynC has discovered a new carbon allotrope at high pressure and room temperature with 100% sp3-bonding in bulk glassy form. Synchrotron inelastic x-ray scattering (IXS) and x-ray diffraction (XRD) probes revealed a pressure-induced sp2-to-sp3 bonding conversion in glassy carbon which was complete above 40 GPa. This high-pressure carbon allotrope remained amorphous, and showed exceptional hardness, withstanding extreme pressure stresses that were previously observed only in diamond. However, unlike diamond and other crystalline forms of carbon, the structure of this new material is amorphous, meaning that its structure lacks the long-range order of crystals. 

This amorphous, superhard carbon allotrope would have a potential advantage over diamond if its hardness turns out to be isotropic—that is, having hardness that is equally strong in all directions. The amorphous carbon created by Mao and Lin can be made thicker than the very thin films previously created by other researchers, and consequently, have more applications. In general, these research findings expand the wealth of pure carbon allotropes and open exciting possibilities for potential applications using superhard amorphous solids.

glassy carbon under pressureFigure 1. Wendy Mao and Yu Lin (from Stanford Report, Oct. 17, 2011) loading samples into a diamond anvil cell.

Figure 1. High pressure IXS carbon K-edge spectra of glassy carbon collected along the compression and decompression cycles. The red spectrum shows the complete σ-bonding in the new high pressure carbon phase. The numbers on the right side indicate pressure in GPa.

Ref: Lin, Yu, Li, Zhang, Ho-kwang Mao, Paul Chow, Yuming Xiao, Maria Baldini, Jinfu Shu, Wendy L. Mao, Amorphous Diamond: A High-Pressure Superhard Carbon Allotrope, Phys. Rev. Lett. 107, 175504 (2011) doi: 10.1103/PhysRevLett.107.175504 (published 19 October 2011).