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Phys Rev Lett. 2011 Oct 21;107(17):175504. Epub 2011 Oct 19.

Amorphous diamond: a high-pressure superhard carbon allotrope.

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1
Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305, USA. lyforest@stanford.edu

Abstract

Compressing glassy carbon above 40 GPa, we have observed a new carbon allotrope with a fully sp(3)-bonded amorphous structure and diamondlike strength. Synchrotron x-ray Raman spectroscopy revealed a continuous pressure-induced sp(2)-to-sp(3) bonding change, while x-ray diffraction confirmed the perseverance of noncrystallinity. The transition was reversible upon releasing pressure. Used as an indenter, the glassy carbon ball demonstrated exceptional strength by reaching 130 GPa with a confining pressure of 60 GPa. Such an extremely large stress difference of >70 GPa has never been observed in any material besides diamond, indicating the high hardness of this high-pressure carbon allotrope.

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