Hard as Diamond: A New Form of Carbon Created under Ultrahigh Pressure

 Diagram of the diamond anvil cell experiment

An amorphous diamond—one that lacks the crystalline structure that makes diamonds cleavable, but is every bit as hard—has been created by a team of researchers using a High Pressure Collaborative Access Team x-ray beamline at the U.S. Department of Energy Office of Science’s Advanced Photon Source.

HPCAT experiments showed that at pressures above 40 GPa, the arrangement of the bonds between the carbon atoms in the glassy spheres had completely shifted to a form that endowed the spheres with diamond-like strength. While the diamonds in the anvil pressed down on the spheres with a pressure of 130 GPa, the pressure on the sides of the spheres was held at 60 GPa, indicating that the amorphous diamond survived a pressure difference of 70 GPa. Nothing else can withstand that sort of stress difference. Although the bonds between atoms in the glassy spheres were altered by the extreme pressure, the amorphous, or disordered structure of the spheres was unchanged. The hardness of the amorphous carbon is tunable; it is soft without pressure, but the stronger it was pressed, the harder it got.
See: Yu Lin1*, Li Zhang2, Ho-kwang Mao2, Paul Chow2, Yuming Xiao2, Maria Baldini2, Jinfu Shu2, and Wendy L. Mao1,3, “Amorphous Diamond: A High-Pressure Superhard Carbon Allotrope,” Phys. Rev. Lett. 107, 175504 (2011).