8.4 ZENG Zhidan

Amorphous diamond: a new form of carbon synthesized under HPHT

Diamond has many unparalleled properties due to its extremely strong sp3 C-C bonding. The search for the same sp3-bonded tetrahedral amorphous carbon which would have properties complementary to diamond has been a long-sought after yet elusive goal. Although other Group-14 elements silicon and germanium have complementary crystalline and amorphous forms consisting of purely sp3 bonds, purely sp3-bonded tetrahedral amorphous carbon has not yet been obtained.

By combining high pressure with in situ laser heating at a specific temperature window, we successfully synthesized a completely sp3-bonded tetrahedral amorphous carbon, i.e., “amorphous diamond”, for the first time. Moreover, this amorphous diamond obtained under high pressure and temperature can be recovered to ambient conditions. We preformed comprehensive studies including x-ray diffraction, high-resolution transmission electron microscopy, electron energy-loss spectroscopy experiments and ab initio molecular dynamics simulations on the amorphous diamond. The results suggest the amorphous diamond has a three dimensional sp3-bonded network structure similar to crystalline diamond, except no long-range order. The amorphous diamond is transparent, and has a density close to that of diamond. In our in situ high pressure x-ray diffraction experiment, this amorphous diamond showed ultrahigh incompressibility (bulk modulus) comparable to diamond.