Pressure turns amorphous carbon to amorphous diamond

Figure caption: Structure and image of the synthesized amorphous diamond compared with the initial glassy carbon sample. XRD patterns of glassy carbon (blue) and the sample recovered from high pressure and temperature. Inset: Optical images of the sample at 49.6 GPa before and after laser heating. The sample size after heating shrinks by approximately 3% in length and width, leading to a volume decrease of nearly 10%.
Figure caption: Structure and image of the synthesized amorphous diamond compared with the initial glassy carbon sample. XRD patterns of glassy carbon (blue) and the sample recovered from high pressure and temperature. Inset: Optical images of the sample at 49.6 GPa before and after laser heating. The sample size after heating shrinks by approximately 3% in length and width, leading to a volume decrease of nearly 10%.

C, Si and Ge all have sp3-bonded crystalline phases with cubic diamond structure. Both Si and Ge display well-known sp3-bonded tetrahedral amorphous forms which have widespread applications. However, the counterpart in C, an amorphous form of diamond, has been conspicuously missing and remained a puzzle. By combining high pressure with in situ laser heating at a specific temperature window, a research group using HPCAT facilities successfully turned amorphous carbon to amorphous diamond. The amorphous diamond has a three dimensional sp3-bonded network structure similar to crystalline diamond, with ultrahigh incompressibility (bulk modulus) comparable to diamond, but lacking long-range periodic order. Moreover, this amorphous diamond is found to be recoverable at ambient conditions. More in Zeng et al., Nature Comm. 8, 322, (2017)