Amorphous Diamond: A High-Pressure Superhard Carbon Allotrope

Compressing glassy carbon above 40 GPa, we have observed a new carbon allotrope with a fully $s{p}^3$-bonded amorphous structure and diamondlike strength. Synchrotron x-ray Raman spectroscopy revealed a continuous pressure-induced $s{p}^2$-to-$s{p}^3$ 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 $>70\mathrm{GPa}$ has never been observed in any material besides diamond, indicating the high hardness of this high-pressure carbon allotrope.

Figure 1

High-pressure XRS carbon $K$-edge spectra of glassy carbon collected along the compression and decompression cycles plotted as normalized scattered intensity versus energy loss (incident energy–analyzer energy). The scattered intensity is normalized to the incident energy. The lower energy peak at approximately 285 eV represents the $\pi$-bonding feature, corresponding $1s$ to ${\pi }^{*}$ transition (labeled ${\pi }^{*}$), and the broad band at higher energy features the $\sigma$-bonding, corresponding $1s$ to ${\sigma }^{*}$ transition (labeled ${\sigma }^{*}$). The gray (red) spectrum with 24-h data collection time shows the complete $\sigma$ bonding in the high-pressure amorphous carbon phase. The numbers on the right-hand side indicate pressure in GPa.

Figure 2

XRD patterns of glassy carbon as a function of pressure up to 45.4 GPa at room temperature where the FSDP and SDP of glassy carbon are labeled at the bottom. No crystallinity was observed throughout the pressure range. The decompression pattern at 13.7 GPa indicates a reversible transition. * denote diffraction peaks from the ruby ball, and # denotes the peak from the tungsten gasket. The numbers on the right-hand side indicate pressure in GPa.

Figure 3

The pressure dependence of the FSDP position of glassy carbon in our study compared to the (002) $d$ spacing of cold-compressed graphite [5, 38]. The solid blue line is the fit to a third-order Birch-Murnaghan equation of state [48]. The error bars associated with the FSDP positions of glassy carbon are comparable to the size of the symbol.

Figure 4

(a) Photomicrograph showing two glassy carbon spheres separated by a small ruby ball surrounded by soft KI medium at 20 GPa. (b) Pressure at the indenting point (spot $A$) of the larger glassy carbon sphere as a function of ruby fluorescence pressure. The surrounding pressure determined by the KI medium at the edge of the sample (represented by spot $B$) was essentially the same as the ruby fluorescence pressure, which is illustrated by the solid line. △ represents the pressure difference generated across the larger glassy carbon sphere in the high-pressure amorphous carbon phase.