Structural transition in cold-compressed glassy carbon

Glassy carbon (GC) distinguishes itself from other carbon materials by its unique atomic structure and properties. Cold-compressed GC gives rise to new physical properties; however, the atomistic mechanism for the transitions remains elusive. In this study, by combining in situ high-pressure x-ray diffraction with first-principles calculations, we observe pressure-induced disappearance of the initial intermediate range order of GC, followed by formation of local tetrahedral structural domains and ${sp}^3$ bonds. Correspondingly, the resistance of GC increases by four orders of magnitude during compression from $\sim 20$ to $\sim 61\mathrm{GPa}$. Both the structural and resistance transitions are partially reversible upon decompression, with noticeable hysteresis. Our results highlight the central role of layer distortions in inducing the ${sp}^2$-to-${sp}^3$ bonding transition and provide the structural underpining for the various transitions observed in cold-compressed glassy carbon.

Figure 1

In situ high-pressure XRD of GC. XRD patterns of GC at representative pressures during compression up to 51.4 GPa (black curves) and decompression to 0 GPa (blue curves). No pressure medium was used in the experiment in order to generate high deviatoric stress. The FDP and SDP correspond to the average interlayer distance of $\sim 3.8\AA$ and intralayer characteristic distance of $\sim 2.05\AA$ in GC at ambient pressure, respectively.

Figure 2

Position (a) and intensity (b) of the FDP (square) and SDP (circle) of GC as functions of pressure during compression up to 51.4 GPa (solid symbols) and decompression (open symbols). The peak position and intensity were derived by fitting the diffraction peaks to Gaussian functions. The experimental data of GC from Ref. [12] (diamond) in which Ne was used as a hydrostatic pressure medium are also included for comparison. The pressure dependence of peak position in compression was fit to the third-order Birch-Murnaghan equation of state (solid line). Dashed line in (b) serves as a guide to the eye.

Figure 3

Atomic configuration of GC at representative pressure 0.6 GPa (a), 19.8 GPa (b), 44.4 GPa (c), and 71.4 GPa (d) during hydrostatic compression obtained by first-principles calculations. The red spheres represent ${sp}^3$-bonded C atoms.

Figure 4

(a) Structure factor $\left[\mathrm{S}\right(q\left)\right]$ of the GC as a function of pressure during hydrostatic compression. The $\mathrm{S}\left(q\right)$ was computed based on the radial distribution function following the Baxter-Dixon-Hutchinson factorization method [31]. (b) The fraction of ${sp}^3$ bonds in GC as a function of pressure during hydrostatic compression (red squares) and uniaxial compression (blue circles), and decompression (open symbols).

Figure 5

Resistance of the GC sample as a function of pressure during compression (solid square) and decompression (open square). A log scale is used for resistance to reveal the change in low-resistance region. Inset a, zoomed-in plot of the resistance data below 27 GPa. Inset b, an optical microscope image of the sample (marked by the blue circle) with four Pt electrode probes for resistance measurement at 61.3 GPa. Ruby was used as the pressure calibrant. The scale bar represents $100\mu \mathrm{m}$.

Figure 6

Calculated EDOS of GC at different pressures during hydrostatic compression.