High-pressure phase of cold-compressed bulk graphite and graphene nanoplatelets

We have studied the high-pressure vibrational and structural behavior of bulk graphite and graphene nanoplatelets at room temperature by means of high-pressure Raman spectroscopic and x-ray diffraction probes. We have detected a clear pressure-induced structural transition in both materials, evidenced by the appearance of new Bragg peaks and Raman features, deviating from the starting hexagonal graphitic structure. The high-pressure phase is identified as a partially disordered orthorhombic structure, consisting of mixed $s{p}^2$- and $s{p}^3$-type bonding. Our experimental findings serve as direct evidence for the existence of a metastable transient modification in cold compressed carbon, lying between the $s{p}^2$-type graphite and ${sp}^3$-type diamond allotropes.

Figure 1

(a) Normalized XRD patterns of GNPs ($\lambda =0.3344\AA , T=300$ K, $Q={\lambda }^{-1}4\pi \sin \theta$, Ne as PTM) and (b) the bulk NG sample ($\lambda =0.2952\AA , T=300$ K, $Q={\lambda }^{-1}4\pi sin\theta$, He as PTM) at various pressures. The black, red, and grey colors correspond to the HG and HP-OG phases, and the coexistence regime, respectively. Background has been subtracted for clarity. The black and red arrows denote the Bragg peaks assigned exclusively to the HG and the HP-OG phase, respectively. Asterisks indicate Ne PTM peaks. Notice that the use of He as PTM does not contaminate our XRD patterns with additional Bragg peaks, thus allowing for the detection of the NG Bragg peaks alone.

Figure 2

(a) Pressure dependence of the HG Bragg peaks for the NG (black stars) and the GNPs (black diamonds). The adjacent Bragg peaks of the HP-OG phase are also plotted (red squares). The violet lines represent the Bragg peaks of cubic diamond (CD) [60]. The respective Miller indices are also provided for convenience. (b) Le Bail refinements of the XRD patterns of bulk natural graphite (NG, $\lambda =0.2952\AA$, He PTM) and GNPs ($\lambda =0.3344\AA$, Ne PTM) collected at 37–38 GPa. The black symbols stand for the experimental XRD pattern, the red and blue lines depicting the best refinement and difference curves, respectively. Vertical ticks mark the HP-OG Bragg peak positions. Asterisks stand for the Ne PTM peaks. The simulated XRD pattern of cubic diamond (CD) at 38 GPa is also drawn (top) [60].

Figure 3

Equations of state for the starting hexagonal graphite (HG, black) and the high-pressure (HP-OG, red) phases of GNPs and NG samples. The red solid lines passing through the $P-V$ data (circles: GNPs, stars: NG) correspond to Birch-Murnaghan equation of state fittings [62, 63], with the respective parameters tabulated in Table 1. Open circles correspond to our LDA-calculated HG data [59]. The EOS of cubic diamond (CD, purple solid line) is also drawn [60].

Figure 4

As-measured Raman spectra of the bulk NG graphite sample at selected pressures, with Ne serving as PTM ($\lambda =457$ nm, $T=300$ K). The spectra are divided between the (a) low-frequency and (b) high-frequency (G-band) regions. The black and red color corresponds to the HG and HP-OG phases, respectively.

Figure 5

Pressure-induced behavior of the Raman mode frequencies for bulk NG and GNPs (solid symbols). The black and red colors stand for the specific HG and HP-OG Raman-related feautures, respectively. The estimated $\mathrm{HG}\text{−}{\mathrm{E}}_{2\mathrm{g}}$ mode (green line) [22], the F-band of glassy carbon (GC-F, violet stars) [40] and our LDA-calculated HG and rhombohedral graphite (RG) mode frequencies (open symbols) are included for comparison. Solid lines passing through the experimental data represent least-square fittings (Table 2). The vertical dashed line marks the onset pressure of the $\mathrm{HG}\to \mathrm{HP}\text{−}\mathrm{OG}$ transition in the NG sample.