In situ analysis of the structural transformation of glassy carbon under compression at room temperature

Room temperature compression of graphitic materials leads to interesting superhard $s{p}^3$ rich phases which are sometimes transparent. In the case of graphite itself, the $s{p}^3$ rich phase is proposed to be monoclinic $M$-carbon; however, for disordered materials such as glassy carbon the nature of the transformation is unknown. We compress glassy carbon at room temperature in a diamond anvil cell, examine the structure in situ using x-ray diffraction, and interpret the findings with molecular dynamics modeling. Experiment and modeling both predict a two-stage transformation. First, the isotropic glassy carbon undergoes a reversible transformation to an oriented compressed graphitic structure. This is followed by a phase transformation at $\sim 35$ GPa to an unstable, disordered $s{p}^3$ rich structure that reverts on decompression to an oriented graphitic structure. Analysis of the simulated $s{p}^3$ rich material formed at high pressure reveals a noncrystalline structure with two different $s{p}^3$ bond lengths.

Figure 1

(a) A schematic of the PP-DAC showing the incident x-ray beam aligned parallel to the compression direction. (b) An XRD pattern showing a symmetric {002} reflection onto the detector. The red areas indicate masking that has been applied to remove interference from the detector and beam stop. The triangular areas drawn on the diffraction patterns indicate the specific regions of the image that are integrated to form the spectra shown in Fig. 2. (c) A schematic of the pan-DAC with a beryllium gasket showing that the x-ray beam is incident perpendicular to the compression direction. (d) An XRD pattern showing a nonuniform {002} reflection onto the detector where the intensity is concentrated in two “arcs” aligned with the DAC compression axis. The sharp rings at the edges of the diffraction pattern are due to the beryllium gasket.

Figure 2

(a) XRD scans of GC recorded in situ in the PP-DAC up to a maximum pressure of 47 GPa. All spectra have been normalized to the intensity of the {100} peak. Note the peak at $\sim 3{\AA }^{-1}$ is sharpened in the 28 GPa scan due to the beam clipping the gasket material. For the recovered and uncompressed scans, the samples were removed from the DAC. The major peaks have been indexed to graphite. (b) Diffraction intensity of the simulated GC structure under compression computed using the Debye scattering equation.

Figure 3

Selected regions of $Q$ space indexed to the {002} peak of graphite from in situ XRD spectra measured in the PP-DAC (a), (b) and the pan-DAC (c), (d) up to a maximum pressure of 47 and 46 GPa, respectively. (e) The intensity of the {002} peak obtained by fitting a Gaussian to the side and bottom spectra shown in (c), (d). This figure shows the gradual onset of preferred orientation of graphitic layers up to 36 GPa, beyond which the {002} peak intensity drops sharply in the bottom direction and is no longer observable in the side direction. The yellow shaded region highlights the pressure range where the sharp change is observed. Error bars fitted in the $y$-axis direction are calculated from the Gaussian fits.

Figure 4

A sequence of snapshots showing the key stages of the uniaxial compression of GC. The compression axis is parallel to the black arrows. Each snapshot shows a slab of 2 nm depth, where red, green, and blue coloring denote $sp, s{p}^2$, and $s{p}^3$ bonding, respectively. The pressure at which the structure is compressed, and its corresponding density are indicated on the top of each snapshot. (a) Uncompressed GC structure. The GC structure compressed to (b) 10 GPa, (c) 25 GPa, (d) 30 (e) 35 GPa, and (f) 45 GPa.

Figure 5

(a) $s{p}^3$ fraction (blue squares, right vertical axis) and the density (black circles, left vertical axis) of the simulated structure as a function of pressure during compression. The yellow shaded region highlights the 30–35 GPa pressure range where the sharp change is observed. (b) Nucleation of diamond crystallites during compression of the simulated structure as a function of pressure. The black circles represent the computed total fraction of atoms in a diamond structure as a function of pressure. The total fraction of diamond is the combination of the fractions of atoms arranged in cubic (brown squares) and hexagonal diamond (orange crosses) structures. (c) Data from the modeling showing the $s{p}^3$ bond lengths parallel (purple circles) and perpendicular (blue open circles) to the compression axis.

Figure 6

(a) Radial distribution functions of the uncompressed simulated GC structure, and of the compressed structure at 10, 25, 35, 45, and 56 GPa. (b) Left axis: First neighbor peak intensity from the $g$($r$) curves in (a) as a function of density during compression. Data are represented in black circles. Right axis: First neighbor peak position from the $g$($r$) curves in (a) as a function of density during compression. Data are represented in blue open circles. The yellow shaded region highlights the 30–35 GPa pressure range where the sharp change is observed in Fig. 5.