Transformation of shock-compressed graphite to hexagonal diamond in nanoseconds

Abstract

The graphite-to-diamond transformation under shock compression has been of broad scientific interest since 1961. The formation of hexagonal diamond (HD) is of particular interest because it is expected to be harder than cubic diamond and due to its use in terrestrial sciences as a marker at meteorite impact sites. However, the formation of diamond having a fully hexagonal structure continues to be questioned and remains unresolved. Using real-time (nanosecond), in situ x-ray diffraction measurements, we show unequivocally that highly oriented pyrolytic graphite, shock-compressed along the c axis to 50 GPa, transforms to highly oriented elastically strained HD with the (100)_HD plane parallel to the graphite basal plane. These findings contradict recent molecular dynamics simulation results for the shock-induced graphite-to-diamond transformation and provide a benchmark for future theoretical simulations. Additionally, our results show that an earlier report of HD forming only above 170 GPa for shocked pyrolytic graphite may lead to incorrect interpretations of meteorite impact events.

Fig. 1. Experimental configuration for real-time, in situ structural determination of shock-compressed graphite.

A 10-mm-diameter LiF(100) single crystal impacts a ZYB-grade HOPG disk (~2 mm thick) along the graphite c axis; the impact generates a shock wave traveling at ~8 km/s through the HOPG, transforming the ambient graphite to HD. Red arrows indicate the propagating shock wave fronts traveling into the graphite sample and into the LiF(100) impactor. Crystal structure of the shocked HOPG is determined in situ from diffraction data obtained using time-resolved synchrotron x-ray pulses (100-ps pulse duration at 153.4-ns intervals) as the shock wave propagates through the material. An area detector records four XRD frames during the impact event.

Fig. 2. Shocked HOPG XRD results.

(A) Single-pulse XRD pattern for ambient HOPG (red spots are from diffraction simulations). Because the x-rays are in the horizontal plane and the HOPG has a fiber texture, the diffraction patterns from the ambient and shocked HOPG all have mirror symmetry about the horizontal blue dashed line containing the direct x-ray beam/detector intersection. The indexed spots refer to families of {hkl} graphite planes. (B to D) Single-pulse XRD patterns obtained after impact; time after impact is listed in the images. In (B) and (C), the shock wave has propagated through 47 and 98% of the HOPG, respectively. In (D), the shock wave has released from the HOPG rear surface. The same HD diffraction spots are observed in all three XRD frames obtained after impact. Bright XRD spots from the LiF(100) impactor are easily recognized because they do not have mirror symmetry about the horizontal line. For reference, the HD spots are labeled with yellow numbers in (C) and indexed in (D). Not all 16 spots are labeled because some spots are masked by LiF diffraction spots and some are not in the detector field of view for this experiment; spots 4, 10, 13, and 15 are mirror images about the blue dashed line of spots 2, 8, 14, and 16, respectively (38). (E) HD orientation and average lattice parameters/distortions at ~50 GPa. (F) Diffraction pattern for HD formed by shock compression of HOPG to 50 GPa; the image was obtained from (C) by subtracting a fraction of the ambient HOPG XRD pattern (A). The yellow lines are XRD simulations of HD using the lattice parameters/distortions shown in (E).

Fig. 3. Stress-volume states of shock-compressed ZYB-grade HOPG.

Results shown are for HOPG below the phase-change threshold (47), at the phase-change threshold (34, 38), and above the phase-change threshold (34, 38). XRD results (red squares) and laser interferometry measurements on shocked ZYB-grade HOPG from the present work (blue circles) (38) and from a previous study (open black circles) (34) are shown. The continuum results have comparable uncertainties. The gray band is the elastic HD stress-volume curve for uniaxial strain normal to the (100)_HD plane calculated using published second-order elastic constants (38). The band represents ±1% uncertainty in the HD density.