$In situ$ observation of a phase transition in silicon carbide under shock compression using pulsed x-ray diffraction

The behavior of silicon carbide (SiC) under shock compression is of interest due to its applications as a high-strength ceramic and for general understanding of shock-induced polymorphism. Here we use the Matter in Extreme Conditions beamline of the Linac Coherent Light Source to carry out a series of time-resolved pump-probe x-ray diffraction measurements on SiC laser-shocked to as high as 206 GPa. Experiments on single crystals and polycrystals of different polytypes show a transformation from a low-pressure tetrahedral phase to the high-pressure rocksalt-type (B1) structure. We directly observe coexistence of the low- and high-pressure phases in a mixed-phase region and complete transformation to the B1 phase above 200 GPa. The densities measured by x-ray diffraction are in agreement with both continuum gas-gun studies and a theoretical B1 Hugoniot derived from static-compression data. Time-resolved measurements during shock loading and release reveal a large hysteresis upon unloading, with the B1 phase retained to as low as 5 GPa. The sample eventually reverts to a mixture of polytypes of the low-pressure phase at late times. Our study demonstrates that x-ray diffraction is an effective means to characterize the time-dependent structural response of materials undergoing shock-induced phase transformations at megabar pressures.

Figure 1

Schematic of the experimental setup. Samples were mounted in a translatable cassette. The x-ray beam (xFEL) was incident at ${15}^{\circ }$ relative to target normal and the two nanosecond laser arms were oriented at $6^{\circ }$ and ${25}^{\circ }$. Diffracted x rays were recorded on CSPAD detectors. The target package, illustrated on the right, shows the CH ablator, SiC sample, and LiF window. The VISAR was oriented normal to the sample, focused on the LiF-SiC interface.

Figure 2

Representative VISAR wave profile collected using drive conditions of the $\sim 114$-GPa polycrystalline time series. The time axis is relative to shock entering SiC. The plateau associated with the phase transformation is at a SiC-LiF particle velocity of $\sim 3.1$ km/s, consistent with SiC stress of $\sim 100$ GPa. Inset: Schematic wave diagram illustrating the elastic wave (red line) arrival along with a second elastic reverberation due to an interaction between the elastic release from the LiF window and the oncoming plastic wave (black line).

Figure 3

(a) Integrated x-ray diffraction patterns for time series collected at a peak stress of $\sim 206$ GPa for polycrystalline 3C starting material. X-ray probe times after the shock enters SiC are listed at the right. B1 peaks are marked with asterisks. Ambient 3C peaks are indexed in the preshot pattern. The feature marked with an inverted triangle at 2.66 Å arises due to stacking fault disorder (Supplemental Material, Fig. S1 [32]). Peaks from the back-transformed 3C phase are indexed in the top pattern. (b) Integrated diffraction patterns for time series collected for a peak stress of $\sim 114$ GPa. B1 peaks and compressed 3C peaks are marked with asterisks and vertical lines, respectively. X-ray probe times after shock enters SiC are listed at the right.

Figure 4

Integrated diffraction patterns for time series collected at a peak stress of $\sim 175$ GPa for single-crystal $\alpha$-SiC starting material (4H). X-ray probe times after the shock enters SiC are listed at the right. B1 peaks are marked with asterisks. For the latest probe time (43.1 ns), the pattern can be indexed with a combination of 3C (red) and 4H (blue) peaks, indicated by tick marks above the pattern.

Figure 5

The $d$ spacings for polycrystalline SiC determined from fits to x-ray diffraction patterns for the time scan shown in Fig. 3 as a function of the time after the shock has entered the SiC. The trends are the result of a linear extrapolation between the discrete probe times (hash marks). Shaded regions indicate the uncertainties in $d$ spacings and encompass the range of $d$ spacings that arise from sampling a nonhomogenous stress state after breakout. The gray area contains results from shots collected on compression, prior to breakout. Ambient 3C peak positions are shown by black triangles at the left.

Figure 6

The present results (red and blue symbols) compared to continuum gas-gun results (purple symbols) [4, 5]. Static 300 K studies are shown as open green symbols; results of Yoshida et al. [16], Daviau et al. [17], Miozzi et al. [19], and Kidokuro et al. [18] are represented by circles, squares, diamonds, and triangles, respectively. The present Hugoniot data are more consistent with the gas-gun results of Vogler et al. [4] than of Sekine et al. [5] and lie close to the calculated theoretical Hugoniot (gray shaded region). The densities of the two components of the mixed-phase region lie close to the 3C Hugoniot curve and the calculated B1 Hugoniot. The 300 K B1 equation of state (EOS) and the 3C EOS calculated from static compression data [19] are shown as dashed black curves.