Diffraction on release

Using an experimental setup similar to Gleason et al.18, we explore the changes in the atomic structure of SiO 2 using transmission in situ XRD with 8 keV X-rays from the XFEL at the Matter in Extreme Conditions (MEC) end-station of the LCLS. Laser ablation from a frequency-doubled Nd:Glass laser system was used to launch a compressive (or shock) wave over the pulse duration, 10 ns. It takes ~10–11 ns for this compressive wave to traverse the sample. Therefore any XRD pattern collected at an X-ray probe time longer than this will include a sampling of material experiencing release (Fig. 1a) due to completion of shock transit and drive laser cessation. Diffraction data presented here were collected during the passage of a shock-release wave. Phase space accessed in this experiment (Fig. 1b) shows the quasi-isentropic release paths and approximate pressure–temperature conditions achieved between 12 and 30 ns.

Fig. 1 Experimental configuration and explored phase space. a Target schematic for sample during the shock-release process. During the onset of release, newly formed grains of stishovite (green sphere-like features) dissolve over a few nanoseconds leaving behind diaplectic glass. b Equilibrium phase diagram32 of SiO 2 showing high-pressure polymorphic phase boundaries and melt curve (black). The fused silica Hugoniot (gray) using data18,33,34. Red points are maximum pressure, temperature conditions achieved for particular ablation drive laser parameters as determined from velocimetry records. The error bars include scatter in the measured transit times, uncertainty in the total sample thickness, uncertainty in the pressure-irradiance scaling law18. Isentropic release paths (blue arrows, determined using Sekine et al.35) show the approximate conditions achieved in this experiment at late time delays, e.g., 12–30 ns (i.e., during release), and release shock temperatures are determined from post shock temperatures for fused silica36 Full size image

The first sharp diffraction peak (FSDP) from ambient fused silica (starting density, 2.20 grams per cubic centimeter) is centered at 2θ ~21.6o (Fig. 2, gray curve), consistent with previous work (e.g., ref. 19). For each peak pressure set, 33.6 ± 5.0, 18.9 ± 3.0, 7.6 ± 1.2, and 4.7 ± 0.8 GPa, XRD patterns are collected on release (i.e., at time delays greater than ~11 ns), Fig. 2. XRD traces collected on release show phases inherently transitioning to lower pressure. Previous work18 shows fused silica transforming to randomly oriented, nanometer-sized grains of stishovite on compression. However, in the lowest two pressures, 4.7 and 7.6 GPa, diffraction shows super-positioning of stishovite peaks with a strong diffuse signal. This diffuse feature is interpreted to reflect the random network of compressed amorphous SiO 2 . At these pressures on release, the stishovite grains are no longer resolvable, and the diffuse feature progressively shifts to lower 2θ indicating the glass is at a lower pressure state at each sequential time-slice. At 18.9 GPa, the 6.3 ns trace shows fused silica converts to stishovite on compression and a diffuse signature centered near 22° 2θ confirming the velocimetry data that the compressive wave has not yet transited the entire sample. For this pressure, we do not have data at the moment of final compression (i.e., ~10 ns). On release from 18.9 GPa stishovite peaks first shift to lower 2θ as the signal of the diffuse feature increases. We interpret this diffuse feature again as a compressed glass, which then progressively shifts to lower 2θ with decreasing pressure. Using a method of background subtraction and normalization19 to estimate phase fraction amorphous vs. crystalline, we find the final compression, highest pressure trace shows full conversion to stishovite on compression and the crystallinity persists for at least 7 ns after onset of release. This 7 ns is markedly longer than any other pressure set which show stishovite crystals only persisting for a few to fractions of a nanosecond. At ~4 ns after onset of release there is an increase in signal of a diffuse feature reflecting the random network structure of a compressed glass. The peak position of this diffuse feature does not change as pressure decreases.

Fig. 2 Multiplot of XRD data. Stishovite peaks are labeled at the top; ambient condition positions (gray dashed lines). Traces are clustered according to maximum applied pressure with time delays listed on shock-release. Offset along the y axis and color scheme of the traces are arbitrary to enable viewing clarity. Discontinuities in the traces are seen at 32.5°, 46.0° and 58.0° 2θ due to spacing between the mosaicked active areas of the detectors Full size image

XRD collected at time infinity (i.e., from recovered debris) still shows a diffuse signal with an FSDP position recording a smaller d-spacing than the starting fused silica, indicative of a compressed amorphous material (Supplementary Fig. 1). The fused silica Hugoniot is plotted in pressure-entropy space (Supplementary Fig. 2) to determine what peak shock pressures intersect the liquidus. We find our highest pressure point of 33.6 GPa may cross the melt bound on release, therefore we cannot rule out a melt state as a contributor to the diffuse signal seen at 25.4 ns; see Supplementary Discussion.

If the peak positions from the amorphous material FSDPs collected on release are plotted as a function of time, there is a striking trend difference between the lowest three pressures, up to 18.9 GPa, compared to the highest pressure, 33.6 GPa (Fig. 3a). Between 4.7–18.9 GPa immediately at onset of release, the FSDP d-spacing decreases sharply at a rate of ~0.05 Å/ns, trending toward the starting FSDP position of ambient fused silica 4.20(1) Å. However, the 33.6 GPa data show a nearly constant FSDP at 3.1 Å up to 25 ns after the onset of release. The FSDP of amorphous shock recovered debris from 33.6 GPa is at a smaller d-spacing, 3.36(2) Å, compared to that of the starting material. The X-ray structure factor S(Q) for the starting fused silica and the recovered material was determined from XRD collected at 25 keV (Beamline 12.2.2, ALS) (Fig. 3b). Ambient condition S(Q) compares well with previous work20. The average pair correlation functions, G(r) for these data (Supplementary Fig. 3) are obtained from the Fourier sine transform of S(Q). Though G(r) does not provide a direct measure of Si-O coordination, it can constrain the nearest-neighbor bond lengths. Ambient starting fused silica shows a <Si-O> bond distance of 1.58(2) Å consistent with a 4-fold coordinated glass. Interestingly, the shock recovered material shows a <Si-O> bond distance of 1.68(5) Å—a marked increase in length consistent with a mixture of 4 and 6 coordination5,21,22.