In situ X-Ray Diffraction of Shock-Compressed Fused Silica

Abstract

Because of its widespread applications in materials science and geophysics, SiO_{2} has been extensively examined under shock compression. Both quartz and fused silica transform through a so-called "mixed-phase region" to a dense, low compressibility high-pressure phase. For decades, the nature of this phase has been a subject of debate. Proposed structures include crystalline stishovite, another high-pressure crystalline phase, or a dense amorphous phase. Here we use plate-impact experiments and pulsed synchrotron x-ray diffraction to examine the structure of fused silica shock compressed to 63 GPa. In contrast to recent laser-driven compression experiments, we find that fused silica adopts a dense amorphous structure at 34 GPa and below. When compressed above 34 GPa, fused silica transforms to untextured polycrystalline stishovite. Our results can explain previously ambiguous features of the shock-compression behavior of fused silica and are consistent with recent molecular dynamics simulations. Stishovite grain sizes are estimated to be ∼5-30 nm for compression over a few hundred nanosecond time scale.

Sound Velocities in Shock‐Synthesized Stishovite to 72 GPa

“…The 44-to 72-GPa stress range to which fused silica was shock-compressed in this study is above the stress at which fused silica is known to transform to stishovite (36 GPa; Tracy et al, 2018). Our measured longitudinal sound velocities range from 11.7-12.3 km/s at 44 GPa to 9.8-10.3 km/s at 72 GPa ( Figure 3).…”

“…The most likely explanation is that shock-synthesized stishovite is partially melting over this stress range. In the XRD experiments of Tracy et al (2018) up to 63 GPa, there is no clear evidence for the presence of melt, but small amounts of liquid would likely not be detected in such experiments. This leads to an eventual complete loss of shear stiffness and complete melting at 72 GPa, as discussed above.…”

“…The results from this and previous studies are compared to the longitudinal and bulk (C B ) sound speeds in stishovite and CaCl 2 -type SiO 2 at high temperature and pressure from theoretical calculations(Yang & Wu, 2014). The approximate lower limit of stishovite formation on the Hugoniot determined byTracy et al (2018) is shown by the vertical black line. The approximate lower limit of stishovite formation on the Hugoniot determined byTracy et al (2018) is shown by the vertical black line.…”

“…Both quartz and fused silica transform to dense, stiffer phases at high stress on the Hugoniot. In the gas-gun experiments of Tracy et al (2018), fused silica remained amorphous before transforming to untextured, polycrystalline stishovite with no evidence of preferred orientation above 34 GPa under shock compression. In the gas-gun experiments of Tracy et al (2018), fused silica remained amorphous before transforming to untextured, polycrystalline stishovite with no evidence of preferred orientation above 34 GPa under shock compression.…”

“…Eulerian longitudinal sound speed (C E ) measured in shock-compressed fused silica. The highest single-shock (SS) and double-shock (DS) stress investigated by in situ XRD(Tracy et al, 2018) are indicated. The highest single-shock (SS) and double-shock (DS) stress investigated by in situ XRD(Tracy et al, 2018) are indicated.…”