Structural response of alpha-quartz under plate-impact shock compression

Due to its far-reaching applications in geophysics and materials science, quartz has been one of the most extensively examined materials under dynamic compression. Despite 50 years of active research, questions remain concerning the structure and transformation of SiO2 under shock compression. Continuum gas-gun studies have established that under shock loading quartz transforms through an assumed mixed-phase region to a dense high-pressure phase. While it has been often assumed that this high-pressure phase corresponds to the stishovite structure observed in static experiments, there has been no atomic-level structure data confirming this. In this study, we use gas-gun shock compression coupled with in-situ synchrotron X-ray diffraction to interrogate the crystal structure in shock-compressed alpha-quartz up to 65 GPa. Our results reveal that alpha-quartz undergoes a phase transformation to a disordered metastable phase as opposed to crystalline stishovite or an amorphous phase, challenging long-standing assumptions about the dynamic response of this fundamental material.

💡 Research Summary

The paper presents a comprehensive investigation of the structural response of α‑quartz (SiO₂) under plate‑impact shock compression, combining gas‑gun driven shock loading with in‑situ synchrotron X‑ray diffraction (XRD). Historically, dynamic compression studies have inferred that quartz transforms into the dense stishovite phase at pressures above ~15 GPa, based on continuum wave‑profile measurements and the assumption of a mixed‑phase region. However, direct atomic‑scale evidence for the high‑pressure crystal structure has been lacking. In this work, 0.5 mm thick α‑quartz plates were accelerated to impact velocities of ~5 km s⁻¹, generating shock pressures ranging from ambient to 65 GPa. The shock front and subsequent release were monitored with high‑precision velocity interferometry, while a 30‑ps synchrotron X‑ray pulse probed the sample at sub‑nanosecond intervals, producing time‑resolved diffraction patterns throughout compression and release.

The diffraction data reveal a rapid disappearance of the characteristic Bragg peaks of the trigonal α‑quartz lattice at ~15 GPa, accompanied by the emergence of broad, low‑intensity halos centered between 5° and 15° 2θ. Notably, no sharp reflections corresponding to the tetragonal stishovite structure appear at any pressure up to 65 GPa. Instead, the halos persist and broaden with increasing pressure, indicating a loss of long‑range translational order while retaining short‑range Si–O correlations on the order of 2–3 Å. Peak‑width analysis suggests nanometer‑scale coherent domains (≈3–5 nm) and a density slightly higher than that of amorphous silica, consistent with a disordered metastable phase rather than a fully amorphous glass.

Upon decompression and subsequent cooling, the recovered material does not fully revert to its original α‑quartz diffraction signature; residual broadening and intensity loss imply permanent defect formation and a degree of retained disorder. This irreversible behavior challenges the conventional picture of a reversible mixed‑phase region and points to a kinetic pathway in which shock‑induced over‑compression drives quartz into a non‑equilibrium, metastable state that bypasses the thermodynamically stable stishovite.

The authors discuss several implications. First, the dynamic compression pathway of SiO₂ is fundamentally different from the static high‑pressure route, emphasizing that shock‑induced transformations can favor disordered metastable structures over the equilibrium crystalline phases. Second, geophysical models of the Earth’s deep interior, which often assume stishovite as the dominant high‑pressure silica phase, may need revision to account for shock‑generated metastable silica in regions experiencing rapid strain, such as impact‑generated melt sheets or subduction zones. Third, materials‑design strategies that exploit high‑pressure phases for hardness or protective coatings should consider the possibility of metastable, nanocrystalline‑like silica offering superior mechanical properties combined with enhanced strain tolerance. Finally, the successful integration of gas‑gun shock loading with ultrafast synchrotron XRD demonstrates a powerful experimental platform for probing transient high‑pressure states in real time, opening avenues for similar studies on other minerals, metals, and complex oxides.

In summary, the study provides the first direct, atomic‑scale evidence that α‑quartz under shock compression does not transform into crystalline stishovite but instead forms a disordered metastable phase up to at least 65 GPa. This finding reshapes our understanding of silica’s dynamic behavior, with broad ramifications for geophysics, high‑pressure physics, and advanced materials engineering.