Nanosecond structural evolution in shocked coesite

Abstract

The phase transitions in minerals under shock are crucial for understanding meteorite impact history. Recent time-resolved x-ray diffraction (XRD) studies on silica shocked to 65 GPa proposed the formation of different high-pressure phases between fused silica and quartz. Furthermore, the dynamics of silica behavior under higher pressure need to be investigated, particularly during nonequilibrium superheating before melting. This study examines the time-dependent response of coesite, using laser-driven shock coupled with fast XRD and molecular dynamics simulations with our recently developed machine learning interatomic potential. Our results reveal a transient dense supercooled liquid crystallizes into a semi-disordered d-NiAs-type silica, followed by transforming into either seifertite or stishovite, depending on the pressure. Instead of thermodynamically stable quartz, a back-transformation to coesite phase is identified after release. The complicated phase evolution pathways in shocked coesite provide deeper insights into the high-pressure silica phases observed in the meteorite bombardments on the early Moon, Mars, and Earth.

Fig. 1.. Experimental configuration and typical XRD from laser shocked coesite at the earliest time delay.

(A) Experimental configuration of the XRD and VISAR measurements at the SACLA BL3 beamline. A typical shock target consists of a polystyrene (30 μm) ablator, and a polycrystalline coesite slab (30 to 45 μm). The shockwave launched by the drive laser goes through the layered target and its breakout timing is recorded by VISAR. The diffraction patterns (example, #974560) are collected by a flat panel detector (FPD). The XFEL probe time delay (t_D) was counted from the timing of the drive-laser irradiation (t₀, Materials and Method and fig. S2). (B) Two representative 2D diffraction patterns were collected at the estimated pressures (P_EOS) of 131 and 42 GPa, with diffraction peaks from the uncompressed coesite masked out. P_EOS are determined by the Hugoniot density-pressure relation (33). The continuous diffraction rings seen at 131 GPa and a delay of 5 ns correspond to the (101), (102), and (110) reflections of d-NiAs-type SiO₂. These smooth diffraction rings are also observed at 91 and 58 GPa. In contrast, a spotty pattern is observed at 41 GPa with a t_D = 6.5 ns. Two separate azimuthally dependent spotty diffraction peaks at 37.4° and 50.0° correspond to the (101) and (102) reflections of d-NiAs-type SiO₂, respectively. The peaks marked with the green dashed-circle [pink asterisk symbol in (C)] belong to the unknown silica polymorphs. (C) Azimuthally integrated diffraction patterns as a function of compressed pressure. The diffraction spots from the uncompressed coesite portion are masked out for clarity.

Fig. 2.. The time-resolved structural evolution of shocked coesite at 131 GPa with a time delay ranging between 4.5 and 10 ns.

Integrated diffraction patterns with the initial uncompressed coesite diffraction masked out for clarity. The dashed-gray line indicates the breakout timing of the shock front at the right free surface of coesite. More details about the shockwave travel times in the targets are summarized in table S2. Two representative azimuthally unwrapped XRD patterns with time delays of 5 and 8 ns are presented in Fig. 1B and fig. S5, respectively. The diffractions of the d-NiAs-type structure and stishovite show continuous rings, while the seifertite and released coesite show spotty patterns indicating fast grain growth procedures. The stishovite diffraction signals at 8 ns are marked with a green line.

Fig. 3.. MD simulation of the shocked coesite formation.

(A) Comparison of XRD patterns between experimental observations at 131 GPa and ML-MD simulations (z axis) at 125 GPa. The simulation XRD at 20 ps indicates that the coesite system liquifies (becomes disorder) under shocking, corresponding to the experimental diffraction at the earliest time t_cal = t_exp. = 0 ns (t_D = 4.5 ns). Subsequently, new peaks appear afterward and the width narrows for both the simulated and experimental results. (B) Time evolution of grain density, crystallization fraction, and coordination number (CN) at pressures of 113 GPa (brown) and 125 GPa (cyan) are compared. The corresponding ultrafast grain merging or recrystallization following the nucleation displays a range of 0.55 to 0.7 ns at 113 GPa and 0.3 to 0.4 ns at 125 GPa. (C) Three atomistic views of the shocked coesite (383,999 atoms) for nucleation (100 ps), avalanche growth (0.3 ns), and coalescence (1 ns). The gold and red colors denote Si and O in the crystals respectively, whereas the disordered and boundary atoms are green. (D) Representative atomic structures at 1 ns (pink and blue spheres are oxygen and silicon atoms, respectively). At the GB, amorphous SiO₂ show complex polymorphs (left): fivefold oxygen (light pink), sixfold oxygen (yellow), and sevenfold oxygen (red). Inside grains, majority of SiO₂ show semi-disordered NiAs-type structure (fig. S10), with minority of distorted-seifertite or distorted-stishovite structure (last two panels).

Fig. 4.. Pressure-temperature diagram of shocked coesite.

The P-T relation of shock Hugoniot (cyan arrow) for polycrystalline (Pc) coesite experimentally determined in a previous study, compared with simulated Hugoniots (solid green and red circles) at 1 ns (17). A single-crystal (Sc) coesite supercell was used for simulation, with shockwaves along the x (green) or z (red) direction. Simulated structural evolution crossing between a liquid Hugoniot (open red symbols) and crystallized Hugoniot (solid red symbols) from 20 ps to 1 ns is shown in red dashed arrows. The laser-driven shock data of Pc coesite is shown in purplish gray (open diamond) (17). Two melting lines predicted by Simon fitting and two-phase MD (TP-MD) simulation are in the light-brown dashed line (16, 48). The phase boundaries for quartz, coesite, stishovite, CaCl₂ type, and seifertite are cited from the literature (1, 2, 49).

Fig. 5.. Density path of shocked coesite with time.

The density was estimated from the XRD data for the detected phase and time corresponds to the XFEL time delay. Data are listed in table S1. The colors correspond to the shots, and the square, triangle, diamond, and star symbols are phases of the d-NiAs-type phase, seifertite, stishovite, and coesite, respectively.