Evidence of shock-compressed stishovite above 300 GPa

SiO2 is one of the most fundamental constituents in planetary bodies, being an essential building block of major mineral phases in the crust and mantle of terrestrial planets (1–10 ME). Silica at depths greater than 300 km may be present in the form of the rutile-type, high pressure polymorph stishovite (P42/mnm) and its thermodynamic stability is of great interest for understanding the seismic and dynamic structure of planetary interiors. Previous studies on stishovite via static and dynamic (shock) compression techniques are contradictory and the observed differences in the lattice-level response is still not clearly understood. Here, laser-induced shock compression experiments at the LCLS- and SACLA XFEL light-sources elucidate the high-pressure behavior of stishovite on the lattice-level under in situ conditions on the Hugoniot to pressures above 300 GPa. We find stishovite is still (meta-)stable at these conditions, and does not undergo any phase transitions. This contradicts static experiments showing structural transformations to the CaCl2, α-PbO2 and pyrite-type structures. However, rate-limited kinetic hindrance may explain our observations. These results are important to our understanding into the validity of EOS data from nanosecond experiments for geophysical applications.

www.nature.com/scientificreports www.nature.com/scientificreports/ explained by either sluggish kinetics as a result of the low compressibility of stishovite or the relatively small volume changes accompanying post-stishovite phase transitions, which cannot be readily distinguished from the stishovite Hugoniot 24 .
Until now, the highest pressures for which in-situ structural information of stishovite was obtained are from static compression experiments with a diamond anvil cell at 128 GPa 5 and there are no structural observations from any shock-compression experiments. The high initial density and low compressibility allowed us to shock-compress stishovite in the solid phase to pressures above 300 GPa. The concurrent use of the brilliant and coherent X-rays at the Linac Coherent Light Source (LCLS) and SPring-8 Angstrom Compact free electron LAser (SACLA), enabled the determination of the structural response under shock loading on the lattice-level of stishovite at these conditions for the first time.

Results
The experiments were carried out at the Matter at Extreme Conditions (MEC) end station of the LCLS-and the BL3:EH5 end station of the SACLA X-ray Free Electron Laser (XFEL). The experimental setups are shown in Fig. 1. Polycrystalline stishovite was synthesized in a large volume press at the P61B end station of PETRA III at the German Electron-Synchrotron (DESY) with an initial density of ρ 0 = 4.30 g/cm 3 (Table 1, run795997). Stishovite samples were cut and polished to 35 μm. Targets were subsequently glued to 50 μm polyimide (kapton) tape and shock compressed using optical drive lasers. Derived from hydrodynamic simulations, steady uniform pressure conditions within the sample are achieved within a time span of ~1-5 ns for the high pressure drives (see Supplementary Material). The samples were probed with an 11.2 keV (LCLS) and 11 keV (SACLA) XFEL X-ray pulse when the majority of it was in the compressed state. This was determined from the shock breakout via the velocity interferometer system for any reflector (VISAR). The VISAR measured the rear free surface velocity of stishovite for each shot from which particle velocity (U p ), shock wave velocity (U s ) and pressure in the shocked state were determined (see Supplementary Material).
The shock and particle velocities (   Table 1. Pressures obtained from the VISAR agree within 2-6% to EOS data from literature 24 and density functional theory (DFT) simulations (see Supplementary Table S2). Bragg reflections of stishovite are observed upon shock loading up to 336 ± 13 GPa (Fig. 2). The stishovite structure from the refined diffraction pattern at 18 ± 2 GPa reveals a relative change in volume of V/V 0 = 0.95 at a density of 4.51 g/cm 3 . The achieved pressure is well within the stishovite stability field found in static compression experiments 2,5,10,27-30 . However, a departure from equilibrium behavior is observed at higher pressures: at 123 ± 5 GPa, Bragg reflections of compressed stishovite are still apparent and refined lattice parameters reveal a relative volume change of V/V 0 = 0.80 at a density of 5.35 g/cm 3 . These conditions are within the equilibrium phase stability field of α-PbO 2 type silica [10][11][12][13][14][15][16] . At higher pressures, Bragg reflections of compressed and ambient (marked with an asterisk) stishovite are observed. The refined stishovite reflections at 301 ± 12 GPa indicate a volume change of V/V 0 = 0.69 at a density of 6.3 g/cm 3 . At 317 ± 15 GPa, density and volume was determined through the VISAR and indicates a relative volume change of V/V 0 = 0.67 and a shock-density of 6.4 g/cm 3 . At 336 ± 13 GPa, the highest achieved pressure within stishovite in this study, exhibits a volume change of V/ V 0 = 0.66 at a density of 6.44 g/cm 3 .
The pressures, determined from VISAR, as a function of density, obtained from XRD, are shown in Fig. 3. They are in excellent agreement with literature data from shock compression experiments [24][25][26] and below 128 GPa comparable to static compression experiments 5,27-30 . We estimated the equation of state (EOS) of stishovite by fitting the data with the EosFit software 31 to the third-order Birch-Murnaghan equation of state 32 and, to compare to static data, corrected the shock wave data to a 300 K isotherm. The 300 K isotherm was calculated by subtracting the thermal pressure from the shocked state at fixed volume 27 . We determined the pressure correction by calculating the temperature along the Hugoniot 26 (see Supplementary Material) and the pressure difference between the Hugoniot and 300 K isotherm using the Grüneisen parameter  www.nature.com/scientificreports www.nature.com/scientificreports/ well as EOS studies of stishovite at lower pressures from static DAC experiments (K 0 = 309.9 GPa, K 0 ′ = 4.59 GPa 5 ; K 0 = 294 GPa, K 0 ′ = 4.85 GPa 27 ).
Pressure evolution of the SiO 2 unit-cell parameters from refined XRD pattern are shown in Fig. 4. Here, a deviation from static data 5,34 is observed for the lattice parameter a at pressures exceeding ~60 GPa (Fig. 4a). The lattice parameter ratio c/a rises with increasing pressures from ~0.638 at ambient conditions to ~0.650 at 123 GPa (Fig. 4b). In this regime the compressibility of the lattice parameter a is higher than for lattice parameter c. However, at ~123 GPa, this trend is reversing and lattice parameter c seems to be more compressible than lattice parameter a with a decreasing lattice parameter ratio c/a from ~0.650 at 123 GPa to ~0.634 at 336 GPa.

Discussion
The results of this study on shock compressed stishovite observed by in-situ X-ray diffraction differ strongly from static compression experiments at respective pressures (Fig. 5). X-ray diffraction studies on stishovite with a DAC show a distinct phase transformation at ~60 GPa to the CaCl 2 type silica 5,34 . This is, however, in contrast to previous shock compression experiments, where the displacive transition towards CaCl 2 does not appear in continuum Hugoniot data [24][25][26] .
Our experimental data is in good accordance with the stishovite EOS of static and shock experiments 5,24 (Fig. 3) and the bulk moduli from our experiment agree within 1-4% to literature data 5,18,24,27 . It seems that this is not only valid at lower pressures, but continues to be apparent at multi-Mbar pressures 25,26 . Past studies have interpreted the lack of evidence for a phase transition as i) not being in the right pressure regime, ii) sluggish kinetics, or iii) relatively small changes in structure and energy attending a phase transition and not detectable with diagnostics 24 . Up to now, however, there is no lattice-level structural information for stishovite under shock compression. By coupling XFEL sources to high-power laser systems, we can now reveal subtle changes in lattice structure during the shock response of materials. The femtosecond diffraction contradicts i) and iii), since Bragg reflections of stishovite can still be observed in the shock compressed state along the respective Hugoniot at pressures exceeding 300 GPa. Furthermore, the measured lattice parameters do not indicate a change in crystal structure as in a first order transition, in contrast to static data 5,34 (Fig. 4). The stishovite structure of our shock compression  www.nature.com/scientificreports www.nature.com/scientificreports/ experiments above 300 GPa is therefore in vast contrast to static compression experiments: at 123 GPa, there is no indication of a α-PbO 2 phase, which was seen in DAC experiments with SiO 2 10,12 and at experimental peak pressures of 301 GPa to 336 GPa there is no indication of the pyrite structure 17 at these timescales.
The differences in relative compression between lattice parameter a and c in our study indicates an anisotropic compression. As predicted by density functional theory, a is initially more compressible than c but the anisotropic behavior of a becomes less pronounced at higher pressure 35 . The relative lattice parameter compression (Fig. 4) even suggests, that this trend eventually reverses and that the polycrystalline stishovite was preferentially orientated within the c-direction during shock transit. Additionally, the broadening of the Bragg reflections in Fig. 2 indicate a pressure gradient and non-hydrostatic stress, with the accuracy of the measured d-values being diminished. It is well known from static (DAC) experiments and molecular dynamic simulations, that (non-) hydrostaticity can fundamentally change the onset, or presence, of phase transitions 36,37 . Previous studies on stishovite in static experiments show, that non-hydrostaticity produces large deviatoric stresses as a result of the high shear strength 7,9 and it was observed, that the preferred orientation of stishovite under compression is the crystallographic c-direction 38 . However, it was suggested, that non-hydrostatic conditions in DACs actually decrease the pressure-induced onset of a ferroelastic phase transitions from the tetragonal (stishovite) to the orthorhombic (CaCl 2 ) structure 8 . This might be different for shock compression experiments in which it is argued, that non-hydrostatic stresses may displace the equilibrium boundaries to higher stresses 39,40 .
In addition to anisotropic compression, due to the absence of phase transformations, shock temperature rises with increasing pressure to adjust to the increase of internal energy due to the compression work 26 . Our calculations indicate, that stishovite is experiencing temperatures ranging from 324 up to 4757 K over a pressure range of 18 to 336 GPa during shock loading (Fig. 5). However, because of the nanosecond timescales during dynamic loading and accommodation of rate-limiting kinetic hinderances, effects can result in significant shifts of equilibrium phase boundaries such that transitions may not be observed or may require significant overpressure. This is a known deviation between shock and static experiments [41][42][43][44][45][46] . However, the extent to which these pressure induced phase transformations can be hindered seems to be much more pronounced for stishovite than for other materials, presumably due to its high initial density and low compressibility. Our results are unexpected as one would not presume crystallization to be inhibited, at least for the CaCl 2 structure, which is differentiated from stishovite only by a small diffusionless displacement of oxygen ions.
The pressure and temperature range encompassed in this study is comparable to Super-Earth (1-10 M E ) interior conditions (Fig. 5). However and contrary to our results, it is well established from experimental observation that a stable stishovite structure within the Earth's mantle at pressures exceeding ~60 GPa is improbable 47 . This can be explained by the short timescales (few ns) involved during our shock compression experiments, which can prevent the observation of long time scale phase transitions as in equilibrated planetary interiors. Indeed, certain phase transitions require at least several nanoseconds to be triggered and can only be observed at longer time scales 20 . Therefore, further time dependence studies under shocked SiO 2 should be foreseen to bring possible constrains on its physical and chemical properties inside Super-Earth interiors.
In summary, we have determined the shock response of stishovite under shock loading via XRD from the LCLS and SACLA XFEL sources. Additional velocimetry diagnostics confirmed shock data from literature, and provided evidence of the departure from equilibrium behavior of stishovite at pressures exceeding 300 GPa -an unprecedented result. A derived stishovite structure from this study at pressures up to 336 GPa implies a skipping of three different second-order equilibrium phase transitions (CaCl 2 , α-PbO 2 and pyrite-type structure) illustrating how complex structural changes are hindered during dynamic compression experiments.  18,26,47 . Shown is experimental data where the structure of SiO 2 was resolved. Indicated are the different equilibrium phase stability fields and the geotherms of Earth and terrestrial planets with a mass of 10 M E . Red diamonds are data from this study and the dashed magenta line indicates the Sesame stishovite EOS 7360 50 . Shown is data with quartz or fused silica as a starting material: black and grey squares from 2 , circles from 10 , right triangles from 12 , lower triangles from 15 and stars from 17 as well as stishovite as a starting material: white squares 4 , upper triangles 27 , crosses 34 and left triangles 28 . Furthermore melting lines are indicated: brown 26 , orange 51 , pink 52 , blue 53 , green 54 and light blue 55,56 . www.nature.com/scientificreports www.nature.com/scientificreports/

Methods
Polycrystalline stishovite was synthesized using the large volume press housed at the PETRA III end station P61B at the German Electron-Synchrotron (DESY). Cylindrical pieces (3 mm in height, 2.5 mm in diameter) were cut from fused silica rods to fit precisely into a Pt-capsule. The Pt-capsule was placed at the center of a 18 mm edge length Cr 2 O 3 doped MgO octahedral multi-anvil assembly. The octahedral assembly was heated to 1500 K for one hour after compressed to a maximum pressure of 12 GPa (see Supplementary Material). Samples were cut and polished to 35 (±1) μm and glued to 50 μm thick black polyimide (kapton) tape. At LCLS, a Nd:glass optical laser (527 nm, 10 ns, quasi flat top pulse) in combination with 250-μm diameter phase plates was used to launch a smooth and well defined laser pulse onto the targets. At SACLA, a ceramic YAG optical laser (532 nm, 5 ns, quasi flat top pulse) in combination with 300-μm diameter phase plates was applied. Detailed information about the experimental platforms can be found in the references 48,49 . Five different maximum drive laser energies were used for the experiments: 19.1 J and 22.1 J at SACLA, and 48.6 J, 50.6 J, 51 J at LCLS. Free surface velocities were recorded through the VISAR and shock velocities were calculated from velocimetry and diffraction data (see Supplementary Material). Two VISAR legs where employed to resolve fringe-jump ambiguity from the compressed sample velocity traces. The LCLS and SACLA FEL sources provided quasi-monochromatic (dE/E = 0.2-0.5%), coherent, 11.2 keV (LCLS) and 11 keV (SACLA) X-ray pulses of ~20 fs (LCLS) and ~7 fs (SACLA) duration. The XFEL beam at LCLS was focused to 50 × 50 μm 2 onto the target and centered to the focal spot of the drive laser. At SACLA, the XFEL beam was focused to 10 × 30 μm 2 . The LCLS experiment was conducted in transmission geometry normal to the target whereas the SACLA experiment was conducted in reflection geometry with a grazing-incidence angle of 18° between sample surface and XFEL beam (Fig. 1). The XFEL probed the samples at shock breakout (±0.5 ns), hence when the sample was completely compressed. Four Cornell-SLAC Pixel Array Detectors (CSPADs) at LCLS and a large flat panel detector (CMOS camera coupled with an X-ray scintillator) at SACLA recorded the resulting diffraction from the XFEL probe beam during shock compression. The diffraction images were azimuthally integrated and are shown as XRD pattern in Fig. 2.