Of the over 6,000 confirmed and candidate extrasolar planets discovered to date, those 1-4 times the radius of the Earth are found to be most abundant. MgO (periclase), is expected to be a major component of the deep mantles of terrestrial planets and exoplanets. Its high-pressure transformation from a rocksalt (B1) structure to the B2 (CsCl) structure is expected to occur in rocky exoplanets greater than about 5 Earth masses in size. In this work, the structure and temperature of MgO upon shock compression over the 200-700 GPa pressure range was examined at the Omega-EP Laser facility. Laser drives of up to 2 kJ over 10 ns were used to shock compress single-crystal MgO. At peak compression, the sample was probed with He-α X-rays from a laser-plasma source. Diffracted X-rays were recorded on image plates lining the inner walls of a box attached to the target package. For each shot we measure pressure (velocity interferometry), density (x-ray diffraction) and shock temperature (pyrometry). We also probe orientation-dependence of the shock Hugoniot by conducting laser-driven decaying shock measurements of single crystal MgO [100], [111] and [110], and will discuss the importance of single crystal experiments to better improve phase diagram models of materials at extreme conditions.

Fast compression in the dynamic diamond anvil cell (dDAC) allows for the study of materials at intermediate strain rates that are not accessible using traditional static and dynamic compression techniques [1]. Previous dDAC studies revealed compression-rate dependent phenomena such as rate-dependent phase transformation pathways [2], the formation of metastable phases [3], and shifts in phase transition boundaries from their equilibrium positions [2,3,4]. The fast diffraction set-up at the Extreme Conditions Beamline (P02.2) at PETRA-III offers time-resolved X-ray diffraction with kHz data collection rates, which allows for phase transition boundaries to be accurately determined at compression rates up to ~1000 GPa/s. Future experiments at the European XFEL will allow for data collection rates up to 4.5 MHz, which will extend these studies to compression rates >100 TPa/s.

In order to develop a full understanding of phase transitions under dynamic compression, it is necessary to investigate sample behaviour on both atomistic (crystal structure) and microscopic (crystal morphology) length scales. This allows for kinetic parameters such as nucleation and growth rates to be determined. When crystallite of the high pressure phase have well-defined phase boundaries, imaging techniques can be used to visualize the growth of the new phase. The X-ray phase contrast imaging platform at P02.2 allows for the visualization of samples that are opaque to visible light, where the simultaneous X-ray diffraction measurements allow for pressure determination, phase identification, and structural refinement. Phase contrast imaging allows us to resolve phase boundaries for grains of similar Z, where conventional absorption-based imaging typically fails.

Here, we present results from X-ray imaging experiments on dynamically-compressed Ga (Fig. 1), where we have successfully imaged pressure-induced melting (Ga-I/liquid) and solidification (liquid/Ga-III). Using an imaging configuration in which the sample is positioned upstream from the focal spot of a CRL-focussed X-ray beam allows for the collection of ‘clean’ diffraction patterns with minimal contribution from the gasket material, and produces clearly-defined solid/liquid phase boundaries in the X-ray images.

[1] Jenei, Zs. et al. Rev. Sci. Instrum. 90, 065114 (2019). [2] Lee, G. W., Evans, W. J. & Yoo, C. S. Phys. Rev. B 74, 134112 (2006). [3] Chen, J. Y & Yoo, C. S. PNAS 108 7685-7688 (2011). [4] Husband. R. J. et al. ‘Compression-rate dependence of pressure-induced phase transitions in Bi’, submitted.

Extreme conditions are ubiquitous in nature. Much of the matter in the universe exists under high pressures and temperatures. Of interest, are the planetary interiors of the icy giants, Uranus and Neptune. Which have particularly complex magnetic fields [1].

To understand these complex magnetic fields the conditions and composition of icy giant planetary interiors need to be determined. The interiors of these planets are understood to contain mixtures of water, ammonia and hydrocarbons [2].

Under compression the phase diagram of ice is rather complex. With several phases determined and predicted under high pressure and temperature conditions [3]. High pressure ice above ~1500K and 50 GPa is predicted to undergo a superionic transition, where the hydrogen atoms diffuse into the oxygen sub- lattice [4,5]. These superionic phases are a possible source of the complex magnetic fields of both Uranus and Neptune.

Several high-pressure phases of water have been observed in the superionic region of the phase diagram. A body-centred cubic (bcc) phase, which if superionic would be analogous to ice X structure and with increasing pressure a phase transition to a face centred cubic (fcc) phase has been reported [5].

Experiments carried out at the MEC end station at the LCLS XFEL in December 2020, utilised reverberating shocks to compress water into Off-Hugoniot states within the superionic region of the ice phase diagram [6]. Liquid water samples were confined between a diamond ablator and a rear window, reaching P-T states ranging from ~40 GPa and 1200K to ~200 GPa and 4000K.

The bcc phase of ice has been observed from ~50 GPa and ~1200 K. A mixed phase region starting at ~90 GPa and ~2500 K, has been of observed with the bcc phase and a second phase. With increasing pressure the second phase becomes more prominent with the loss of the initial bcc phase.

The higher-pressure ice initially appears to be the fcc phase as described by Millot et al. However, further examination of the diffraction revealed misfits to the fcc lattice and a lack of refinement has suggested that that this may in fact be a different structure. The structure of this phase has yet to be determined. However, several candidates are proposed from predicted high pressure ices [7].

Ongoing work aims to determine these structures of ice under superionic P-T conditions and with comparison with simulation, understand the magnetic field behaviour of icy giant type planets.

Acknowledgements: The work was supported by the Helmholtz Association under VH-NG- 1141 and ERC-RA-0041. Use of the Linac Coherent Light Source (LCLS), SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC02-76SF00515. The MEC instrument is supported by the U.S. Department of Energy, Office of Science, Office of Fusion Energy Sciences under Contract No. SF00515.

[1] W.J. Nellis, J. Phys.: Conf. Ser. 950, 042046 (2017)

[2] M. D. Hofstadter et al., Ice Giants: Pre- Decadal Survey Mission Study Report, NASA-JPL report JPL-D-100520 (2017)

[3] C. G. Salzmann, J. Chem. Phys 150, 06091 (2019)
[4] I. A. Ryzhkin, Sol. Stat. Com. 56, 1 (1985)

[5] M. Millot et al., Nat. 569, 7755 (2019)

[6] M. Millot et al., Nat. Phys. 14, 3 (2018)

[7] A. Hermann et al., PNAS 109, 3 (2012)

Au has long been regarded as an important calibration standard in the high-pressure diffraction community, especially for experiments involving diamond anvil cells. The face centred cubic phase of Au is believe to be stable for hundreds of GPa at room temperature [1,2]. Recent dynamic-compression work has shown that the high-pressure behaviour of Au is not as simple at higher temperatures, and under laser-driven shock-compression, Au was found to transform, on-Hugoniot, from its ambient face centred cubic phase to a body centred cubic phase at 223 GPa before melting around 320 GPa [3].

As well as being used as a calibration standard, Au is also a commonly used material in target packages for laser-driven, dynamic-compression experiments. For experiments that explore the behaviour of various materials at the highest pressures and temperatures achievable (such as the experiments conducted at the National Ignition Facility or at the Omega laser facility) a layer of Au may be placed before the material of interest to act as a shield to prevent x-ray heating of the material of interest before the compression wave has reached the sample. For many of these experiments, the compression wave is not necessarily a shock wave, but the target may instead be ramp-compressed meaning that the compression state does not lie on the Hugoniot.

Given the frequent use of Au in diffraction experiments at extreme conditions, it is important that its high-pressure, high-temperature behaviour is well constrained off-Hugoniot so that we may correctly identify its contribution to diffraction data collected in this regime. To this end, a series of shock and ramp compression experiments have been conducted across various laser-compression platforms to explore the extent of the high-pressure bcc phase of Au. These experiments involve the compression of Au to previously unexplored pressures and temperatures, utilizing diffraction and velocity interferometry as the primary diagnostics. This talk presents a discussion of these results and reconciles this new, unpublished data with existing findings within the field.

This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

[1] Dubrovinsky, L., Dubrovinskaia, N., et al. (2007). Phys. Rev. Lett. 98, 045503
[2] Boettger, J.C. (2003) Phys. Rev. B. 67, 174107
[3] Briggs, R.J., et al., (2019) Phys. Rev. Lett. 123, 045701

Non-hydrostatic stress is known to change the evolution of unit cell parameters [1] and the compression of bond lengths and angles in the structures of crystals, e.g. [2]. The resulting modifications in the structures can lead to changes in the physical and thermodynamic properties of crystals, and thus change their thermodynamic stability. As a consequence, both reconstructive phase transitions [3] and displacive-type symmetry-breaking phase transitions [4] under deviatoric stress can occur at different temperatures and different mean stress than under hydrostatic pressure. Despite its importance, the effect of non-hydrostatic stress on crystal structures is still poorly understood, because it is challenging to perform experiments under controlled deviatoric stress conditions. On the other hand, mineral host-inclusion systems composed of a mineral entrapped inside another mineral provide the perfect example to characterize a crystal under deviatoric stress. Because the inclusion is entrapped inside another crystal, it will not be under hydrostatic pressure and the deviatoric stress imposed on it will be the result of the difference in the elastic properties of the two crystals and their mutual crystallographic orientations. In this contribution, we describe a methodology to characterize the effect of deviatoric stress on inclusion crystal structures using synchrotron x-ray diffraction, including how to deal with the experimental challenges in the collection of intensity data from a host-inclusion system, and the evaluation of the quality of the results. The quartz in garnet system is an ideal candidate for this study. Quartz has a simple and well-known structure, whose variation with pressure and temperature has been widely characterised, while garnet, being cubic, imposes an almost isotropic strain on the inclusions, thus providing a relatively simple case study. Furthermore, quartz is one of the most common mineral inclusions in different types of rocks, so it qualifies as an interesting case for geological applications.

[1] Bassett, W. A. (2006). J. Phys.: Condens-Mat., 18(25), S921.
[2] Gatta, G. D., Kantor, I., Ballaran, T. B., Dubrovinsky, L., & McCammon, C. (2007). Effect of non-hydrostatic conditions on the elastic behaviour of magnetite: an in situ single-crystal X-ray diffraction study. Phys, Chem. Miner., 34(9), 627-635.
[3] Richter B., Stünitz H. & Heilbronner R. (2016) J. Geophys. Res. -Solid Earth, 121, 8015-8033.
[4] Bismayer U., Salje E. & Joffrin C. (1982) J. Phys., 43, 1379-1388.

The strong interest in MOCl (M = Ti, V, Cr, Fe) compounds stems from their nonlinear optical properties in the IR band (CrOCl) [1], their use as intercalation compounds for cathode materials (FeOCl) [2], their use as parent structures for van der Waals heterostructures [3] and especially from their low-dimensional magnetic phenomena [4-6].

MOCl-type compounds are isostructural at ambient conditions with the space group Pmmn and consist of double layers of distorted MO₄Cl₂ octahedra, which are connected by van der Waals forces. It has been shown that the magnetic behaviour and the dimensionality of MOCl-type compounds is determined by orbital order of the 3d electron of the transition metal [4-6]. For M = V, Cr, Fe orbital order leads to strong intra- and interchain exchange couplings, which results in quasi-two-dimensional (2D) magnetic systems that exhibit antiferromagnetic (AFM) order at low temperatures [4,5]. The transition to the AFM state is characterized by a magneto-elastic coupling in the form of a monoclinic lattice distortion that lifts the geometric frustration of the magnetic order on the orthorhombic crystal structure as well as by the formation of an incommensurate modulation of the structure [4,5].

Applying hydrostatic pressure to those compounds allows us to continuously adjust the intra- and interchain exchange parameters through the modification of the octahedral geometry and the metal-to-metal distances. This provides a unique opportunity to study the interplay between magnetic order and pressure-induced structural changes in dependence of the electronic configuration of the transition metal within one single structure type. Pressurizing MOCl compounds to approximately 15 GPa leads to a normal-to-incommensurate phase transition, characterized by an optimization of the interlayer packing, which is not associated with changes in the electronic or magnetic structure [7]. This gives us, in addition, the possibility to investigate the effect of the high-pressure structural transition on the magnetic order and vice versa.

The high-pressure (HP) low-temperature (LT) single crystal X-ray diffraction experiments, which were conducted at P02.2/PETRA III above and below T_N,1bar for pressures up to 40 GPa and temperatures down to 6 K, provides an insight into the HP-LT mechanisms of FeOCl: The magneto-elastic coupling is governed by a monoclinic lattice distortion below T_N,1bar, whereas an interplay between lattice distortion and significant structural changes takes place above T_N,1bar. These changes enhance, from a geometrical perspective, superexchange interactions up to a pressure of ≈ 15 GPa where the structural HP phase transition gets superimposed on the further structural development. We will present the sequence of phase transitions and the structural development of FeOCl in detail and compare it, where applicable, with the quasi-2D compound CrOCl.

With the described approach and an in-depth analysis of structural changes, we aim at disentangling the magneto-structural correlations in the model system of MOCl as a function of composition, temperature and pressure in order to facilitate the understanding of low-dimensional magnetic systems in general.