Session P23: Materials in Extremes: Phase Transitions

Live

Accurate equations of state (EOS) and phase diagrams for silicate materials are key to understand the interiors of Earth, Super-Earths and the cores of giant planets. While recent shock wave experiments have provided new data on the principal Hugoniot curve, accessing off-Hugoniot states is much more challenging. Ab initio computer simulations offer the possibility to complement the experiments in this regard. Here we report results of an extensive set of ab initio simulations of MgO in solid B1 and B2 and liquid phases over a wide range of parameters relevant for planetary interiors. Using molecular dynamics methods in combination with a thermodynamic integration technique, we derive a consistent EOS including free energy and entropy information. We will discuss the relative stability of the different phases and compare methods to quantify anharmonic effects in the solid phases at elevated temperatures.   

Live

Electrical conductivity of materials under extreme conditions may provide the necessary information to obtain bulk temperatures, the location of phase boundaries, and pertinent knowledge for ongoing electromagnetic responses for planetary interiors. We introduce a means to study the electrical conductivity of metals, in this study tin, with an easily accessed melt boundary between 45 and 70 GPa on the principal Hugoniot. A plate impact methodology is implemented to drive the tin sample to high pressure and temperature states while the electrical resistivity and conductivity of the sample are recorded. We observe a stark jump in the electrical resistivity and conductivity associated with the melt boundary, further constraining the tin melt boundary. Through the Wiedemann-Franz law, we will address implications of whether the law holds and what the predicted temperature and thermal conductivity are under shock conditions.   

Live

Ramp-driven compression-release experiments offer possibilities to explore material response under conditions distinct from those accessed by shock-driven loading conditions. For a material undergoing phase transformation, the problem of material model identification from experimental measurement is made substantially more complex by the need to untangle not only elasticity and plasticity, but also features introduced by the phase transformation. Tin exhibits a complex phase diagram within a relatively accessible range of temperature and pressures and the characterization of its phases is considered an open problem with significant scientific merit. Moreover, under extreme loading conditions, equilibrium phase transition modeling appears insufficient, suggesting the presence of important kinetic processes. In this study, we construct a full forward model of the experiment and simulation results are compared to recent observations of Sn response in ramp-driven compression-release experiments.   

Live

Dynamic responses of SiC under compression are investigated using large-scale molecular dynamics simulations.The ramp wave compression applied to single crystal 3C-SiC uses ramp rise times from 10 to 100 ps. Wave-free quasi-isentropic compression loading is also applied with strain rates varying from 10⁸ to 10¹¹ s^-1. Results show that the plastic deformation and phase transition are strongly strain-rate dependent. With increasing strain rate, the threshold strain and longitudinal stress for deformation twinning is anisotropically increased. The threshold longitudinal and shear stresses triggering plasticity are lowest in [001] direction, followed by [110], and highest in [111] SiC at the same strain rate. The threshold pressure for structural phase transition from zinc-blende (ZB) to rock-salt (RS) structure increases with the applied strain rate. As a result, the transition from ZB to RS structure is incomplete and inhomogeneous mixed-phase structures are observed over a wide range of applied stresses, even up to ~180 GPa, which agrees well with experimental observations.   

Live

Multi-megabar (Mbar) phase transitions of MgO are essential for planetary and high-pressure research but have remained formidable for both theory and experiment (as an example, previously reported[1,2,3] B1–B2 transition pressures vary by ~20%). In this presentation, we show our latest research from first-principles calculations based on density functional theory using an optimal exchange-correlation functional,[4] quantum Monte Carlo,[5] and quantum molecular dynamics.[6] By accurately taking the cold curve and ion and electron thermal contributions into account, our data provide a theoretical benchmark for the multi-Mbar phase diagram of MgO.

[1] J. Bouchet et al., Phys. Rev. B 99, 094113 (2019).
[2] N. Dubrovinskaia et al., https://arxiv.org/abs/1904.00476 (2019).
[3] F. Coppari et al., Nat. Geosci. 6, 926 (2013).
[4] S. Zhang et al., Bull. Am. Phys. Soc. 64, BAPS.2019.MAR.P17.00003 (2019).
[5] S. Zhang et al., Bull. Am. Phys. Soc. 65, BAPS.2020.MAR.M03.00002 (2020).
[6] R. Paul et al., Phys. Rev. Lett. 122, 125701 (2019).   

Live

The study of metal ejecta microjet interactions has broad applicability to fields ranging from particle dynamics modeling to materials physics [1]. Recent experiments at laser facilities have begun to study microjet formation [2], but there exist few examples of interaction studies. We present the first movies of jet-jet interactions from experiments performed on the OMEGA EP laser. Lasers drive shocks through two tin metal foils with angular trenches machined on their back surfaces. As the shocks break out, the trench features invert to form planar jets moving towards each other at speeds of several km/s. We use point-projection radiography to image the interacting jets. Jets emerging from tin releasing into solid have densities of 6 mg cm^-3, whereas jets emerging from tin releasing into liquid have densities nearly three times greater. We discuss the observed interaction dynamics for both conditions.
[1] W. T. Buttler et al., J. Dyn. Behav. Mater. 3(2), 151–155 (2017).
[2] T. de Rességuier et al., J. Appl. Phys. 124, 065106 (2018).   

Live

We have studied room temperature electrical resistance and the Raman spectra of Ce₂S₃ up to pressures of 60 GPa as well as low temperature electrical resistance. Three phase transitions were observed at room temperature at 15 and 25 GPa based on changes in Raman spectra. The initial transitions are followed by and insulator to metal transition at 40 GPa, determined by a seven order of decrease in electrical resistance, the disappearance of Raman modes, and the change in appearance from opaque to reflective. The temperature dependence electrical resistance measurements support the conclusion that Ce₂S₃ is an insulator at low pressures and a metal above 40 GPa.   

Live

We discuss a new five-phase equation of state of tin, referred to as SESAME 2162. Density functional theory (DFT) calculations of the solid phases and liquid phase are performed, including calculations of the cold curves of the solid phases, phonon calculations in the quasi-harmonic approximation over a range of volumes for each solid phase, and DFT-based molecular dynamics (DFT-MD) calculations of the liquid phase. Using the DFT results and existing experimental data, we construct a multiphase SESAME equation of state for tin. Comparisons to experiments show an overall high level of agreement in isobaric, isothermal, shock, and phase boundary data, including measurements of the melt curve.   

Live

One of the unanswered questions in physics is how much time does a shock-driven phase transition need - i.e. its kinetics? And how does this time influence the end-result of the shock process?
Traditionally, phase transitions in dynamic compression are inferred from continuum data and compared to results from static compression experiments, shock-recovery, or calculations.
With the development of time-resolved synchrotron dynamic x-ray diffraction (DXRD) combined with shock compression a new dawn is rising for the field of shock physics, as one can now probe atomic-scale changes in situ, with nanosecond resolution.
We will illustrate the atomic- and nanosecond-scale quantification of kinetics of shock-driven phase transitions in example materials. We will also show how we leverage two user facilities of the Advanced Photon Source: DCS and HPCAT, in synergistic dynamic/static compression experiments.   

Live

Magnesium difluoride (MgF₂) is an archetypal simple ionic crystal that is extensively used as an optical material. It is also of significant interest to geophysics, and therefore, well studied under static high-pressure loading, because its ambient rutile-type structure is isomorphic to SiO₂ (stishovite). In this work, plate impact experiments were carried out to shock compress single crystal MgF₂ samples to 130 GPa along the c-axis. Resulting wave profile and shock velocity through the sample were measured using laser velocimetry (PDV). Shocked MgF₂ exhibits a nonlinear elastic response up to ~10 GPa, beyond which a two-wave structure, consisting of an elastic wave and an inelastic wave, was observed. A single overdriven wave was observed for peak longitudinal stresses greater than 90 GPa. Measured wave profiles and the calculated longitudinal stress – particle velocity Hugoniot do not indicate any phase transition, such as those reported under static high-pressure loading (rutile → orthorhombic → distorted fluorite → cotunnite). The optical response of shocked MgF₂ along the c-axis will also be discussed in this talk. LA-UR-20-29023   

On Demand

Iron is one of the most studied chemical elements due to its socio-technological and planetary importance, hence understanding its structural transition dynamics is of vital interest. By combining a short pulse optical laser and an ultra-short free electron laser pulse available at PAL-XFEL, we have observed, for the first time, the sub-nanosecond structural dynamics of iron from high quality X-ray diffraction data measured at 50 ps intervals up to 2500 ps. We unequivocally identify a three-wave structure during the initial compression and a two-wave structure during the decaying shock, involving all of the known structural types of iron (α, γ and ε-phases). In the final stage, negative lattice pressures are generated by the propagation of rarefaction waves leading to the formation of expanded phases and the recovery of γ-phase. Our observations demonstrate the unique capability of measuring the atomistic evolution during the entire lattice compression and release processes at unprecedented time and strain rate.   

On Demand

The phase diagrams of binary alloys have been found to vary with pressure. Within the framework of solution-type models, the variation is
controlled by the pressure dependence of the elemental end members, which is relatively well known, and that of the excess interaction, which is relatively unknown. Exact thermodynamic relationships for the pressure and temperature dependence of the interaction parameters in an alloy solution are developed and related to the composition dependence of the excess thermodynamic quantities measured at ambient conditions. The effects of pressure modification of selected phase diagrams, including transformation from isomorphous to eutectic forms and shifts of the eutectic point are explored. Model predictions are compared with experimental data.   

On Demand

The development of the dynamic diamond anvil cell (dDAC) has created the ability to probe potential kinetic effects on the high-pressure behavior of different materials [Jenei et al., RSI, 2019]. The addition of resistive heating to the dDAC adds an additional degree of freedom for probing a materials thermodynamic properties under controlled dynamic conditions. By precisely tuning compression rates from millisecond timescales up to second timescales and temperatures up to 1000 °C, we can begin to systematically probe phase transition mechanisms and help to bridge the gap between static and shock compression experiments. In this talk I will discuss our recent work on Sn dynamically compressed in a resistively heated dDAC. I will present a systematic study of the β-Sn → BCT transition pressure as a function of compression rate, as well as discuss the effect of transition rate and temperature on the large phase co-existence region of the kinetically hindered BCO → BCC phase transition.