Session L03: Materials in Extremes: Phase Transitions

Probing liquid-liquid phase transitions under dynamic compression: an X-ray diffraction and ab initio MD study of selenium

One of the most interesting phenomena of non-crystalline materials is polyamorphism, or the ability of an amorphous material, with medium range order, to exist in several distinct modifications. In liquids, the transition between these states has been termed the liquid-liquid phase transition (LLPT) and has been observed experimentally under static compression experiments in several materials. In selenium, ab initio calculations have shown significant changes in the liquid structure with increasing pressure, yet experimental data at high-pressure is limited. Even less is known regarding the evolution of the liquid structure under rapid shock compression and whether a LLPT between complex liquid structures can occur on nanosecond timescales.
Here we report on femtosecond X-ray diffraction measurements of the LLPT in selenium following shock compression and release and find that the liquid structure transformations are dramatic and we show that they can occur on nanosecond timescales. The results are supported by molecular dynamic simulations of the liquid state at a range of pressure, temperature, density states covering the LLPT. Experiments were performed at the Matter at Extreme Conditions end-station of the Linac Coherent Light Source (LCLS). This work was performed under the auspices of the US Department of Energy by Lawrence Livermore National Laboratory under Contract No. DE-AC52-07NA27344.   

Ab-initio calculations of the phase diagram of Gold

We will present first principles calculations on the phase diagram of Gold up to 1TPa. To calculate the Gibbs free energies of the different structures (fcc, hcp, bcc...) we have performed calculations on the unit cells to obtain the cold curves and ab-initio molecular dynamics coupled with the temperature dependent energy potential (TDEP) method to calculate the vibational contribution to the free energy and capture the anharmonicity at high temperature. We compare our results to previous calculations and experimental data. We will also show results on thermodynamic data : thermal dilatation, Gruneisen parameter... and our calculations of the melting curve.   

A new metastable phase for shock compressed Copper

Copper is utilized in many different applications within the shock compression community (such as pistons in NIF) primarily because it is believed that it does not undergo any solid-solid phase transition prior to melting. However, studies have demonstarted coordination number loss behind shock front. A recent MD study by the authors group demonstrated that solid-solid phase transition occurs in Cu under shock compression to that of the BCT phase [1]. However, any MD calculations are dependent on the interatomic force potentials used, which for the previous study was taken as EAM-Mishin potential for Cu having valence shell containing “s” orbitals. To rule out any forcefield issues, DFT studies were carried out. DFT studies are for 0K temperature, whereas under shock compression there is a significant rise in temperature for Cu samples. Thereby, Gibbs free energy calculation within the quasiharmonic approximation is carried out to demonstrate the feasibility of existence of BCT phase for Cu shock compressed along 001 direction [2], supporting previous postulation by Friedel.
Ref.
[1] Neogi, A., and Mitra, N. (2017). Scientific Reports. 7, 7337.
[2] Dey, U., Mitra, N., and Taraphder, A. (2019). Computational Materials Science, 170, 109154.
  

Measurement of high pressure crystal structure and the pressure-temperature melt conditions in shock-compressed silicon carbide

Silicon Carbide (SiC) has many attractive properties including low density, high strength, high melting point, low wear coefficient and high chemical stability that lead to its extensive use in a wide range of industrial applications including as abrasives, shielding material on space craft,^, personal armor and as a potential ablator material for fusion capsules. Silicon carbide is also important in geology and planetary science. SiC grains - found in meteorites and impact sites – have an unusual isotopic signature which indicate that they are pre-solar in origin and provide constraints on stellar nucleosynthesis and on the stellar sources for the origin of the solar system. Several studies have also explored possible interior structure of carbon-rich planets in which SiC is a likely main constituent, but experimental data for the high-pressure properties of SiC is currently unavailable to test these models. Here we report on laser-driven nanosecond x-ray diffraction and shock-decay experiments on the Omega-EP laser facility located at the Laboratory for Laser Energetics (NY, USA). We present new data on the high pressure crystal structure, and pressure-temperature conditions for melt in single-crystal SiC samples.   

Hurry up or take your time: kinetics of shock-driven phase transitions and dynamic x-ray diffraction.

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.
  

Experimental data analysis methods for the physics-based description of phase transition kinetics

Experimental data analysis techniques for the determination of phase transformation kinetics under ramp compression are presented. The new techniques, which do not require any simulations, are described in three parts: (I) estimation of nucleation and growth rates via analysis of the velocimetry wave profile “loop”, (II) bounding of the metastability limit or “maximum over-pressurization” and (III) application of a universal scaling relation that sheds light on the time-dependence of the phase transition. The relatively simple analysis techniques presented are made available in an open source code for the community.
  

Experiments and Simulations of Shocked and Ramp-Compressed Metals to 5 Mbars

In this report, we present a series of shocked and ramp compressed data on various metals including tantalum, iron and tin. These samples were shocked and ramp-compressed to pressures as high as 5 Mbars with graded density impactors (GDI). To analyze these data, we utilize both backward (characteristics) and forward analyses. The former method does not require a priori knowledge of a pressure drive, and often fails in the presence of strength or phase transition. By employing simulations in analysis of these experiments, we can improve experimental uncertainty as well as gain better understanding of phase transition and strength during ramp compression. In particular, we will examine the alpha-epsilon and solid-liquid transitions in iron. We will also report on recent efforts to characterize GDI in situ impedance profile.
This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344
  

Experimental measures of the orientation dependence of the B1-B2 transformation in shock-compressed MgO

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 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.   

Metastable conducting solid hydrogen at high pressures.

The posibility of formation of metastable metallic hydrogen at normal pressure was predicted in [1]. In the present work, the density functional theory is applied for the calculation of the equation of state, the pair correlation function and density of states of solid hydrogen in the region of the possible formation of the conducting phase. A hysteresis of the dependence of pressure on density is observed in the pressure range from 350 GPa to 625 GPa. During compression, the transition of molecular hydrogen with the C2/c symmetry to a conducting atomic state with the C222₁ symmetry through an intermediate conducting molecular phase with Cmca-4 symmetry is observed. The results of calculation of the band structure of the molecular state Cmca-4 point to the semimetalic mechanism of conductivity. The possibility of the existence of conductive atomic solid hydrogen with P2₁/c symmetry under expansion up to a pressure of 350 GPa is shown.

1. Kagan, Yu.; Brovman, E. G. // Sov. Phys. Usp. 1971, 14, 809 - 811.   

Ionization of H in iron oxy-hydroxide

The symmetrization of O-H bonds in hydroxide often induces a slightly first order phase transition. However, the phase transition causes dramatic reconfiguring of electronic properties. In this work, we pressurized a piece of iron oxy-hydroxide (FeO₂H) up to 60 GPa. Upon the asymmetric O-H bonds symmetrize to O-H-O, the measured electric conductivity surged a few orders of magnitude higher. Combining with first-principle simulation, the symmetrization separates H⁺ ion from the hydroxyl OH^-1 group by forming ionic-type double O-H bonds. Although the pressure range of symmetrization coincides with the high-low spin transition of Fe, the sudden boost of electric conductivity is against conventional spin-transition effect, which reduces conductivity. Therefore, the ionic phase transition of H is likely to be responsible for the change of conductivity. The ionization of H is among the many intriguing behaviors of H under extreme pressure conditions and may have profound influences for the fundamental processes in Earth’s deep interiors.   

High-Pressure-Induced Phase Transition in 1,3-Diphenylurea: the Approaching of N−H...O Hydrogen-Bonded Chains

Under extreme pressure conditions, supramolecular materials can undergo many novel phenomena such as phase transition, polymerization, piezochromism, and negative compression. In our research plan, we selected a series of low-dimensional supramolecular materials to study the evolution of crystal structure and intermolecular interactions under high pressure. In this study, the crystal structure and hydrogen-bonded chains of 1,3-diphenylurea were found to appear abrupt changes when the pressure increased to 1.8 GPa. The intermolecular interactions, the vibrations of chemical groups and the molecular conformation were clearly distinguished from the initial state. The anisotropic compression of the original crystal structure and the occurrence of structural phase transition were derived from the remarkable shrink of distance between hydrogen-bonded chains. This study demonstrates that excessively reducing the space between of hydrogen-bonded chains will rearrange the self-assembly of supramolecular materials.

Reference:
1. Y. Dai, et al. J. Raman Spectrosc. 2019, DOI: 10.1002/jrs.5706.
2. Y. Dai, et al. J. Phys. Chem. C 2018, 122, 11747-11753.
3. Y. Dai, et al. RSC Adv. 2018, 8, 4588-4594.
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