Session G24: Matter at Extreme Conditions: Phase Transitions

High-pressure melting of Pt, Fe, and H2O using microsecond Joule-heating calorimetry in diamond anvil cells

Improved measurements of melting curves at megabar pressures would advance geophysics and condensed matter physics by reducing uncertainties in the melting curve of iron alloys and by providing benchmarks for atomistic models of melting. In this talk, I will describe two recently-developed high-pressure Joule heating techniques designed specifically to study melting temperatures and physical properties of materials near melting and other temperature-driven phase transitions: AC calorimetry and pulsed calorimetry in diamond anvil cells. Using these techniques, we have unambiguously identified melting of Pt up to 100 GPa, Fe up to 80 GPa, and several order-disorder transitions in H2O up to 10 GPa. These measurements also provide constraints on the changes in heat capacity, entropy, thermal conductivity, and electrical conductivity across melting.   

Shock and melting behaviour of beryllium and magnesium oxide at megabar pressures

We used ab initio molecular dynamic simulations to study the phase diagrams of beryllium and magnesium oxide at megabar pressures. We employ the thermodynamic integration (TDI) method to obtain the free energies of the liquid and the solid phases and to derive phase boundaries. We find that both materials exhibit very similar properties. There is a solid-to-solid phase transition that is strongly affected by anharmonic effects that shift the triple point to high pressure, which makes it more difficult to observe the higher-pressure solid phase with single-shock experiments. Conversely quasi-harmonic calculations underestimate the stability of lower-pressure phase.

 

Furthermore we calculated the principal and secondary shock Hugoniot curves of both materials and determine the offset between solid and liquid branches in pressure-temperature space. We find that this offset is much larger for secondary Hugoniot curves that start from a higher initial pressure. We also compare our secondary Hugoniot curves with recent shock experiments of MgO. We find good agreement in liquid region only.

 

We also make predictions for ramp compression experiments assuming they are isentropic. We determine the pressure-temperature interval where they follow the melting line while remaining in solid-liquid mixed state. Finally, we compare the melting line that we computed with TDI method with predictions based on Lindemann's law.   

Machine learning assisted molecular dynamics study of the superhydride LaH10

The study of metallic, Hydrogen-rich compounds in recent years has revived efforts toward room temperature superconductivity. One of these compounds is a Lanthanum superhydride, successfully synthesized and shown to superconduct up to 260 K [1,2]. First-principles electron-phonon calculations show that fcc-LaH₁₀ and slight rhombohedral distortions thereof are consistent with high temperature superconductivity. However, these calculations do not fully account for the dynamics of the Hydrogen sublattice, which is highly anharmonic. Using a machine learned interatomic potential, we characterize diffusion and distortion in LaH₁₀ with molecular dynamics. With access to larger length and time scales, we highlight the importance of finite-size effects and quantum effects. We find that proton diffusion begins at a lower temperature than previously expected. We also see how a rhombohedral distortion can appear as pressure is lowered, which experiments have observed.

[1] Somayazulu, Maddury et al. "Evidence for Superconductivity above 260 K in Lanthanum Superhydride at Megabar Pressures". Phys. Rev. Lett. 122. (2019): 027001.

[2] Drozdov, A. P. et al. "Superconductivity at 250 K in lanthanum hydride under high pressures". Nature 569. 7757(2019): 528-531.   

Single crystal x-ray diffraction, synchrotron infrared spectroscopy, and first-principles calculations of oP32-type Ge under pressure

Single-crystal diffraction of the oP32-structured allotrope of Ge was performed under hydrostatic and quasihydrostatic pressures up to 6 GPa. Analysis of unit cell parameters, systematic absences, and structure factors show no evidence for structural transitions, as a continuous compression of the structure was observed. In addition, occurrence of multiple domains as well as diffuse scattering rods between Bragg peaks along some directions are observed, which could be the result of stacking disorder. The phase transforms to the β-Sn type structure near 9 GPa, as indicated by powder diffraction which was conducted to at least 32 GPa. Synchrotron infrared and optical absorption measurements up to 8.5 GPa indicate widening of the band gap of the op32 phase prior to collapse to the highly reflecting metallic β-Sn type phase. The increased band gap energy with pressure of oP32 Ge and the phase transition to the b-Sn phase agree with first-principles density functional theory calculations. The effects of pressure on the structural and electronic properties of oP32 Ge will also be discussed.   

Chemical interactions between helium and VF3-type compounds.

Helium is the second most abundant element in the universe and coincidentally, the least reactive. It is a relatively scarce element in Earth's atmosphere, nevertheless, non-negligible quantities of this element may still be trapped in the interior parts of our and other rocky planets. On the other hand, helium can be found in larger amounts in giant plants like Jupiter and Saturn. As consequence, the chemistry of helium under extreme condition is of great importance in planetary science. Computational methods and experiments have revealed that this element that is traditionally thought of as being inert, in combination with other compounds, like pure metals or ionic salts, can form a variety of interstitial, stable, systems.

In our work, we have studied the reactivity and interactions between helium and VF-type compounds, such as AlF3, a perovskite like structure having a vacant A-site, with first principles calculations. We observed that not only He can form stable compounds occupying the vacancy in the VF-type compounds, increasing notably properties like the bulk modulus, but it is also activated by the chemical environment. The possibility for chemical bond formation with He is discussed.   

Dynamic Diamond Anvil Cells as a Tool For Kinetic Studies

Understanding the kinetics of pressure-induced phase transitions is of broad interest to the fields of both materials science and geoscience; however these studies have previously been complicated by difficulties in achieving both the rapid change from initial conditions and stable pressure conditions that are required in classical kinetic studies. This work focuses on the use of a piezo-electrically driven dynamic diamond anvil cell (dDAC) to achieve a rapid pressure change followed by a steady pressure that enables assessment of a materials transformation kinetics. This technique has been applied to both covalently bonded CdSe and metallically bonded iron and measurements of kinetics have been made during in-situ time-resolved X-ray diffraction at Sector 16-IDB of the Advanced Photon Source.   

A DFT prediction of an epsilon-zeta phase transition of solid oxygen: The SCAN+rVV10 calculation

In the previous studies [1, 2], we carried out a DFT calculation for solid oxygen using meta-GGA+van-der-Waals functionals (SCAN+rVV10). This method enables us to reproduce the experimental interatomic distances between the two O₂ molecules in the (O₂)₄ structure of the epsilon phase at 10~20 GPa which implies that the magnetic epsilon phase is stable in that pressure range [1]. Moreover, the epsilon-zeta phase transition was studied using SCAN+rVV10 and consistently compared with our XRS measurement [2]. In this study, we discuss possibilities of a pre-phase of the zeta superconducting phase suggested by SCAN+rVV10 calculation which was not obtained by using neither PBE [3] nor hybrid-functionals [4]. The vibrational spectra and superconducting properties will be analyzed to clarify this phase from the epsilon and zeta phases.   

Nucleating a Different Coordination in a Crystal under Pressure: A Study of the B1-B2 Transition in NaCl by Metadynamics

Prediction of crystal structures has reached a high level of reliability, but much less is known about the mechanisms of structural transitions and pertinent barriers. The barriers related to nucleation of crystal structure inside another one are critically important for kinetics and eventually decide what structure will be created in experiment. 

We demonstrate an NPT metadynamics simulation scheme [Phys. Rev. Lett. 127, 105701 (2021)] employing coordination number and volume as collective variables and illustrate its application on a well-known example of reconstructive structural transformation B1-B2 in NaCl. Studying systems with size up to 64 000 atoms we reach beyond a collective mechanism and observe the nucleation regime. We reveal the structure of the critical nucleus and calculate the free-energy barrier of nucleation and also uncover details of the atomistic transition mechanism and show that it is size-dependent.

Our approach is likely to be applicable to a broader class of structural phase transitions induced by compression/decompression and could find phases unreachable by standard crystal structure prediction methods as well as reveal complex nucleation and growth effects of martensitic transitions.   

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The martensitic transformation is a rapid displacive fundamental process in material science, specifically renowned for giving rise to the unique functional properties of shape memory alloys (SMA). Yet, little is known about how fast it can be under high driving forces. We study the transformation kinetics in the large thermodynamic driving force regime, by superheating an SMA wire via an abrupt high-voltage heating-pulse. We track the evolution of the transformation by microsecond-scale multi-frame time-resolved X-ray diffraction at synchrotron radiation with simultaneous high-bandwidth stress measurements. The diffraction data provides local measures for the austenite content with a temporal resolution of one microsecond. We reveal three stages occurring at different times and length-scales. During the dominant stage, a single phase-front propagates from the wire periphery inwards. Fitting a theoretical model to the measured results provides the kinetic relation between the velocity of the phase front and the driving force propagating it. The revealed high-specific-energy front releases energy faster than low-energy fronts forming under low driving forces.   

Researches on pressure-induced structural evolution using multiple synchrotron X-ray techniques

       The researches on behavior of crystalline and non-crystalline materials under high pressure extreme conditions are long time hot topics in condense matter physics. Many interesting behavior, such as pressure-induced phase transitions, amorphization of solid forms, liquid-to-liquid transformation, pressure-induced polyamorphism from low- to high-density states of glasses, and the dynamics of pressure-induced crystallization were reported but not fully understood. In this presentation, several selected noncrystalline systems were in situ studied under high pressure using cutting-edge synchrotron microtomographic, x-ray diffraction (XRD) as well as pair distribution function (PDF) techniques in diamond anvil cell and large volume press. XPCS technique will be used to study the relaxation behaviors of metallic glasses upon compression and heating. Several typical crystalline materials, including powder and single crystal samples, will be presented, for the research topic on phase transitions, isostructural transition, pressure tune charge transfer and mixed valance modifications behaviors, and negative linear compression at over 100 GPa conditions. 3 dimensional X-ray diffraction (3D-XRD) technique will be introduced for the study on the pressure induced phase transition in powder Fe sample within diamond anvil cell (DAC). The micro focusing beam size at range of couple microns could help us to create diffraction tomography of the different phase in sample during phase transition process under pressure conditions.

    

Returning to close packed structures and core bonding in Cs metal under high-pressure

Numerous experimental and theoretical studies showed that alkali and alkaline metals transform into various structures with large open interstitials under pressure, an intricate behavior that is caused by the electron transfer among s, p, d atomic and quasi-atomic orbitals. However, little is known to the behavior of these metals beyond this pressure region. Using first principles calculations and crystal structure search methods, we show that Cs returns to close packed structures under ultrahigh pressure, including the transition from DHCP to FCC structure at 183 GPa and a further transition into HCP at 655 GPa. More significantly, in these structures, the 5p electrons are activated and form strong chemical bonds between neighboring Cs atoms that governs the properties of Cs under very high pressure, including the transition from FCC to HCP.   

Simultaneous inference of plasticity and phase transition kinetics in tin

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. We employ Bayesian statistical techniques to explore the interactions between inelasticity and phase transition kinetics in Sn. The degree to which these different kinetic processes can be distinguished given velocimetry data is discussed.