Session RR05: V: Matter at Extreme Conditions

PBE-GGA predicts the B8↔B2 phase boundary of FeO at Earth's core conditions

FeO is a crucial component of the Earth’s core, and its thermodynamic properties are essential to developing more accurate core models. It is also a notorious correlated insulator in the NaCl-type (B1) phase at ambient conditions. It undergoes two polymorphic transitions at 300 K before it becomes metallic in the NiAs-type (B8) structure at ~100 GPa. Although its phase diagram is not fully mapped, it is well established that the B8 phase transforms to the CsCl-type (B2) phase at core pressures and temperatures. Here we report a successful ab initio calculation of the B8↔B2 phase boundary in FeO at Earth’s core pressures. We show that fully anharmonic free energies computed with the PBE-GGA coupled with thermal electronic excitations reproduce the experimental phase boundary within uncertainties at P > 255 GPa, including the largely negative Clapeyron slope of –52 MPa/K. This study validates the applicability of a standard DFT functional to FeO under Earth’s core conditions and demonstrates the theoretical framework that enables complex predictive studies of this region.   

Pressure and temperature dependent ab-initio quasi-harmonic thermoelastic properties of tungsten

In this presentation we show the ab-initio temperature and pressure dependent thermoelastic properties of body-centered cubic tungsten. The temperature dependent quasi-harmonic elastic constants are computed at several reference volumes and interpolated at different temperatures and pressures. Our results are compared with room pressure experimental elastic constants on a single crystal and with the recent measurements of the pressure and temperature dependent compressional and shear sound velocities of polycrystalline tungsten, finding a reasonable agreement. Moreover, we present the phonon dispersions, the thermal pressure-volume equations of state, and the temperature dependent volumetric thermal expansion, isobaric heat capacity, adiabatic bulk modulus, and average Gr"uneisen parameter at ambient and high pressures. All these quantities are computed with three exchange and correlation functionals: LDA, PBE, and PBEsol. The differences among functionals decrease with pressure and the ab-initio results are in agreement with empirical high pressure extrapolations of the experimental data.   

Theory and Simulation of Transient Absorption in Warm Dense Matter Created by an X-Ray Free-Electron Laser

The understanding of  warm dense matter (WDM) is important in many fields, including planetary science, inertial confinement fusion, and  astrophysics. Although recent advances in experimental techniques using x-ray free electron lasers have led to probes of WDM at extreme temperature- and time-scales, theoretical interpretations of these out of equilibrium states of matter are only now being developed. Here we describe an approach for theoretical simulations of x-ray absorption in XFEL generated warm dense copper based on the combination of a kinetic Boltzmann equation model and  finite-temperature real-space Green's function theory. Calculations of the L₃ near edge spectra reproduce the main features and trends of recent experimental data [1], including a transition from reverse-saturable to saturable absorption over a wide range of ultra-short fs duration x-ray pulse intensities. The interpretation of these results links   features in the spectra to a combination of core- and valence-level shifts, band narrowing, and changes in the photoelectron inelastic mean free path that result from depletion of the 3d band  with increased pulse intensities.    

Phase behaviour of the 1:1 Ammonia-Hydrogen sulphide mixtures at high pressures and temperatures

Although the major constituents of the gaseous atmosphere of the ‘ice giants’ are H and He, their mantle regions are postulated to be comprised of hot dense ices of CH₄, H₂O, NH₃ and H₂S. While the individual ices and their mixtures e.g., CH₄-H₂O¹, NH₃-H₂O^2,3  are studied under extreme conditions, the complete phase diagram of NH₃-H₂S mixtures remains unexplored, yet will contribute to accurate models of the interiors of ice giant planets. Using ab initio molecular dynamics, we construct the phase diagram of ammonia monosulphide (NH₃:H₂S = 1:1) up to 165 GPa and 3500K, focusing on the emerging plastic and superionic states, and the melting curve (MC).  Our calculated MC demonstrates a maximum in 40-70 GPa range followed by a dip at high-pressure regime as the mixture gives rise to short-lived di-sulphur molecules. Tracing the MC with the isentropes of Uranus and Neptune indicates that in the shallow mantle of these planets, H₂S can stabilize by forming NH₃-H₂S molecular fluid that could phase-separate from superionic NH₃-H₂O mixtures. Our study reveals that increasing temperature lowers the band-gap which is most prominent in the Cc crystalline phase at 20 GPa, indicating that this phase can switch from semi-conductor to conductor in the high-temperature liquid state beyond 3000K.

 ¹Pruteanu et al., Sci. Adv. 3, e1700240 (2017).

²Robinson et al., PNAS, 114, 9003 (2017).

³Robinson and Hermann, JPCM 32, 184004 (2020).   

Isobaric heat capacity and density of liquid lead : Ab initio calculation and Bayesian approach

Isobaric heat capacity (Cp) is an essential property for characterizing the thermal behavior of liquids. This quantity is used to constrain equations of state models.

In the case of liquid lead, inconsistencies in the experimental evaluation of Cp with different measurement techniques have been observed. To address these discrepancies, we employ Quantum Molecular Dynamics (QMD) simulations to assess the theoretical Cp.

Cp is not a direct output of QMD simulations. In this study, a method is proposed. We determine the enthalpy at fixed temperature, H(T), using QMD NPT simulations. Its statistical uncertainty is determined using the bootstrap method. Subsequently, a Bayesian approach with a polynomial model is applied to H(T) with uncertainties. Cp is obtained by derivation of this polynom. Our calculations revealed that Cp remains constant (142±3 J/kg/K) between 650-2000K. A sensitivity study on Cp and density with respect to the functional, spin-orbit coupling, and pseudo PAW is currently underway. Preliminary results indicate that the calculated Cp is less sensitive to methodological choices than density.   

Unveiling the effect of Ni on the formation and structure of Earth's inner core

Ni is the second most abundant element in the Earth's core. However, its effects on the inner core's structure and formation process are often overlooked due to its similar atomic number to Fe. Using ab initio simulations, we have found that the bcc phase can spontaneously crystallize in liquid Ni at temperatures above Fe's melting point under inner core pressures. The phase relations among hcp, bcc, and liquid differ between Fe and Ni. Ni can act as a bcc stabilizer for Fe at the inner core's temperatures and pressures. A small amount of Ni can significantly accelerate the crystallization of Fe under core conditions. These results suggest that Ni may have a substantial impact on the structure and formation process of the solid inner core.   

Thermoelastic properties of bridgmanite using Deep Potential Molecular Dynamics

The Earth's lower mantle is dominated by the high-pressure Pbnm-perovskite polymorph of MgSiO₃, known as bridgmanite (Bm). The elucidation of Bm's structural and elastic properties is of significant geophysical importance, yet the extreme conditions within the mantle pose challenges for experimental investigations. To overcome this hurdle, ab initio-based methods have emerged as indispensable tools. In this study, we present deep neural network potential models for Bm, developed using density functional theory (DFT) with various functionals. These models enable extensive molecular dynamics (MD) simulations, comprehensively exploring a wide range of pressure-temperature (P-T) conditions. Our research focuses on the compressional behavior and elastic moduli of Bm at high P-T, shedding light on the remarkable performance of the DP-SCAN functionals in accurately predicting high-temperature equations of state and elastic properties. By merging advanced computational techniques with DFT, we offer a robust solution to the accuracy-efficiency dilemma, facilitating precise investigations of elastic properties for minerals like MgPv at high P-T conditions. Our findings signify a significant leap forward in understanding the Earth's internal state and processes, providing an innovative avenue for further exploration.

    

Phase diagram of CaSiO3 perovskite from deep potentials with ab initio accuracy

Thermodynamic properties of minerals are crucial in revealing the structure and properties of planetary interiors. CaSiO3 perovskite (CaPv) is believed to be the third most abundant mineral in the Earth’s lower mantle. A previous study suggested that its tetragonal-to-cubic transition can occur at lower-mantle conditions, potentially causing seismic anomalies, particularly in the Large Low-Shear-Velocity Provinces (LLSVPs) (Thomson et al., Nature 572, 643, 2019). However, ab initio methods for describing the tetragonal-cubic phase transitions in CaPv were limited by their high computational cost and difficulties in addressing a nearly second-order phase transition. We utilized a machine-learning method and novel thermodynamic integration techniques to investigate the phase diagram of CaPv at temperatures ranging from 300-3,000 K and pressures up to 130 GPa. Our simulations using a deep-learning potential with ab initioaccuracy indicate that the tetragonal-to-cubic transition in CaPv happens at ~1,000 K below mantle temperatures. Therefore, the lower mantle should have only cubic-CaPv. This suggests LLSVPs are caused by other mechanisms.