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Register a new userShock Response and Phase Transitions of MgO at Planetary Impact Conditions
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The moon-forming impact and the subsequent evolution of the proto-Earth is strongly dependent on the properties of materials at the extreme conditions generated by this violent collision. We examine the high pressure behavior of MgO, one of the dominant constituents in the earths mantle, using high-precision, plate impact shock compression experiments performed on Sandia National Laboratories Z-Machine and extensive quantum simulations using Density Functional Theory (DFT) and quantum Monte Carlo (QMC). The combined data span from ambient conditions to 1.2 TPa and 42,000 K, showing solid-solid and solid-liquid phase boundaries. Furthermore our results indicate under impact that the solid and liquid phases coexist for more than 100 GPa, pushing complete melting to pressures in excess of 600 GPa. The high pressure required for complete shock melting places a lower bound on the relative velocities required for the moon forming impact.
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Most water in the universe may be superionic, and its thermodynamic and transport properties are crucial for planetary science but difficult to probe experimentally or theoretically. We use machine learning and free energy methods to overcome the limitations of quantum mechanical simulations, and characterize hydrogen diffusion, superionic transitions, and phase behaviors of water at extreme conditions. We predict that a close-packed superionic phase with mixed stacking is stable over a wide temperature and pressure range, while a body-centered cubic phase is only thermodynamically stable in a small window but is kinetically favored. Our phase boundaries, which are consistent with the existing-albeit scarce-experimental observations, help resolve the fractions of insulating ice, different superionic phases, and liquid water inside of ice giants.
We show that under rather general assumptions on the form of the entropy function, the energy balance equation for a system in thermodynamic equilibrium is equivalent to a set of nonlinear equations of hydrodynamic type. This set of equations is integrable via the method of the characteristics and it provides the equation of state for the gas. The shock wave catastrophe set identifies the phase transition. A family of explicitly solvable models of non-hydrodynamic type such as the classical plasma and the ideal Bose gas are also discussed.
Theoretical calculations of sound-wave velocities of materials at extreme conditions are of great importance to various fields, in particular geophysics. For example, the seismic data on sound-wave propagation through the solid iron-rich Earths inner core have been the main source for elucidating its properties and building models. As the laboratory experiments at very high temperatures and pressures are non-trivial, ab initio predictions are invaluable. The latter, however, tend to disagree with experiment. We notice that many attempts to calculate sound-wave velocities of matter at extreme conditions in the framework of quantum-mechanics based methods have not been taking into account the effect of anharmonic atomic vibrations. We show how anharmonic effects can be incorporated into ab initio calculations and demonstrate that in particular they might be non-negligible for iron in Earths core. Therefore, we open an avenue to reconcile experiment and ab initio theory.
Studies of the behaviour of solids at ultra-high pressures, those beyond 200 GPa, contribute to our fundamental understanding of materials properties and allow an insight into the processes happening at such extreme conditions relevant for terrestrial and extra-terrestrial bodies. The behaviour of magnesium oxide, MgO, is of a particular importance, as it is believed to be a major phase in the Earth lower mantle and the interior of super-Earth planets. Here we report the results of studies of MgO at ultra-high static pressures up to ca. 660 GPa using the double-stage diamond anvil cell technique with synchrotron X-ray diffraction. We observed the B1-B2 phase transition in the pressure interval from 429(10) GPa to 562(10) GPa setting an unambiguous reference mark for the B1-B2 transition in MgO at room temperature. Our observations allow constraining theoretical predictions and results of available so far dynamic compression experiments.
In surface catalysis, the adsorption of carbon monoxide on transition-metal electrodes represents the prototype of strong chemisorption. Notwithstanding significant changes in the molecular orbitals of adsorbed CO, spectroscopic experiments highlight a close correlation between the adsorbate stretching frequency and equilibrium bond length for a wide range of adsorption geometries and substrate compositions. In this work, we study the origins of this correlation, commonly known as Badgers rule, by deconvoluting and examining contributions from the adsorption environment to the intramolecular potential using first-principles calculations. Noting that intramolecular anharmonicity is preserved upon CO chemisorption, we show that Badgers rule for adsorbed CO can be expressed solely in terms of the tabulated Herzberg spectroscopic constants of isolated CO. Moreover, although it had been previously established using finite-cluster models that Badgers rule is not affected by electrical conditions, we find here that Badgers rule breaks down when the electrified surface is represented as a periodic slab. Examining this breakdown in terms of anharmonic contributions from the effective surface charge reveals limitations of conventional finite-cluster models in describing electrical conditions at metal electrodes.
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