Ab initio spectroscopy and ionic conductivity of water under Earth mantle conditions

Abstract

The phase diagram of water at extreme conditions plays a critical role in Earth and planetary science, yet remains poorly understood. Here we report a first-principles investigation of the liquid at high temperature, between 11 GPa and 20 GPa-a region where numerous controversial results have been reported over the past three decades. Our results are consistent with the recent estimates of the water melting line below 1,000 K and show that on the 1,000-K isotherm the liquid is rapidly dissociating and recombining through a bimolecular mechanism. We found that short-lived ionic species act as charge carriers, giving rise to an ionic conductivity that at 11 GPa and 20 GPa is six and seven orders of magnitude larger, respectively, than at ambient conditions. Conductivity calculations were performed entirely from first principles, with no a priori assumptions on the nature of charge carriers. Despite frequent dissociative events, we observed that hydrogen bonding persists at high pressure, up to at least 20 GPa. Our computed Raman spectra, which are in excellent agreement with experiment, show no distinctive signatures of the hydronium and hydroxide ions present in our simulations. Instead, we found that infrared spectra are sensitive probes of molecular dissociation, exhibiting a broad band below the OH stretching mode ascribable to vibrations of complex ions.

Temperature effects on the ionic conductivity in concentrated alkaline electrolyte solutions

“…Simulations of electric properties, such as ionic conductivity, necessitate long timescales and, except for cases at extreme conditions 16 , are normally beyond reach of density functional theorybased MD. One way to tackle this time-scale challenge is to explore finite-field methods to speed up the convergence of the polarization P, which has been successfully applied to compute the dielectric constant of polar liquids and the capacitance of electrified solid-electrolyte interfaces 17 .…”

Large Presence of Carbonic Acid in CO2-Rich Aqueous Fluids under Earth’s Mantle Conditions

“…It has been reported that generalized gradient approximations (GGAs) may not be able to describe doubly charged anions in water accurately at ambient conditions [35]. However, PBE performs better at HP-HT conditions than at ambient conditions, as shown in many of our previous studies [14,[22][23][24]. Our previous finding that CO 2 (aq) is absent at ∼11…”

“…Our total simulation time exceeds 3.8 nanoseconds. We found that at ∼10 GPa and 1000 K, [22], dielectric constant [23], Raman and IR spectra, and ionic conductivity [24], as well as the reactions of CO 2 and carbonates in HP-HT water [14].…”

Yttrium speciation in subduction zone fluids from <i>ab initio</i> molecular dynamics simulations

“…This observation cannot be explained by the increased self-dissociation of water, which increases with pressure (see e.g. Rozsa et al (2018); Goncharov et al (2005)). According to Marshall and Franck (1981) the OH − molality is in the range of 10 -6 to 10 -5 under the investigated P/T conditions.…”