PNNL

Abstract

Water is known to play a controlling role in directing mineralization pathways and stabilizing metastable amorphous intermediates in hydrous carbonate mineral MCO3·nH2O systems, where M2+ is a divalent metal cation. Despite this recognition, the chemical mechanisms at play and the nature of the controls on crystallization time scales are poorly understood, largely owing to the difficulty in characterizing and analyzing the dynamically disordered structures of both the reactants and products of the precipitation reactions at the atomic scale. Here we focus on the products and present detailed theoretical simulations of the dependence of the X-ray pair distribution function (PDF) on the hydration level n in amorphous solids incorporating M=Ca, Mg, Sr. The simulations use ab initio molecular dynamics (AIMD) to theoretically account for thermal disorder and chemical dynamics such as proton transfer reactions. These simulations inform a reciprocal-space treatment of the photon scattering problem that includes experimental parameters in the simulated signals. The resulting simulated PDFs and structural models are compared to available experimental data. We discuss the implications of the simulations for understanding the role water plays in structuring amorphous carbonates. We present a series of atomistic models across a range of experimentally relevant cations and hydration levels that simultaneously obey quantum mechanics and reproduce available experimental data to facilitate the interpretation of measurements in terms of the underlying atomic motions. We find that water forms an extensive hydrogen bond network among both water and CO32- groups that exhibits frequent proton transfers for all three cations considered.