Uncovering the true atomic structure of disordered materials: The structure of a hydrated amorphous magnesium carbonate (MgCO₃·3D ₂O)

Amorphous calcium/magnesium carbonates are of significant interest in the technology sector for a range of processes, including carbon storage and biomineralization. Here, the atomic structure of one hydrated amorphous magnesium carbonate (hydrated AMC, MgCO₃·3D₂O) is investigated using an iterative methodology, where quantum chemistry and experimental total scattering data are combined in an interactive iterative manner to produce an experimentally valid structural representation that is thermodynamically stable. The atomic structure of this hydrated AMC consists of a distribution of Mg²⁺ coordination states, predominately V- and VI-fold, and is heterogeneous due to the presence of Mg²⁺/CO ₃^2--rich regions interspersed with small 'pores' of water molecules. This heterogeneity at the atomic length scale is likely to contribute to the dehydration of hydrated AMC by providing a route for water molecules to be removed. We show that this iterative methodology enables wide sampling of the potential energy landscape, which is important for elucidating the true atomic structure of highly disordered metastable materials.