Structural evolution and superionic state of MnOOH under high-pressure and high-temperature

Abstract

Hydrous minerals are essential for elucidating the mechanisms and forms of water storage in the Earth’s deep interior. Using crystal structure prediction and first-principles calculations, we identify a previously unreported Pbca-MnOOH phase that is both thermodynamically and dynamically stable in the 34–75 GPa pressure range. This structure consists of edge-sharing MnO6 octahedra, further linked via corner-sharing to form a three-dimensional framework. Electronic structure analysis indicates the coexistence of O–H covalent bonding and Mn–O ionic interactions, with the phase remaining semiconducting throughout the stability range. Ab initio molecular dynamics simulations reveal that Pbca-MnOOH enters a superionic state under high-pressure and high-temperature conditions relevant to the Earth’s geotherm. These results not only point to a possible mechanism for deep-Earth water transport but also provide critical insights into the pressure-induced structural and transport properties of Mn-bearing hydrous phases.