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

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.