Superionic Al─O─H Compounds With Unconventional Stoichiometries Under Extreme Planetary Conditions

ABSTRACT Extreme pressure and temperature induce novel chemical behaviors, including unconventional stoichiometries and superionic states. Here, by combining crystal structure prediction with first‐principles calculations, we identify two unconventional stoichiometries in the ternary Al─O─H system, AlOH and AlH 3 O 4 , which lie on the convex hull at 300 GPa. Representative formation reactions become thermodynamically stable above 65 and 245 GPa after including ZPE, respectively. Both compounds are wide‐gap insulators and AlH 3 O 4 exhibits pronounced site‐dependent charge differentiation between inequivalent oxygen environments under high pressure. Crucially, ab initio molecular dynamics simulations reveal protonic superionic transitions at elevated temperatures, where protons diffuse like a liquid through rigid Al─O sublattices. High‐pressure and high‐temperature dynamical‐state diagrams further indicate that the solid, superionic, and liquid regimes overlap with conditions expected in deep planetary interiors, suggesting that they could play important roles in chemical behaviors inside planetary bodies. These findings significantly enrich the high‐pressure phase diagram of Al─O─H system and provide valuable insights into chemical behaviors under extreme conditions.