Pressure-driven dimensional evolution and two-dimensional superionic state in AlI3

Iodide ions can form crystal lattices with large interstitial spaces, making them archetypal systems for investigating superionic phase transitions. Understanding how iodine-based lattices evolve under different thermodynamic conditions is therefore a central problem in condensed matter physics and functional materials design. Aluminum iodide (AlI3) is a molecular solid crystal with low ionic conductivity under ambient conditions, and it plays important roles in batteries and catalytic applications, motivating exploration of its pressure-tunable ionic transport behavior. Here, we reveal the pressure-induced structural dimensionality evolution in AlI3 through first-principles structural searches and synchrotron X-ray diffraction (XRD). We identify a sequence of phase transitions: from the molecular P21/c phase to a two-dimensional layered rhombohedral (R-3) phase above 1.3 GPa, and subsequently to a one-dimensional chain-like orthorhombic (Cmcm) phase beyond 49 GPa. Notably, in situ laser-heating XRD and ab initio molecular dynamics simulations reveal that the R-3 phase undergoes a transition to a superionic state at high temperatures, where Al3+ ions undergo partially disordered, rapid diffusion within the rigid iodine layers. We further demonstrate that the introduction of Al3+ vacancies substantially reduces the superionic transition temperature. Our work not only maps the structural evolution of AlI3 under pressure, but also provides a key reference for the structural design of metal halides under high pressure.