Self-Adaptive Layering Structure of Nanoconfined Fe-Ni Liquids: Origin of the Liquid-Liquid Phase Transition

Metallic liquids under confinement exhibit different properties compared to those of their corresponding bulk phases, such as miscibility, diffusion, and phase transitions. Unfortunately, the challenges in experimentally characterizing Fe-Ni liquids at the nanoscale and the high cost of first-principles simulations hindered the atom-level understanding that is necessary for controlling Fe-Ni liquids. Here, we report a comprehensive molecular dynamics study of the liquid Fe-Ni alloy confined within nanoslits. Driven by the slit size, the confined Fe-Ni liquid experiences the liquid-liquid phase transition (LLPT) that is characterized by layering transitions. Interestingly, during the LLPT, a transition liquid phase appears, separating two layering phases, which is accompanied by abnormal variations in density, potential energy, and pressure perpendicular to the wall. The Voronoi cluster analysis reveals a self-adaptive local structural evolution in confined Fe-Ni liquids during the LLPT. The effect of temperature, pressure, and composition on the LLPT is investigated. Based on the pressure-confinement phase diagram, the LLPT is mainly induced by confinement under high pressure, while under low pressure, the LLPT is mainly pressure-driven. Our research will stimulate more interest in the phase transition under confinement in a metallic system.