Hexagonal InOI monolayer: A 2D phase-change material combining topological insulator states and piezoelectricity

Two-dimensional (2D) phase-change materials (PCMs) with moderate transition barriers and distinctly contrasting properties are highly desirable for multifunctional devices, yet such systems remain scarce. Using first-principles calculations, we propose a hexagonal InOI monolayer as a promising 2D PCM. This material exhibits two distinct polymorphs: an energetically favorable T′ phase and a metastable T phase, differentiated by iodine atom positions. The T′-to-T structural phase transition features a moderate energy barrier Eb of 72.1 meV per formula unit, facilitating reversible switching. Notably, strain engineering tailors the electronic transition, inducing either a metal-to-topological insulator or a metal-to-normal insulator transformation. Additionally, this phase transition modulates the piezoelectric response and shifts optical absorption from the infrared to the visible range. These multifunctional properties make 2D hexagonal InOI highly promising for applications in non-volatile memory, low-contact resistance spintronics, and optical switching devices.