Inverse Design of Photoinduced Structural Phase Transitions in Monolayer Metal Diiodides MI₂ (M = Sn, Pb, Hg, Mg)

Photoinduced structural phase transitions (PISPTs) in two-dimensional (2D) materials offer an ultrafast and noninvasive route to access structures and properties unavailable under dark conditions. However, discovering PISPTs remains challenging because illumination fundamentally reshapes the energy landscape and invalidates conventional ground state searches. Here, we develop an inverse-design method for the unbiased exploration of photostabilized phases by using the photomodified Gibbs free energy as the global variable function. Applying this method to 2D metal diiodides, we identify nonvolatile terahertz (THz)-driven PISPTs in monolayer MI₂ (M = Sn, Pb, Hg, Mg). First-principles calculations show that the THz-stabilized SnI₂-Pmmn and HgI₂-C2/m phases exhibit significantly enhanced and anisotropic optical absorption. Moreover, the THz-stabilized HgI₂-P4m2 and PbI₂-Pmm2 phases host switchable fractional quantum ferroelectric states with in-plane polarizations and low switching barriers, suggesting nonvolatile and reversible phase transitions. Our work reveals rich opportunities for photoinduced structural phase engineering in 2D materials with emergent optical and ferroelectric properties.