Emergent ferroelectricity in the two-dimensional Janus MoSSe monolayer driven by nondegenerate phonon instability

We report the discovery of bistable polar states with switchable polarization in the Janus monolayer 1T-MoSSe, induced by symmetry breaking in its chalcogenide atomic layers. Our results demonstrate that Janus 1T-MoSSe exhibits two out-of-plane bistable polar states with switchable polarization, rather than a polarization emerging from a non-polar phase, which is an unconventional form of ferroelectric-like behavior. First-principles calculations and phenomenological models reveal that the inequivalent stacking of sulfur and selenium (S/Se) atoms breaks the central inversion symmetry, activating non-degenerate phonon modes at the K-point (K₂/K₃) that drive the structural transformation of metastable d1TS and d1TSe phases. This coupling leads to bipolar control of out-of-plane polarization through atomic displacements and charge redistribution, resulting in a polarization change of Δ P ≈ ± 0.3\ μC/cm². The Landau free energy model indicates that anharmonic terms and inter-mode coupling give rise to an asymmetric double-well potential, which is crucial for bistable polar states. Molecular dynamics simulations show that the d1TS-phase remains stable at high temperatures, whereas the d1TSe-phase undergoes an irreversible phase transition near 300 K, accompanied by a Peierls-like distortion of the Mo atomic chain. This phase transition is driven by differences in electronegativity, atomic radius, and d–p orbital hybridization between S and Se. Our findings provide a theoretical framework for designing nonlinear responses in 2D ferroelectrics and suggest that low-energy polarization reversal at room temperature can be achieved through strain or electric field control, with significant implications for non-volatile memory and piezoelectric sensors.