Highly degenerate 2D ferroelectricity in pore decorated covalent/metal organic frameworks.

Two-dimensional (2D) ferroelectricity, a fundamental concept in low-dimensional physics, serves as the basis of non-volatile information storage and various electronic devices. Conventional 2D ferroelectric (FE) materials are usually two-fold degenerate, meaning that they can only store two logical states. In order to break such limitation, a new concept of highly degenerate ferroelectricity with multiple FE states (more than 2) coexisting in a single 2D material is proposed. This is obtained through the asymmetrical decoration of porous covalent/metal organic frameworks (COFs/MOFs). Using first-principles calculations and Monte Carlo (MC) simulations, Li-decorated 2D Cr(pyz) ₂ is systematically explored as a prototype of highly degenerate 2D FE materials. We show that 2D FE Li _0.5 Cr(pyz) ₂ and LiCr(pyz) ₂ are four-fold and eight-fold degenerate, respectively, with sizable spontaneous electric polarization that can be switched across low transition barriers. In particular, the coupling between neighbouring electric dipoles in LiCr(pyz) ₂ induces novel ferroelectricity-controlled ferroelastic transition and direction-controllable hole transport channels. Moreover, three-fold and six-fold degenerate ferroelectricity is also demonstrated in P-decorated g-C ₃ N ₄ and Ru-decorated C ₂ N, respectively. Our work presents a general route to obtain highly degenerate 2D ferroelectricity, which goes beyond the two-state paradigm of traditional 2D FE materials and substantially broadens the applications of 2D FE compounds.