First-principles study of ferroelectricity, antiferroelectricity, and ferroelasticity in two-dimensional γ -AlOOH

The exploration of two-dimensional (2D) ferroic materials and the investigation of ferroic couplings are highly desired in view of the design of next-generation functional devices. Herein, we report through first-principles calculations that the single-layer \ensuremath{\gamma}-AlOOH exhibits intrinsic ferroelectric (FE), antiferroelectric (AFE), and ferroelastic properties. The polarization and antipolarization originate from the orientational displacements of hydrogen atoms actuated by two soft phonon modes of the centrosymmetric phase. We studied a possible FE switching process and the FE-AFE transformation in the cases of monolayer and bulk, resulting in the prediction of a stable bulk AFE phase with similar energy to the FE ground state. Moreover, the \ensuremath{\gamma}-AlOOH monolayer shows a giant ferroelastic phase transition capable of efficiently tuning the ferroelectricity, which is able to accomplish a 90 \ifmmode^\circ\else\textdegree\fi{} switching of polarization and leads to an FE/AFE quadruple state. Our work adds a unique candidate to the family of 2D ferroics, broadening the platform for the design of ferroic-based devices.