Intrinsically scale-free ferroelectricity in two-dimensional M2X2Y6

Discovery of novel two-dimensional (2D) ferroelectric materials and understanding the mechanism are of vital importance for the design of nanoscale ferroelectric devices. Herein, we report the distinct geometric evolution mechanism of the newly reported M2Ge2Y6 monolayers and find out a large group of 2D ferroelectric candidates based on this mechanism. The origination of the ferroelectricity of M2Ge2Y6 is the vertical displacement of Ge-dimer in the same direction driven by a soft phonon mode of the centrosymmetric configuration. Interestingly, we find another centrosymmetric configuration which is dynamically stable but higher in energy comparing with the ferroelectric phase. The metastable centrosymmetric phase of M2Ge2Y6 monolayers allows a new two-step ferroelectric switching path and may induce novel domain behaviors. Moreover, the ferroelectric M2Ge2Y6 monolayers exhibit independently switchable dipoles and maintain their ferroelectricity after contacting with graphene electrodes, indicating their high application potentials in high-density storage. Furthermore, 16 ferroelectric (FE) M2Ge2Y6 and 65 potential FE M2Sn2Y6 monolayers are identified through high-throughput calculations. Our findings provide a new strategy for future discovery of novel 2D ferroelectric materials and also platforms for experimental design of related functional devices.