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 M 2 Ge 2 Y 6 monolayers and find out a large group of 2D ferroelectric candidates based on this mechanism. The origination of the ferroelectricity of M 2 Ge 2 Y 6 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 M 2 Ge 2 Y 6 monolayers allows a new two-step ferroelectric switching path and may induce novel domain behaviors. Moreover, the ferroelectric M 2 Ge 2 Y 6 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) M 2 Ge 2 Y 6 and 65 potential FE M 2 Sn 2 Y 6 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.