Moreover, the ferroelectric M 2Ge 2Y 6 monolayers exhibit independently switchable dipoles and maintain their ferroelectricity after contacting with graphene electrodes, indicating their high application potentials in high-density storage. The metastable centrosymmetric phase of M 2Ge 2Y 6 monolayers allows a new two-step ferroelectric switching path and may induce novel domain behaviors.

Interestingly, we find another centrosymmetric configuration which is dynamically stable but higher in energy comparing with the ferroelectric phase. The origination of the ferroelectricity of M 2Ge 2Y 6 is the vertical displacement of Ge-dimer in the same direction driven by a soft phonon mode of the centrosymmetric configuration. Herein, we report the distinct geometric evolution mechanism of the newly reported M 2Ge 2Y 6 monolayers and find out a large group of 2D ferroelectric candidates based on this mechanism. Discovery of novel two-dimensional (2D) ferroelectric materials and understanding the mechanism are of vital importance for the design of nanoscale ferroelectric devices.

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