Insights into the origin of multiferroicity and large in-plane piezoelectricity in AlXY ( X=S,Se ; <mml:math xmlns:mml…

Understanding the interplay of properties in two-dimensional (2D) multiferroic materials is of paramount importance for crafting the blueprint of cutting-edge functional devices in the next generation. In the present study, we report a family of stable multiferroics AlXY (X=S,Se; Y=Cl,Br,I) with the coexistence of ferroelectricity and ferroelasticity, using density functional theory (DFT) calculations. The AlXY monolayers (MLs) exhibit large in-plane ferroelectric polarization (Py) ranging from 148 to 177 pC/m with a moderate switching barrier of 0.102--0.192 eV/atom. The polarization in these MLs owes its origin to the repositioning of Al atoms, actuated by soft B₂ᵤ phonon mode in the paraelectric phase (Pmmn). These MLs exhibit robust ferroelasticity with a large reversible strain of 38%--45.1% and moderate switching barriers of 0.175--0.213/eV/atom. The ferroelectric and ferroelastic (FA) phases differ in the electric polarization direction by 90^∘ rotation. Besides a strong anisotropy in mechanical properties, in-plane piezoelectricity and carrier mobilities are observed in the AlXY MLs. Moreover, FA switching provides a highly effective way for finely tuning these anisotropic properties of AlXY MLs. Complementing these findings, we devised an empirical predictive model built on descriptors derived from linear regression analysis, linking atomic polarizability, Bader charge, lattice constant (b), layer thickness (h), bandgap, and effective mass of electron in order to estimate polarization (Py) and in-plane piezoelectric constants (d₂₂,0.16em0exd₂₁) of AlXY MLs, which is in excellent agreement (R²=0.95-0.98) with results obtained from DFT.