Strain tunable phase transition in two-dimensional CuInP2X6 (X = S and Se) compounds

This study explores the strain-mediated ferroelectric phase transition in monolayer CuInP2X6 (X = S and Se) using first-principles calculations. The CuInP2X6 compounds exhibit proper ferroelectricity, characterized by a spontaneous out-of-plane polarization. Biaxial compressive strain effectively tunes the energy difference between the polar and nonpolar phases. For CuInP2S6, a critical compressive strain of 7%–8% induces a transition where the polarization vanishes, accompanied by a discontinuity in internal stress, indicating a first-order character. Similar strain-induced transitions are observed in the CuInP2X6 compounds. These findings establish biaxial strain as a powerful and reversible tool for controlling ferroelectric order in this two-dimensional material family, offering prospects for tunable nanoscale ferroelectric devices.