Emergence of an Abnormal Polar Phase Controlled by Cu Content in Cu _<i>x</i> InP ₂ Se ₆

Two-dimensional (2D) CuInP₂X₆ (X = S, Se) materials have emerged as promising candidates for a range of technological applications due to their layered structure and ferroelectric properties. By synthesizing Cu_<i>x</i>InP₂Se₆ with an excessive copper content, we observe an unusual polar phase at room temperature. In this study, Cu_<i>x</i>InP₂Se₆ (<i>x</i> = 1.1, 1.4, and 1.95) samples were prepared via chemical vapor transport, and all components exhibited similar X-ray diffraction and Raman characteristics. Cu_<i>x</i>InP₂Se₆ (<i>x</i> = 1.1, 1.95) samples exhibit negligible second-harmonic generation (SHG) response at room temperature. In contrast, the Cu_<i>x</i>InP₂Se₆ (<i>x</i> = 1.4) sample shows an asymmetric structure due to remarkable SHG response at room temperature. Theoretical calculations demonstrate that it arises from symmetry breaking induced by lattice distortion due to excessive doping of Cu. These results demonstrate that modulating the copper content provides an effective means of stabilizing the polar structure in CuInP₂Se₆, offering a novel strategy for the artificial control of phase symmetry in van der Waals materials.