Atomic-scale insights into microstructural complexity of van der Waals ferroelectric CuInP₂S₆

The rapid advancement of technology has positioned devices based on two-dimensional materials as a key solution to overcoming the limitations of traditional semiconductor materials. Among these, the ferroelectric compound CuInP₂S₆ (CIPS) has attracted significant attention for its multiple ferroelectric polarization states. However, its structural complexity, particularly its susceptibility to copper deficiency, presents notable challenges for further research. This study investigates copper-deficient CIPS (Cu: CIPS) to explore the structure-property relationship between ferroelectricity and structural composition at macroscopic and microscopic levels. Using spherical aberration-corrected scanning transmission electron microscopy, copper deficiency is shown to induce phase separation in CIPS, resulting in the coexistence of CIPS (Cc), CIPS (Cc′) (where the two layers experience a sliding displacement of 1/6b along the b-axis) and an In_4/3P₂S₆ structure containing small amounts of Cu (Cu: IPS) structure. Moreover, the intermediate phase exhibits a lack of ferroelectric switching response in piezoelectric force microscopy measurements. This work elucidates the microstructural evolution of Cu: CIPS, identifies the mechanism underlying the failure of ferroelectric switching in CIPS, and provides valuable insights for advancing CIPS-based ferroelectric memory devices.