Unique and unusual properties of layered ferrielectric CuInP2S6 - I. Theory*

CuInP₂S₆ (CIPS) is a van der Waals layered ferrielectric with out-of-plane polarization and an ionic conductor. By combining density-functional-theory calculations and piezoelectric-force-microscopy experiments (next abstract by S. Neumayer et al.), we demonstrate that CIPS features a quadruple-well potential with four polarization states, corresponing¹ to Cu atoms displacing within or just outside the CIPS layers. Under an applied voltage, the Cu atoms can go beyond the HP state and cross the vdW gaps, at which point the polarization aligns against the electric field.² Further, because CIPS has two centrosymmetric planes, middle of layers and middle of vdW gaps, the polarization curve as a function of Cu displacements has a negative slope within the vdW gaps, signaling a regime of negative capacitance by a novel mechanism. This talk will describe the theoretical findings. It is followed by a talk (S. Neumayer et al.) on experimental validation. The unique and unusual properties of CIPS present new opportunities for novel applications.
¹J. A. Brehm et al., Nat. Mater. 19, 43 (2020).
²S. M. Neumayer et al., Phy. Rev. Appl. 13, 064063 (2020).
³S. M. Neumayer et al., Adv. Energy Mater. 2020, online.