Unique and unusual properties of layered ferrielectric CuInP₂S₆– II. Experimental

The multifunctionality and van der Waals (vdW) layered structure of CuInP₂S₆ provides opportunities to achieve ultrathin electromechanical devices. The orientation and magnitude of the polarization in CuInP₂S₆ is determined by the position of Cu ions, which following theoretical predictions of 4 polarization states can either reside within layers or displace into the vdW gap (theoretical abstract by L. Tao).¹ These four stable uniaxial polarization states can be identified using a combination of density functional theory and scanning probe microscopy. Cu is also the most mobile ion, which leads to a unique interplay of ionic and ferroelectric properties that allows polarization alignment against an electric field.² This type of switching is underpinned by Cu motion across the vdW gap activated by the electric field. Based on this phenomenon, a polarization switching regime associated with negative capacitance is accessible.³ Similar functionality is expected in other materials where polar and ionic properties manifest in the same ionic sublattice.
¹Brehm et al, Nat Mater 19:43, 2020
²Neumayer et al, Phys Rev Appl 13:064063, 2020
³Neumayer et al, Adv Energy Mater 10:2001726, 2020