II: negative electrostriction and pressure-induced switching*

Layered ferroelectric CuInP₂S₆ (CIPS) is highly unusual in light of its negative longitudinal piezoelectric coefficient the recently predicted quadruple-well potential (previous abstract, Brehm et al.). Here we show an even larger spectrum of peculiar behaviors, obtained via systematic analysis of nanoscale electromechanical properties with scanning probe microscopy [1]: giant negative electrostriction, coexistence of high and low polarization phases with four distinct polarization orientations, and intrinsic pressure-induced polarization switching. These properties derive from the quadruple potential well for polar displacements, verifying its existence. The combination of unique properties enable deeper insight from microscopic measurements, such as imaging of inhomogeneous strain distribution. More broadly, the peculiarities of CIPS are in large part engendered by its van der Waals structure, thus opening new prospects at the intersection of ferroelectric and 2D materials. [1] J. A. Brehm et al. “Tunable quadruple-well ferroelectric van-der-Waals crystals” Nature Mater., in press.