I: a quadruple-well potential*

CuInP₂S₆ (CIPS) is a van der Waals solid that is ferrielectric below room temperature and has polarization in the stacking direction. Using density-functional-theory calculations, we discovered that, instead of the usual double-well potential, CIPS features a unique quadruple-well potential, with two low-polarization and two high-polarization states, with the latter corresponding to large Cu displacements to the layer surfaces, where they bond to the adjacent layers.[1] The quadruple well is tunable by strain, which can eliminate one or the other of the polar states. Quantum molecular dynamics shed light on the nature of polarization switching in the CIPS environment. The predicted features have been verified by experiments utilizing scanning probe microscopy (next abstract, Maksymovych et al.). The new results and the propensity of CIPS for ionic substitution opens new opportunities to control and generate ferroelectric properties in layered materials.
[1] J. A. Brehm et al. “Tunable quadruple-well ferroelectric van-der-Waals crystals”, Nature Mater., in press.