Accessing Uniaxial Four-Well Landscape of CuInP₂S₆ via Unconventional Polarization Switching Pathways

CuInP₂S₆ (CIPS), a van der Waals ferroelectric, features a unique four-well energy landscape and strong polarization-ion coupling. However, its polarization switching pathways remain unclear, impeding access to metastable states for multistate memory. By employing switching spectroscopy piezoresponse force microscopy (SS-PFM) and novel 3D domain mapping, we identify three unconventional switching pathways alongside a classical one. These originate from combinations of three fundamental Cu-ion switching modes, modulated by the driving voltage. We reveal Cu-ion transformations among intralayer stable sites, interlayer metastable sites, and interlayer displacement and elucidate a dual alteration mechanism involving both polarization direction and piezoelectric coefficient sign. These insights enable deterministic, repeatable, and reversible access to all four energy wells via precise electric bias control, thereby overcoming a key barrier to practical multistate CIPS applications.