  The van der Waals layered material CuInP2S6 features interesting functional
behavior, including the existence of four uniaxial polarization states,
polarization reversal against the electric field through Cu ion migration, a
negative-capacitance regime, and reversible extraction of Cu ions. At the heart
of these characteristics lies the high mobility of Cu ions, which also
determines the spontaneous polarization. Therefore, Cu migration across the
lattice results in unusual ferroelectric behavior. Here, we demonstrate how the
interplay of polar and ionic properties provides a path to ionically controlled
ferroelectric behavior, achieved by applying selected DC voltage pulses and
subsequently probing ferroelectric switching during fast triangular voltage
sweeps. Using current measurements and theoretical calculations, we observe
that increasing DC pulse duration results in higher ionic currents, the
build-up of an internal electric field that shifts polarization loops, and an
increase in total switchable polarization by ~50% due to the existence of a
high polarization phase which is stabilized by the internal electric field.
Apart from tuning ferroelectric behavior by selected square pulses, hysteretic
polarization switching can even be entirely deactivated and reactivated,
resulting in three-state systems where polarization switching is either
inhibited or can be performed in two different directions.
