  Interlayer stacking order has recently emerged as a unique degree of freedom
to control crystal symmetry and physical properties in two-dimensional van der
Waals (vdW) materials and heterostructures. By tuning the layer stacking
pattern, symmetry-breaking and electric polarization can be created in
otherwise non-polar crystals, whose polarization reversal depends on the
interlayer sliding motion. Herein, we demonstrate that in a vdW layered
ferroelectric, its existing polarization is closely coupled to the interlayer
sliding driven by hydrostatic pressure. Through combined structural,
electrical, vibrational characterizations, and theoretical calculations, we
clearly map out the structural evolution of CuInP2S6 under pressure. A tendency
towards a high polarization state is observed in the low-pressure region,
followed by an interlayer-sliding-mediated phase transition from a monoclinic
to a trigonal phase. Along the transformation pathway, the displacive-instable
Cu ion serves as a pivot point that regulates the interlayer interaction in
response to external pressure. The rich phase diagram of CuInP2S6, which is
enabled by stacking orders, sheds light on the physics of vdW ferroelectricity
and opens an alternative route to tailoring long-range order in vdW layered
crystals.
