  Using Landau-Ginzburg-Devonshire (LGD) phenomenological approach we analyze
the bending-induced re-distribution of electric polarization and field, elastic
stresses and strains inside ultrathin layers of van der Waals ferrielectrics.
We consider a CuInP2S6 (CIPS) thin layer with fixed edges and suspended central
part, the bending of which is induced by external forces. The unique aspect of
CIPS is the existence of two ferrielectric states, FI1 and FI2, corresponding
to big and small polarization values, which arise due to the specific four-well
potential of the eighth-order LGD functional. When the CIPS layer is flat, the
single-domain FI1 state is stable in the central part of the layer, and the FI2
states are stable near the fixed edges. With an increase of the layer bending
below the critical value, the sizes of the FI2 states near the fixed edges
decreases, and the size of the FI1 region increases. When the bending exceeds
the critical value, the edge FI2 states disappear being substituted by the FI1
state, but they appear abruptly near the inflection regions and expand as the
bending increases. The bending-induced isostructural FI1-FI2 transition is
specific for the bended van der Waals ferrielectrics described by the eighth
(or higher) order LGD functional with consideration of linear and nonlinear
electrostriction couplings. The isostructural transition, which is revealed in
the vicinity of room temperature, can significantly reduce the coercive voltage
of ferroelectric polarization reversal in CIPS nanoflakes, allowing for the
curvature-engineering control of various flexible nanodevices.
