  The interest in ferroelectric van der Waals crystals arises from the
potential to realize ultrathin ferroic systems owing to the reduced surface
energy of these materials and the layered structure that allows for
exfoliation. Here, we quantitatively unravel giant negative electrostriction of
van der Waals layered copper indium thiophosphate (CIPS), which exhibits an
electrostrictive coefficient Q33 as high as -3.2 m4/C2 and a resulting bulk
piezoelectric coefficient d33 up to -85 pm/V. As a result, the
electromechanical response of CIPS is comparable in magnitude to established
perovskite ferroelectrics despite possessing a much smaller spontaneous
polarization of only a few uC/cm2. In the paraelectric state, readily
accessible owing to low transition temperatures, CIPS exhibits large dielectric
tunability, similar to widely-used barium strontium titanate, and as a result
both giant and continuously tunable electromechanical response. The persistence
of electrostrictive and tunable responses in the paraelectric state indicates
that even few layer films or nanoparticles will sustain significant
electromechanical functionality, offsetting the inevitable suppression of
ferroelectric properties in the nanoscale limit. These findings can likely be
extended to other ferroelectric transition metal thiophosphates and (quasi-)
two-dimensional materials and might facilitate the quest towards novel
ultrathin functional devices incorporating electromechanical response.
