  Using the Landau-Ginzburg-Devonshire approach, we study light-induced phase
transitions, evolution of polar state and domain morphology in
photo-ferroelectric nanoparticles (NPs). Light exposure increases the free
carrier density near the NP surface and may in turn induce phase transitions
from the nonpolar paraelectric to the polar ferroelectric phase. Using the
uniaxial photo-ferroelectric Sn2P2S6 as an example, we show that visible light
exposure induces the appearance and vanishing of striped, labyrinthine or
curled domains and changes in the polarization switching hysteresis loop shape
from paraelectric curves to double, pinched and single loops, as well as the
shifting in the position of the tricritical point. Furthermore, we demonstrate
that an ensemble of non-interacting photo-ferroelectric NPs may exhibit
superparaelectric-like features at the tricritical point, such as strongly
frequency-dependent giant piezoelectric and dielectric responses, which can
potentially be exploited for piezoelectric applications.
