Light-induced transitions of polar state and domain morphology of photoferroelectric nanoparticles

Using the Landau-Ginzburg-Devonshire approach, we study light-induced phase transitions, evolution of polar state, and domain morphology in photoferroelectric 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 photoferroelectric ${\mathrm{Sn}}₂{\mathrm{P}}₂{\mathrm{S}}₆$ 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 noninteracting photoferroelectric 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.