Polarization fluctuations in the perovskite-structured ferroelectric AgNbO3

Perovskite-structured $\mathrm{AgNb}{\mathrm{O}}_{3}$ is a promising lead-free ferroelectric material that at room temperature exhibits weak ferroelectric behavior with a large polarization under an applied electric field. Here we report first-principles molecular dynamics (FPMD) simulations of monocrystalline $\mathrm{AgNb}{\mathrm{O}}_{3}$ over a range of temperatures to examine the microscopic polarization-switching mechanism. Polarization switching is found to occur at temperatures around 200 K and above; regardless of whether the simulations commence from the antiferroelectric Pbcm structure or ferroelectric $Pmc{2}_{1}$ structure, above 200 K the crystal fluctuates between the two forms. The FPMD are consistent with the coexistence of the two phases at room temperature, which can explain the mixed ferroelectric/antiferroelectric behavior, such as double P-E hysteresis loops, observed experimentally.