Polarization rotation by external electric field in two-dimensional antiferroelectric squaric acid, H$_2$C$_4$O$_4$

Abstract

A pseudospin model for description of the influence of the electric field, confined to the plane of sublattice polarization, on the two-dimensional squaric acid antiferroelectrics is developed. The system behavior is analyzed in terms of the parameters of ferroelectric and antiferroelectric ordering, as well as the non-collinearity angle $\theta$, which is the angle between sublattice polarizations. The temperature-electric field $T-E_1$ phase diagram is constructed. Most of the field-induced transitions are found to be associated with polarization rotation. The observed phases include the collinear ferroelectric and non-collinear ferrielectric with almost antiparallel and perpendicular polarizations of the sublattices, respectively. The diagram also contains two regions, where the non-collinearity angle varies continuously between, nominally, 0 and 90$^\circ$ and between 90$^\circ$ and 180$^\circ$. The first and second order transition lines, supercritical lines, critical end points, and critical points on the $T-E_1$ phase diagram are detected.