Microscopic origin of the two-sublattice model for antiferroelectric state

The macroscopic model proposed by Kittel for antiferroelectrics assumes two interpenetrating sublattices with antiparallel polarizations. Although the model can explain many properties, it does not provide microscopic lattice displacement patterns associated with these sublattices since the order parameter in the model is a volume-averaged quantity. We propose a model based on microscopic positioning of symmetry elements and use an order parameter field directly associated with the local dipoles, which can provide a more transparent definition for the sublattices used in the Kittel model. We also include nonlocal gradient interactions to model multidomain structures and domain walls in ferroelectrics.