Hidden improper ferroelectric phases for design of antiferroelectrics

Strong anomalous increase of the dielectric constant across a structural phase transition between two centrosymmetric phases, commonly observed in various crystals including prominent antiferroelectrics, is shown to originate from the hidden improper ferroelectric phases. In the vicinity of the phase transition double hysteresis loops of electric polarization vs electric field should be observed, which can be used for targeted design of antiferroelectric compounds. The suggested mechanism is illustrated by theoretical explanation of the recently discovered antiferroelectricity in the Ruddlesden-Popper compound ((CH₃)₂CHCH₂NH₃)₂CsPb₂Br₇. Implications of the suggested models for the phase transition between the R and P phases in NaNbO₃ are discussed.