B23-O-08Microstructures in improper ferroelectric compounds revealed by electron microscopy

In improper ferroelectric materials, the order parameter of the phase transition is not the polarization but another physical quantity, where transformation properties are different from those of the polarization. The spontaneous polarization arises in the phase transition as a secondary effect [ 1 ]. For example, geometric ferroelectric such as the hexagonal manganites are improper ferroelectrics in which geometric structural constrains induce ferroelectric polarization [ 2 ]. An another example is a hybrid improper ferroelectricity such as (Ca,Sr) 3 Ti 2 O 7 , whichresults from the combination of two or more non-ferroelectric structural order parameters [ 3 ]. The coupling between the spontaneous ferroelectric polarization and other physical quantities should result in unique domain structures in the improper ferroelectric materials. Figure 1 is a domain structure in the improper ferroelectric compound BaAl 2 O 4 with the hexagonal structure [ 4 ]. Characteristic structural antiphase domains are observed, in which structurally modulated superstructure with 2 a × 2 a × c . In the presentation, unique charged domain walls found in some improper ferroelectric compounds will be reported.