Recently a large number of functional materials have been proposed based on first-principles calculations. For example, hybrid improper ferroelectric compounds such as double layered Ca3Ti2O7, Ca3Mn2O7 and (Ca/Sr/Ba)3(Sn/Zr/Ge)2O7 with the Ruddlesden-Popper structure were predicted to show ferroelectricity from first-principles calculations [1]. Recently Oh et al. succeeded in preparing single crystals of hybrid improper ferroelectric (Ca,Sr)3Ti2O7 and found the intriguing ferroelectric domain structure consisting of abundant charged domain walls with conducting head-to-head and insulating tail-to-tail configurations, even though they usually accompany a large electrostatic energy cost. In this work, using state-of-the-art aberration-corrected high-angle annular-dark-field (HAADF) scanning transmission electron microscopy (STEM), we investigated ferroelectric domain wall structures in (Ca,Sr)3Ti2O7 crystals at the atomic scale. We succeeded in observing directly three distinct types of ferroelectric domain walls at the sub-atomic scale, i.e., 180° domain walls, head-to-head type and tail-to-tail type charged domain walls. The FE 180° domain wall is atomically sharp between two adjacent FE domains with antiparallel ferroelectric polarization. On the other hand, it is revealed that charged domain walls are stably and abundantly present and they have unique domain wall structures.
References
[1] N. A. Benedek and C. J. Fennie, Phys. Rev. Lett. 106, 107204 (2011).
[2] Y-S. Oh et al., Nature Mater. 14, 407 (2015).
[3] F-T. Haung et al., Nat. Commun. 7, 11602 (2016).