Charged domain boundaries stabilized by translational symmetry breaking in the hybrid improper ferroelectric Ca3-xSrxTi2O7

Charged domain walls and boundaries in ferroelectric materials display distinct phenomena, such as an increased conductivity due to the accumulation of bound charges. Here, we report the electron microscopy observations of atomic-scale arrangements at charged domain boundaries in the hybrid improper ferroelectric Ca2.46Sr0.54Ti2O7. Like in the prototype improper ferroelectric YMnO3, we find that charged domain boundaries in Ca2.46Sr0.54Ti2O7 correspond to out-of-phase boundaries, which separate adjacent domains with a fractional translational shift of the unit cell. In addition, our results show that strontium ions are located at charged domain boundaries. The out-of-phase boundary structure may decrease the polarization charge at the boundary because of the ferrielectric nature of Ca2.46Sr0.54Ti2O7, thereby promoting the stabilization of the charged state. By combining atomic-resolution imaging and density-functional theory calculations, this study proposes an unexplored stabilization mechanism of charged domain boundaries and structural defects accompanying out-of-phase translational shifts. Charged domain walls in ferroelectrics are interesting as they may enhance electrical conductivity. Here, atomic-resolution imaging of Ca3-xSrxTi2O7 reveals that charged domain boundaries are stabilized by an out-of-phase translational shift of crystallographic domains and that Sr ions accumulate at the boundaries.