Charged domain boundaries stabilized by translational symmetry breaking in the hybrid improper ferroelectric Ca$_{3-x}$Sr$_x$Ti$_2$O$_7$

TL;DR

This study reveals atomic-scale structures and stabilization mechanisms of charged domain boundaries in a hybrid improper ferroelectric, highlighting the role of out-of-phase boundaries and strontium ions in charge stabilization.

Contribution

It uncovers a novel stabilization mechanism for charged domain boundaries involving out-of-phase boundaries and ionic arrangements in hybrid improper ferroelectrics.

Findings

Charged domain boundaries correspond to out-of-phase boundaries with fractional translational shifts.

Strontium ions are located at charged domain boundaries.

Out-of-phase boundaries may reduce polarization charge, stabilizing the charged state.

Abstract

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 Ca$_{2.46}$Sr$_{0.54}$Ti$_2$O$_7$. Like in the prototype improper ferroelectric YMnO$_3$, we find that charged domain boundaries in Ca$_{2.46}$Sr$_{0.54}$Ti$_2$O$_7$ 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 Ca$_{2.46}$Sr$_{0.54}$Ti$_2$O$_7$, 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.