Antiferroelectric-like Switching inside Ferroelastic Domain Walls

Ferroelastic materials (materials with switchable spontaneous strain) can be centrosymmetric, but their domain walls are always polar, as their internal strain gradients cause flexoelectricity. This flexoelectric polarization of the walls is not switchable by voltage, because reversing their polarity would require switching the spontaneous strain of the adjacent domains, annihilating the domain wall in the process. However, polarization can also arise from biquadratic coupling between polar and nonpolar order parameters (e.g., octahedral tilts in perovskites). Being quadratic, such coupling is independent of the sign of the polarization and thus allows switching between +P and -P. In this Letter, we seek to answer the question of whether the polarization of domain walls in an archetypal ferroelastic perovskite (CaTiO_{3}) is switchable, as per biquadratic coupling, or nonswitchable due to unipolar flexoelectricity. Using molecular dynamics calculations, we find that high electric fields gradually suppress tilts, broaden the ferroelastic domain walls, and eventually enable the onset of switchable polarization. Such switching can happen also in the bulk of the crystal, but the critical field is markedly lower in the domain walls, which are thus independently switchable. We also find that the polarization switching is antiferroelectric-like, with a double hysteresis loop. The analysis suggests that double-hysteresis switching can also occur in other ferroelastic perovskites.