Polar twin boundaries and nonconventional ferroelectric switching

Polar twin wall properties in non-polar or weakly polar matrices can be switched by electric fields with a well-defined ferroelectric hysteresis. While the bulk is nonpolar or weakly polar, the ferroelectric hysteresis stems from the interplay of the field-induced polarization in the bulk and the rotation of the dipole moments in the twin walls. While each tilt of a dipole would be a linear function of the applied field, their combination leads to frustration: The boundary polarity hinders the development of the bulk polarity for weak fields perpendicular to the twin walls. Only when the boundary polarity is sufficiently rotated under stronger fields do we find a sudden collapse of the frustration and the rapid development of bulk polarity. This effect suggests that wall polarity (as observed in CaTiO3 and SrTiO3) may lead to nonconventional ferroelectric switching including the bulk rather than exclusively inside domain walls.