  We present in-situ transmission electron microscopy observations of domain
wall motion in thin freestanding potassium niobate single-crystals. We observe
that not all domains of a given polarization orientation are equally switchable
in applied electric fields. Tilted and curved 90 degree domain walls are common
and display field-induced mobility, whereas untilted domain boundaries resist
such motion. The sensitivity of the domain wall response to field direction
suggests that electrostatic energy contributions play a crucial role in the
observed switching properties. We can explain our results in terms of the
polarization charges and the resulting depolarization fields associated with
angled domain walls. A phenomenological Landau-Ginzburg analysis of a tilted
domain boundary indicates that such electrostatic effects reduce its local
switching barrier, thereby enhancing its field-induced mobility compared to
that of its untilted counterpart. Consequently not all domain walls are equally
mobile. Furthermore the switching efficiency of a particular domain is
determined by its allowed final states, as defined by its neighbors. Switching
will be inhibited if the relative energetics are unfavorable, and we call this
phenomenon domain interlocking. Any increase in density of such field-resistant
wall configurations with cycle time represents an inherent contribution to
ferroelectric fatigue. Uniaxial ferroelectrics, with polarizations parallel to
the field, should not support such interlocked domains.
