We present transmission electron microscopy observations of domain wall motion in thin freestanding KnbO_3 crystals under applied electric fields. Since there is no substrate, there is no elastic clamping of 90 deg domains. We observe that curved and tilted 90 deg domain walls are the most mobile, whereas untilted 90 deg domain walls are resistant to field-induced motion. We explain this result in terms of two factors. First, the switching pressure on a domain wall (P_2-P_1) centre dot E is determined by the relative electrostatic energies of the neighboring polarizations P_1 and P_2. Consequently, some 90 deg domain walls are immobile under certain field directions, leading to domain interlocking. Second, do-main walls experiencing a high switching pressure move by a ripple mechanism, and do not move as rigid sheets. The tilted wall region in such a ripple has a polarization charge, and an associated depolarization field, which reduces the local switching barrier. An accumulation of polarization charge can result in a tilted or curved wall, as occurs at the mobile tips of 90 deg domain needles. Any increase in density of immobile wall configurations with cycle time represents an inherent contribution to fatigue. Uniaxial ferroelectrics, with polarizations parallel to the field, should not experience such domain interlocking.
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