Control of specific domain structure in KNbO₃ single crystals by differential vector poling method

The controllability of specific domain structures in KNbO₃ single crystals was investigated by electric poling to several different orientations at room temperature. By applying electric field to the direction corresponding to the differetial direction between the original and intended spontaneous polarization directions (differential vector poling method), 60°-, 90°-, and 180°-domain pairs were successfully fabricated under control in KNbO₃. The measured threshold electric field for 90°-domain pairs was the lowest (140 V/mm), which could be related to the difference between the piezoelectric coefficients of the poling direction. For 60°- and 90°-domain pairs, theoretical uncharged domain walls fabricated under control without the generation of other domain structures. For 120°-domain pairs, (011)_pc uncharged domain wall was not observed, which suggests the difficulty in generating uncharged 120°-domain walls without passing through the phase transition.