A BaTiO3 single crystal, whose dimensions were 3.9 x 3.9 x 3.6 mm3, was mechanically and electrically poled to remove its 90-degrees and 180-degrees ferroelectric domain walls, respectively. After mechanical poling, four-step electrical poling was applied to the crystal, which did not include 90-degrees domain walls, with the aim of removing the 180-degrees domains gradually. In each of these steps, the dielectric constant of the crystal was measured using an impedance analyzer. Prior to the electrical poling, the dielectric constant in the [0011 direction was 137 at 100 kHz (unclamped) and 133 at 10 MHz (clamped). By applying four-step poling, these values varied gradually with the progress of poling, and became 130 and 58, respectively, after the final step, i.e., in a single-domain state.
