  <001> oriented 0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 single crystals were poled under
different electric fields, i.e. Epoling=4 kV/cm and Epoling=13 kV/cm. In
addition to the temperature-dependent dielectric constant measurement, X-ray
diffraction was also used to identify the poling-induced phase transitions.
Results showed that the phase transition significantly depends on the poling
intensity. A weaker field (Epoling=4 kV/cm) can overcome the effect of random
internal field to perform the phase transition from rhombohedral ferroelectric
state with short range ordering (microdomain) FESRO to rhombohedral
ferroelectric state with long range ordering (macrodomain) FElRO. But the
rhombohedral ferroelectric to tetragonal ferroelectric phase transition
originating from <111> to <001> polarization rotation can only be induced by a
stronger field (Epoling=13 kV/cm). The sample poled at Epoling=4 kV/cm showed
higher piezoelectric constant, d33>1500 pC/N, than the sample poled at
Epoling=13 kV/cm.
