The phase transition between the ferroelectric and relaxor states for (1-x)Pb(Mg1/3Nb2/3)O-3-xPbTiO(3) (PMN-PT) single crystals at low PT (x = 0.21) was identified using high-resolution X-ray diffraction. The characterization of the dielectric and pyroelectric currents determined the properties of the polar state. The temperature stability region of the polar phase was enlarged with increasing bias field, and the corresponding polarization and piezoelectric properties were enhanced. Non-linearity of the electric displacement and loss under high ac fields was investigated by laser interferometry at a frequency of 40 kHz well below the resonant frequency. A linear displacement was obtained up to a large alternating current-field of 125 V/mm with an almost negligible second order harmonic component. The optimal d(33) improvement was 1,339 pm/V found for the [001] orientation. Such disclosure of phase stability and enhanced performances under high external field will be useful to determine their practical applications for resonant power transducers and quasi-static actuators.