Structural transition in unpoled (1 - x)PMN-xPT ceramics near the morphotropic boundary

The phase relation and the structural transition in (1 - x)Pb(Mg_1/3Nb_2/3)O₃-xPbTiO₃ ceramics for 0.28 ≤ x ≤ 0.4 were investigated by high resolution X-ray powder diffraction (HR-XRD), temperature-dependent X-ray powder diffraction (T-XRD) and dielectric analysis. Unexpected broad and frequency dispersive dielectric anomalies were observed, just below the tetragonal (T)-cubic (C) transition dielectric peaks in temperature dependent dielectric permittivity in the unpoled ceramics for x = 0.32 and 0.34. Structural analysis revealed that the monoclinic (M_B) and tetragonal (T) structures coexisted in the composition range 0.32 ≤ x ≤ 0.36 at room temperature and in a wide temperature range for x = 0.32 and 0.34, however, in a non-equilibrium state. The sluggish M_B-T transition is closely related to the dielectric anomalies. The metastable ferroelectric phase diagram near MPB was re-constructed based on XRD and dielectric analysis. The phase transition procedure was discussed.