Abstract: The mechanism of the B-site ordering and disordering in Lead Magnesium Niobate (PMN) was examined by using both heat treatment and La-doped methods. The B-site structural ordering characteristics were exposed by
X-ray diffraction (XRD), selected area electron diffraction (SAED) and high-resolution electron microscopy (HREM). The reason of the pyrochlore
phase presenting with the La-doped increased was studied. Based on the analysis of dielectric properties, the increase in the degree of diffuse
phase transition (DPT) in annealed and La-doped PMN observed was interpreted as the Mg²⁺:Nb⁵⁺=1:1 nonstoichiometric short-range
ordered domain accompanied with the segregation of the Nb⁵⁺ in the disordered area. Furthermore, it was demonstrated that the kinetic mechanism
of the ordering micro-domain growing was ion diffusion equilibrium by the effects of the heat treatment on DPT properties of the various amounts
of La-doped.

Key words: relaxor ferroelectrics, order-disorder structure, diffuse phase transition, dielectric relaxtion