Abstract: Compositions in the perovskite family, having the general formula Pb（B₁,B₂）O₃ and known as ferroelectric relaxors are of considerable interest. However, such materials are difficult to be fabricated without the appearance of a pyrochlore phase. In this paper, recent works on the kinetics and thermodynamics of the perovskite vs pyrochlore formation specially for PMN are analysed, and several methods of preparing pure perovskite phase are presented.