Pii: S0022-3697(99)00275-9

High temperature thermal treatments were used to modify the cation order in several tantalate and niobate members of the Pb(Mg1/3Nb2/3)O3 (PMN) family of relaxors. The observation of complete 1:1 structural order in several compositions, and the refined cation occupancies of well-ordered samples conflict with the predictions of the “space charge” model, and support the “random site” description for the B-site order. In this charge balanced model one of the positions in the ordered structure is solely occupied by Ta (Nb), while the other contains a random distribution of Mg and the remaining Ta (Nb) cations. The stability of the order and magnitude of the domain growth is strongly influenced by solid solution additives. For Pb(Mg1/3Ta2/3)O3 (PMT), ordering-enhancing Zr, Sc, or La substituents increase the cation order–disorder transition temperature (,13758C in pure PMT) and promote extensive domain coarsening. For pure PMN a low temperature (,10008C) order–disorder transition prevents any structure modification, and domain growth could only be realized with additives (Tb, Sc, or La) that stabilize the order to temperatures where the samples are “kinetically active”. The retention of relaxor behavior in all the fully 1:1 ordered, large-domain PMT and PMN-based ceramics suggests that the disorder on the random site is critical in frustrating ferroelectric order. By systematically controlling the concentration of ferroelectrically active cations on this position in fully ordered (1 2 x)Pb(Mg1/3Ta2/3)O3–(x)Pb(Sc1/2Ta1/2)O3 solid solutions, a crossover from relaxor to normal ferroelectric behavior was induced at x ˆ 0:5: q 1999 Elsevier Science Ltd. All rights reserved.