Growth of the chemically ordered domains in PMN-type relaxor ferroelectrics

The "space charge" models for the structure of the Pb(Mg1/3Nb2/3)O-3 (PMN) family of relaxors are based, in part, on the apparent absence of any change in the degree of chemical order with extended thermal treatment and the observation of an increase in the ordering in donor-doped PLMN solid solutions. Through experiments on the tantalate relaxer, Pb(Mg1/3Ta2/3)O-3(PMT), We have found that the as sintered microstructures do not represent a thermally equilibrated state and the degree of cation order can be extensively modified by thermal treatments. For PMT the modification of the ordering is also particularly sensitive to small levels of cation substitutions (e.g. Zr4+) on the B-site sub-lattice, and relaxors containing only 5% Zr could be fully 1:1 ordered in > 30 nm domains after annealing at 1325 degrees C. These results, coupled with the observation that the fully 1:1 ordered ceramics retain a relaxer type response, cannot be reconciled by the existing crystal chemical models and support a charge balanced "random site" model for the chemical order. Experiments on the niobate members of this relaxer family revealed that the ordering in PMN can also be modified by thermal treatment, though the conditions and the substituents required to promote ordering are quite different. In this case extensive chemical order was only observed after annealing samples of PMN that contained small concentrations of larger size B-site dopants such as Tb4+ or Sc3+.