Controlling the Chemical Order in PMN-Type Relaxors

Abstract

AbstractInvestigations of pure and chemically substituted tantalate and niobate members of the Pb(Mg1/3 Nb2/3)O3 (PMN) family of perovskite relaxors demonstrate that the degree of cation ordering and size of the chemically ordered domains can be extensively modified by thermal treatment. These observations and refinements of the cation occupancies in well ordered samples conflict with the predictions of the widely accepted “space charge model”, and instead support a “random site” model for the 1:1 B-site ordering. In this model one of the positions in the ordered structure is occupied by Ta (or Nb) and the other contains a random distribution of Mg and the remaining Ta cations. The stability of the order and magnitude of the domain growth can be enhanced by relatively small concentrations of selected solid solution additives (e.g. Zr in Pb(Mg1/3Ta2/3)O3 (PMT); Th or Sc in PMN). Correlations between the stability of the 1:1 order and the size of the solid solution additive can be used to understand the different responses of the cation order in PMN and PMT to thermal treatment. Results are also presented for the dielectric response of fully ordered relaxors in the (l–x) Pb(Mg1/3Ta2/3)O3 – (x) Pb(Sc1/2 Ta1/2)O3 system. While large domain samples of PMT-rich compositions exhibit a relaxor response, a crossover to normal ferroelectric behavior is observed for x = 0.5. The change in the dielectric response can be rationalized in terms of the effect of the chemistry of the random site sub-lattice on the correlation length of the ferroelectric coupling.