W0127

Control and Modification of the Cation Ordering in "1:2" Mixed-Metal Perovskite Relaxor Ferroelectrics and Microwave Dielectrics. Peter K. Davies & M. A. Akbas, Department of Materials Science & Engineering, University of Pennsylvania, Philadelphia, PA 19104-6272 USA

The extensive studies that have been conducted on the Pb(Mg_1/3Nb_2/3)O₃ (PMN) family of perovskite relaxor ferroelectrics have led to the establishment and acceptance of the chemical segregation/space charge model as a basis for explaining their structures and dielectric properties. In this model it is assumed that the 1:1 chemically ordered nano-domains contain a 1:1 distribution of Mg:Nb and that the resultant charge imbalance is compensated by an Nb-rich disordered matrix. The primary experimental support for this interpretation of the B-site order has come from the apparent absence of any change in the degree of order with extended thermal treatment. Through a series of experiments on Pb(Mg_1/3Ta_2/3)O₃ (PMT) tantalate relaxors and PMN, we have found that the degree of cation ordering and size of the ordered domains can be modified by thermal treatment, but that the temperatures required to induce the increase in order are significantly higher than those previously explored. We have also discovered that the driving force for domain growth and the stability of the 1:1 order is significantly enhanced by the introduction of small levels of selected tetravalent dopants (e.g. Zr⁴⁺) onto the B-site sub-lattice. In contrast to other ferroelectric systems such as Pb(Sc_1/2Ta_1/2)O₃ (PST), the large domain, fully 1:1 ordered samples of PMT and PMN have been found to retain a relaxor type response. It is difficult to rationalize these new results in terms of the accepted space charge models, and the crystal chemical features of the PMN-type perovskites are more consistent with a charge balanced "random site" model for the chemical order. In this model for the 1:1 ordered Pb([beta]'_1/2[beta]"_1/2)O₃ structure, the [beta]" sites are exclusively populated by Ta(or Nb) while the [beta]' positions contain a random distribution of the Mg and remaining Ta cations. Results will also be presented on the modification of the cation ordering in the corresponding Ba-based systems, which are widely utilized as frequency filters in wireless communication devices. While systems such as Ba(Mg_1/3Ta_2/3)O₃ adopt a trigonal 1:2 ordered structure comprised of ordered layers of Mg and Ta, a transformation to a 1:1 ordered "PMN-type" doubled perovskite is induced by very small levels of Zr or Ce. The role of the substituent cations in promoting these transformations will be discussed.