Processing and characterization of lead magnesium tantalate ceramics
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Processing and characterization of lead magnesium tantalate ceramics Mehmet A. Akbas and Peter K. Davies Department of Materials Science and Engineering, University of Pennsylvania, 3231 Walnut Street, Philadelphia, Pennsylvania 19104-6272 (Received 10 October 1996; accepted 24 February 1997)
Using a processing route that employed platinum crucibles, single phase ceramics of Pb(Mg1/3 Ta2/3 )O3 (PMT) relaxor ferroelectrics were prepared with densities greater than 95% of their theoretical value. The improvements in the sintering characteristics of this 0 system that result from this route were reflected by the dielectric properties, emax 6300 at 182 K, which are similar to those reported for single crystal PMT. Contrast originating from nanosized polar clusters was evident in dark-field TEM images collected from the PMT ceramics at room temperature and showed little change upon cooling through the permittivity maximum. The electron diffraction patterns contained weak superlattice reflections at sh 6 1y2, k 6 1y2, l 6 1y2d that originate from a 1 : 1 ordering of the B-site cations. High resolution imaging indicated that the length scale of the chemical ordering in PMT is essentially identical to niobate relaxors such as PMN, with the 1–2 nm ordered domains being surrounded by a disordered matrix.
I. INTRODUCTION
Lead-based perovskite relaxor ferroelectrics have been the subject of extensive research in recent decades. As a result these systems, which are characterized by a diffuse and frequency dependent maximum in their relative permittivity,1 have been investigated for their suitability in applications ranging from multilayer capacitors, transducers, and electro-optic devices. In the Pb(BI1/3 BII 2/3 )O3 family of perovskites that contain a 1 : 2 distribution of divalent (BI ) and pentavalent (BII ) cations on the octahedral sites, Pb(Mg1/3 Nb2/3 )O3 (PMN) is perhaps the most well known and widely studied relaxor ferroelectric. Although the chemical ratio of the B-site cations in PMN is 1 : 2, several investigators have shown the structure of this and other related niobate relaxors comprises 1 : 1 ordered nanosized domains surrounded by a disordered matrix.2–4 Most investigators have interpreted these observations in terms of a “space charge” model2–4 in which the chemically ordered nanodomains are assumed to contain a 1 : 1 ratio of BI and BII , and the resultant charge imbalance is compensated by an equal and oppositely charged Nb-rich disordered matrix. Support for this “two-phase” model has come primarily from the apparent lack of coarsening of the ordered domains upon annealing.2,5 In comparison to the large number of papers that have appeared on the niobate perovskite relaxors, relatively few reports have appeared on their tantalate counterparts. Single crystal Pb(Mg1/3 Ta2/3 )O3 (PMT) was one of the first perovskite systems shown to exhibit relaxor behavior,6 but it is only recently that higher permittivity ceramics have been prepared.7 One of the J. Mater. R
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