Local structure of Pb(Sc-1/2,Ta-1/2)O-3 and related compounds

The atomic structure of Pb(Sc1/2Ta1/2)O-3 (PST), and related Pb(A,B)O-3 typ e mixed perovskites was studied by diffraction using synchrotron X-rays and pulsed neutrons. The Rietveld refinement was carried out to determine the average, long-range crystallographic structure, while the pair-density func tion (PDF) analysis was used in studying the local atomic structure. The co mpounds examined, relaxer ferroelectric Pb(Sc2/3W1/3)O-3 (PSW) and Pb(Mg1/3 Ta2/3)O-3-PbZrO3 (PMT-PZ) in addition to PST, all exhibit long range 1:1 B- site cation ordering. The Rietveld refinement showed that the degree of lon g range 1:1 (NaCl-type) chemical ordering on the B-sites is up to 90%. For all samples including the nominally ordered PST the local atomic structure determined by the PDF analysis, was found to be significantly different fro m the average crystallographic structure determined by the Rietveld analysi s. Local deviations are greater for O and Pb atoms than the B-site cations. The direction of the local Pb displacement in PST is [100], even though th e macroscopic easy axis of polarization appears to be [111]. These results show that the conflict between the local structural preference and the aver age structure is not limited to relaxer ferroelectric oxides, but may be wi dely prevalent in mixed-ion ferroelectrics. (C) 1999 Elsevier Science Ltd. All rights reserved.