Pii: S0022-3697(99)00286-3

The atomic structure of Pb(Sc1/2Ta1/2)O3 (PST), and related Pb(A,B)O3 type 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 function (PDF) analysis was used in studying the local atomic structure. The compounds examined, relaxor ferroelectric Pb(Sc2/3W1/3)O3 (PSW) and Pb(Mg1/3Ta2/3)O3–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 long range 1:1 (NaCltype) 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 from the average crystallographic structure determined by the Rietveld analysis. 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 the macroscopic easy axis of polarization appears to be [111]. These results show that the conflict between the local structural preference and the average structure is not limited to relaxor ferroelectric oxides, but may be widely prevalent in mixed-ion ferroelectrics. q 1999 Elsevier Science Ltd. All rights reserved.