X-RAY AND ELECTRON-DIFFRACTION STUDIES OF THE STRUCTURES OF PSEUDO-PEROVSKITE COMPOUNDS PB2(SC,TA)O6 AND PB2(MG,W)O6

Electron diffraction patterns, X-ray precession patterns and synchrotron Rietveld powder diffraction profiles were used to study the crystal structure of the pseudo-perovskite compound Pb2(Sc,Ta)O6 (PST). The results of a Rietveld refinement and single-crystal X-ray precession studies showed that PST has a lower symmetry than the cubic Fm3m in the paraelectric state. The remarkable similarities between the crystal structures of the antiferroelectric Pb2(Mg,W)O6 (PMW) and ferroelectric PST are studied in detail by electron diffraction and it is suggested that PST is a weak or frustrated antiferroelectric oxide. The influence of the degree of structural long-range order on the existence of an antiferroelectric phase transition in PST and PMW is discussed.