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.