X-ray Analysis and Computer Modeling of the Structure of `Relaxor' Ferroelectrics Pb₃MgNb₂O₉ and Pb₂ScTaO₆ in the Paraelectric State

A structural study of Pb₃MgNb₂O₉ and ordered Pb₂ScTaO₆ single crystals is made by X-ray diffraction at high temperatures. Displacements of the Pb atom from the special site are clearly observed. Disordered atomic positions are uniformly distributed over a sphere with radius 0.2-0.3 Å and with center at the special position. These atomic displacements are probably caused by local fields due to random statistical distribution of the (Mg, Nb) atoms in Pb₃MgNb₂O₉ and, partially, the (Sc, Ta) atoms in Pb₂ScTaO₆. Computer modeling, based on the total-energy-minimization method, gives satisfactory agreement with experimental results.