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 t he Pb atom from the special site are clearly observed. Disordered atom ic positions are uniformly distributed over a sphere with radius 0.2-0 .3 Angstrom and with center at the special position. These atomic disp lacements are probably caused by local fields due to random statistica l distribution of the (Mg, Nb) atoms in Pb3MgNb2O9 and, partially, the (Sc, Ta) atoms in Pb2ScTaO6. Computer modeling, based on the total-en ergy-minimization method, gives satisfactory agreement with experiment al results.