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 Angstrom 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.