Dept. of Chem., Moscow State University, 119899, Moscow, Russia; A.F.Ioffe Phys.Tech.Inst. St.Petersburg, 194021, St.Petersburg, Russia
The disordered perovskite-like ferroelectrics with nonisovalent ions in the equivalent positions reveal a number of unusual physical properties [1]. Unfortunately the microscopic nature of the observed anomalies is not clear yet. So it is obvious, that the precise structural investigation of these crystals should be done. In this paper the results of X-ray diffraction study of Pb3MgNb2O9 (PMN) at 20°C and 300°C and partly ordered Pb2ScTaO6 (PST) at 60°C and 200°C are presented.
It is confirmed that even at high temperatures the lead ions are shifted from their symmetry sites and these shifts are random both in length and direction. To describe adequately the experimental data sets the model of the uniform disordering of the lead atom over a sphere with a radius 0.25(1)-0.29(1) A is proposed.
To clarify the reason for the appearance of discussed shifts the simple physical model based on pair atomic potentials is suggested for the crystals with randomly distributed cations. The refinement of the atomic positions to minimize the total energy show the significant shifts of the lead atoms in the range 0.1-0.15 A. The directions of the lead displacements are statistically distributed over space. The small displacements of the other cations, less than 0.05 A, are obtained as well.
1. M.E. Lines and A.M. Glass. Ferroelectrics and Related Materials, 1981, NY, 763 p.