Features of the Structural Model of the Low-Temperature Phase of Lead Magnesium Niobate—Relaxor Ferroelectric

Abstract

In this work, we discuss a model of the behavior of the crystal lattice in lead magnoniobate (PMN) crystals, beeing the classical ferroelectric relaxor , according to X-ray diffraction at temperatures of 103, 183 and 203 K. We have analyzed changes in the electron density in different cross-sections using difference synthesis with a temperature change below the phase transition temperature (T_m = 240 K). The obtained results of atomic displacements in PMN at temperatures below the phase transition temperature in this method of analysis are explained. The results are explained, if it is assumed that the deviations from 180° of the angles between the Mg/Nb–O–Mg/Nb–O bonds exist and because of the occurrence of some rotation of the chains Mg/Nb–O–Mg/Nb–O relative to their positions in an ideal cubic structure. The corresponding changes in the positions of the atoms must lead to some distortion of the oxygen octahedra in the chain, owing to the difference in the lengths of the interatomic Mg–O and Nb–O bonds forming at the apices of the octahedra. The maps of the electron density cross-sections and the diagrams of its distribution along the bonds in the PMN reveal changes in the displacements of atoms with decreasing temperature, and a correlation is observed in the directions of these displacements. The possible structural models of the low-temperature PMN contingency corresponding to the obtained electron density distributions are discussed in this work.