Polarized Raman study of the phonon dynamics in Pb(Mg_1/3Nb_2/3)O₃ crystal

Pb(Mg_1/3Nb_2/3)O₃ is one of the simplest members of the class of lead relaxors and often serves as a model system for more complicated compounds. In this paper, we analyze both polarized and depolarized Raman-scattering spectra, measured in the temperature range between 1000 and 100 K, using a multiple-peak decomposition. Based on this analysis, we propose a comprehensive picture of the structural transformations in the crystal and associated dynamics. According to our model, the formation of the Fm3¯m symmetry in the chemically ordered regions as well as the appearance and freezing of the polar nanoregions are the consequences of the same phenomenon: the off-centered displacements of ions and their fast reorientational thermal motion. Short-lived dynamic lattice distortions are present even at the highest measured temperatures. From the Burns temperature, T_d∼620 K, their motion becomes progressively more restricted. Temperature T*∼350 K marks the beginning of the freezing process which continues down to the temperature of the electric-field-induced phase transition, T_d0∼210 K. Raman scattering exists due to the presence of local lattice distortions. It is characterized by phonons with different wave vectors interacting with dynamic and static disorder.