Anomalous Behavior of Long-Wavelength Optical Phonons in a Relaxor Ferroelectric PbNi1/3Nb2/3O3

The paper presents the results of studying the vibrational spectrum of the PbNi_1/3Nb_2/3O₃ (PNN) crystal using Raman scattering in the temperature range from 100 to 650 K, including the region of the “diffuse phase transition” with a maximum permittivity at \({{T}_{m}} = 150{\kern 1pt} \) K at a frequency of 10 kHz. Polarized light scattering spectra in PNN are similar to those observed in a number of studied relaxor ferroelectrics with a perovskite structure \(AB{\kern 1pt} '{\kern 1pt} B{\kern 1pt} ''{\kern 1pt} {{O}_{3}}\). Analysis of the temperature evolution of phonon modes allowed us to identify special points in the dynamics of the PNN crystal lattice at \({{T}_{1}} = 280{\kern 1pt} \) K and \({{T}_{m}} = 150{\kern 1pt} \) K. It is shown that anomalies in the temperature behavior of optical phonons in the vicinity of \({{T}_{1}}\) can be associated with the dynamics of polar nanoregions, the nature of which changes from dynamic to static. In the studied temperature range, a sequence of phase transformations is proposed for PNN crystals.