Anomalous behavior of long-wavelength optical phonons in a relaxor ferroelectric PbNi$_{1/3}$Nb$_{2/3}$O$_3$

The paper presents the results of studying the vibrational spectrum of the PbNi$_{1/3}$Nb$_{2/3}$O$_3$ (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 ${{T}_{m}} = 150$ 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 $AB' B'' O_3$. Analysis of the temperature evolution of phonon modes allowed us to identify special points in the dynamics of the PNN crystal lattice at $T_1 = 280$ K and $T_m = 150$ 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.