Raman scattering at phase transformations in the relaxor ferroelectric PbCo$_{1/3}$Nb$_{2/3}$O$_3$

In this paper, we present the results of studies of the phonon and relaxation subsystems in the relaxor ferroelectric crystal PbCo$_{1/3}$Nb$_{2/3}$O$_3$ (PCN) using Raman scattering in the temperature range from $650$ to $80$ K, including the regions of “diffuse phase transition” with a maximum permittivity ${{T}_{m}} \approx 250$ K at a frequency of $1$ kHz and charge phase separation in the vicinity of $170$ K. It has been shown that the polarized spectra of PCN are similar to those observed in relaxor ferroelectrics with a perovskite structure. The analysis of the temperature behavior of the main phonon modes and anisotropic quasielastic light scattering revealed anomalies in the region corresponding to the onset (${{T}_{2}} \sim 230$ K) and final formation (${{T}_{3}} \sim 170$ K) of charge phase separation, determined by the presence of Co$^{2+}$ and Co$^{3+}$ ions in the PCN crystal. Anomalies in the evolution of the Raman spectra near the temperature of the relative permittivity maximum ${{T}_{m}}$, which are characteristic of relaxor ferroelectrics, were not observed in PCN. The dynamics of polar nanoregions, which determine the behavior of quasielastic light scattering, differ from those observed in the model relaxor ferroelectric PbCo$_{1/3}$Nb$_{2/3}$O$_3$.