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 1/3 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 $${{T}_{m}} = 150{\kern 1pt} $$ 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.