Phonon and Relaxation Dynamics in the Relaxor Ferroelectrics PbMg _1/3 Nb _2/3 O ₃ Using Raman Light Scattering.

ABSTRACT This paper presents the results of a study on the vibrational spectrum of the PbMg 1/3 Nb 2/3 O 3 (PMN) crystal using Raman light scattering across the temperature range of 100 to 1150 K. At T > 1050 K, polarized weak modes are observed in the Raman spectra of PMN. These weak modes can be associated with local distortions of the perovskite structure. When cooling PMN, in the vicinity of T 1 ≈ 1050 K, the intensity of the observed modes in the Raman spectrum increases abruptly by more than an order of magnitude. The appearance of polar nanoregions (PNRs) around T d ≈ 640 K is manifested only in the behavior of the intensities of Raman modes. The change of regimes in the evolution of PNRs from dynamic to static ( T* ≈ 400 K) is accompanied by anomalies in the temperature dependences of the frequencies of low‐lying optical phonons. The temperature behavior of the high‐frequency mode is governed by anharmonicity, with deviations observed around T* ≈ 400 K. The temperature dependence of the deviations correlates well with a critical behavior of the strictly elastic neutron scattering. Frequency dispersion in the temperature behavior of low‐lying phonons in the PMN has been discovered. The anisotropy of quasi‐elastic light scattering (QELS) is shown. The temperature dependences of the integrated intensity and width of the vertical–vertical and vertical–horizontal polarization components of QELS differ, but have anomalies near the Burns temperature, T* , and at the “freezing” of PNRs ( T vf ≈ 250 K), thus revealing the entire sequence of PMN phase transformations. The obtained results are discussed in the context of modern concepts of the physics of relaxor ferroelectrics.