Abstract:
In this paper, we present the results of a study of the optical phonon behavior of the PbMg$_{1/3}$Nb$_{2/3}$O$_3$ (PMN) crystal using Raman scattering in the temperature range from 1100 to 300 K. At $T>$ 1050 K, polarized “weak” modes and a quasi-elastic scattering component are observed in the Raman spectra of PMN. These “weak” modes can be associated with local distortions of the crystal lattice caused by disorder in the B-sublattice of the perovskite structure. When cooling PMN, in the vicinity of $T_1\approx$ 1050 K, the intensity of the observed modes in the Raman spectrum increases abruptly by more than an order of magnitude, the frequencies of these modes do not demonstrate visible anomalies. Further cooling of the PMN crystal leads to the restoration of the polarized Raman spectrum, well known from the literature. In the temperature evolution of the main modes, the Burns temperature ($T_\mathrm{d}\approx$ 640 K) can be distinguished, in the vicinity of which the intensities of the modes in the scattering spectra with VV polarization begin to increase linearly with decreasing temperature, in the absence of anomalies in the frequency behavior. Anomalies in the temperature dependences of the optical phonon frequencies are observed in the vicinity of $T^*\approx$ 400 K, where a change in the short-range order dynamics is assumed. The results obtained are discussed within the framework of modern physics of relaxor ferroelectrics.
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