IMF2017

PbNi_1/3Nb_2/3O₃ (PNN) belongs to cubic relaxor ferroelectrics. Its lattice dynamics is poorly studied. It is known that a broad frequency-dependent anomaly of the dielectric and acoustic response is observed near 153 K [H. J. Fan, et.al. JAP, 91, 2262 (2002)]. The crystal symmetry changes near 153 K from the cubic to a rhombohedral ferroelectric phase. Investigations of PNN magnetic properties have shown that it is paramagnetic up to helium temperatures [T.Shirakami et.al. JJAP, 39, L678 (2000)]. This report presents Raman studies of vibrational spectrum of the PNN crystal in a wide temperature range, 600-90 K. The experiments used a T64000 spectrometer (Joben Ivon) in the backscattering geometry. It has been found that at high temperatures a well-polarized Raman spectrum is similar to the scattering spectra for PMN crystals. A temperature decrease affects the Raman spectrum. Additional lines appear, the low-frequency region considerably changes. This behavior of the Raman spectrum for the PNN crystal points to a crystal symmetry change. By using group-theoretical analysis of the experimental spectra, possible models of the PNN structure in the low-temperature phase are considered. Analysis of temperature dependences of frequencies of the first-order scattering lines revealed a significant "softening" of the low-frequency vibration in the VV polarization near 150 K and also the anomalous behavior of hard modes in this temperature range. Fitting of the low-frequency region of the spectra showed the existence of quasi-elastic light scattering the behavior of which exhibited a strong dependence on temperature.