Multi-component quasi-elastic light scattering in Na$_{1/2}$Bi$_{1/2}$TiO$_{3}$ as studied by broadband Brillouin scattering

Brillouin light scattering studies of the low-frequency region of vibrational spectra of a partially disordered ferroelectric crystal Na$_{1/2}$Bi$_{1/2}$TiO$_3$ (NBT) from 300 to 850 K are presented. It is shown that the light scattering spectra contain a multi-component quasi-elastic scattering (QELS) observed in a broad frequency range, from 800 to 0.7 GHz. Reconstruction of the inelastic light scattering spectra in NBT in the frequency region from 0.7 to 800 GHz did not reveal relaxation processes ($\alpha$ and $\beta$ relaxations) typical of disordered compounds (glasses, supercooled liquids, etc.). The fractal approach also proved to be inapplicable to the description of QELS in NBT. Detailed analysis of the Brillouin spectra with different free spectral ranges (“frequency windows”) showed that different contributions to the quasi-elastic light scattering manifest themselves in different “frequency windows”. These contributions are associated with the structural phase transitions and other processes responsible for the emergence of QELS (domain structure evolution, heterophase fluctuations, etc.). Thus, we suggest a new approach to the analysis of the temperature behavior of quasi-elastic scattering. It allows one to study the critical dynamics of the crystal lattice and the nonphonon contributions into vibrational spectra of partially disordered crystals.