| Lead perovskite ferroelectric relaxors (PbMg1/3Nb2/3 O3, PbZn1/3Nb2/3O3 and their mixtures with PbTiO3) are promising materials for usage in electronic devices, like capacitors, memory elements, transducers, light modulators, etc. Despite significant research efforts, due to the high internal structural complexity of these compounds, the nature of the relaxor behavior is still puzzling. This Raman spectroscopy study is aimed at gaining a deeper understanding the relaxor properties. It starts with the relatively simple case of KTa 1−xNbxO 3. Depending on x, this material can exhibit either classical ferroelectric or relaxor properties. It is characterized by the internal orientational motion of ions, which becomes progressively more restricted with each transition to a lower symmetry phase. Our results demonstrate the presence of a strong coupling between this motion and phonon modes. We show that this coupling is the main reason for the existence of the depolarized component in the second order Raman spectra and that it is also responsible for the frequency decrease (softening) of the transverse acoustic mode down to the third and lowest transition, below which the reorientational motion is no longer allowed. The data presented here suggest that, in KTa1− xNbxO3 , each phase is preceded by the appearance of precursors. The knowledge gained from this model system has been applied to lead relaxors. Basing on the multiple peak fit decomposition of the polarized and depolarized scattering spectra in the temperature range of 1000–100 K, we present a new picture of the structural transformations in a relaxor crystal and of the nature of its Raman scattering. As in the case of KTa1− xNbxO3, lowering the temperature leads to the appearance of polar distortions and to progressive restriction of the reorientational motion. In lead relaxors, phonon Raman scattering is a product of multiphonon interaction processes mediated by the presence of dynamic and static disorder.; The results of this work may also be helpful for understanding other types of materials with internal orientational disorder, including glasses. |
