thèse Al Zein

Thèse Ali Al Zein (LCVN et ICG)

« Order and Disorder of Relaxor and Ferroelectric Materials: Structural and Vibrational Studies »

Abstract
Among piezoelectric materials, lead-based ferroelectric perovskites are known to have the largest piezoelectric coefficients and electromechanical coupling. They are widely used in different industrial and technological applications. The so-called relaxors belong to this family. Their structure is characterized by the presence of randomly oriented polar nanoregions. In this thesis, we are interested in studying the structural and dynamical properties of prototypical ferroelectric materials and relaxors such as PbTiO₃, PbZr_0.52Ti_0.48O₃, PbMg_1/3Nb_2/3O (PMN), PbZn_1/3Nb_2/3O₃, and PbMg_1/3Ta_2/3O₃(PMT). The long and short range structure has been investigated by neutron diffraction and X-ray absorption fine structure (XAFS), while hyper-Raman scattering (HRS) is used to probe the vibrations. The local structure analysis of complex perovskite materials AB’B »O₃shows that pressure reduces the static disorder of the large cation occupying the B-site, while an applied electric field has an opposite effect. This field-induced distortion might relate to the large piezoelectric coefficient in such materials. HRS in PMN and PMT allows the first observation of the primary soft mode responsible for the temperature dependence of the dielectric constant. The selection rule analysis reveals the nature of the HRS active vibrational bands and enables us to get insights about the involvement of each atom in the structural modications upon temperature.