Raman investigation on electric field induced polarization frustrations in Ba-based relaxors

Relaxor ferroelectrics (RF) are widely known for their attractive electrical and electromechanical properties, which can be useful for energy storage devices or piezoelectric actuators. They are ferroelectric systems with fragmented or disordered states induced by chemical substitution. However, the origin of relaxor behaviour is still controversial. We attempt here to clarify this in homovalent (Zr4+) and heterovalent (Nb5+) B-site substituted barium titanate (BT) by carefully designed field cooling (FC) experiments. Both homovalent and heterovalent substitutions introduce relaxor behavior in BT over certain concentrations, but the mechanism is expected to be different. Homovalent substituents induce difference in the local polar order as a consequence of local strain fields (due to ionic radii mismatch), whereas heterovalent substituents directly introduce charge imbalances due to the different valence state(s). The effect of the latter in introducing relaxor behavior is expected to be stronger. Being sensitive to the material's short-range order/disorder, Raman spectroscopy (RS) as a function of temperature (T) is a suitable method to study the local structures in the vicinity of vacancies (only in heterovalent substitution) and foreign atoms (any substituted systems). FC experiments have the goal to evidence the presence of long-range ferroelectric states promoted by defect reorientation resulting from the applied electric field, which is expected to occur only in heterovalent-substituted materials. Results on phase transitions sequence and domain orientations will be presented and discussed for both Nb- and Zr-doped BT, in presence and absence of field cooling respectively. Correlation of Raman results with the local structure induced by substituents in the respective materials will be attempted.