Defect structure of oxide ferroelectrics - valence state, site of incorporation, mechanisms of charge compensation and internal bias fields

The defect structure of aliovalent transition-metal and rare-earth functional centers in ferroelectric perovskite oxides is characterized by means of multifrequency electron paramagnetic resonance spectroscopy, assisted by density-functional theory calculations. The review is mainly focused on lead zirconate titanate (Pb[ZrxTi1- x]O-3, PZT) compounds. However, where available also results on ferroelectric 'lead-free' compounds are discussed. The results include the formation of charged (Fe'(Zr,Ti) - V-O center dot center dot)center dot defect dipoles, causing internal bias fields, multivalence manganese centers, acceptor-type copper functional centers creating isolated oxygen vacancies that promote ionic conductivity, as well as Gd-Pb center dot Pb donor-type centers. Moreover, the impact of the defect structure on macroscopic material properties is discussed.