Complex bonding in perovskite ferroelectrics

Perovskite ferroelectrics are an important class of functional ceramics. For more than 60 years they are well established materials for charge storage devices and all kinds of piezoelectric components. Increasing demands still drive their development, e.g. currently the quest for lead-free piezoelectric ceramics. Properties of perovskite ferroelectrics can be tailored taking into account the atomic mass of its constituents, which influences the ferroelectric/paraelectric phase transition, and their ionic radii, which are responsible for the symmetry of the lattice. By proper choice of ionic radii advantage of a morphotropic phase boundary can be taken. Donor- or acceptor-doping influences the defect thermodynamics and decides over “hard” or “soft” ferroelectrics. For the further development of ferroelectric ceramics the bonding situation may gain increasing importance. In this paper concepts for covalent or better complex bonding in perovskites are introduced and their consequences for the stability of structures and other material properties are outlined.