What is a ferroelectric-a materials designer perspective

Ferroelectrics are usually defined as polar materials that display polarisation hysteresis. This is a functional definition. A structural definition exists too, based solely on symmetry arguments. The structural definition, useful e.g. for design of new materials, says that a ferroelectric is a material that underwent a phase transition from a parent higher-symmetry phase into a ferroelectric phase, such that the polar directions in the ferroelectric phase are linked to each other by lost symmetry operations of the parent phase.Ferroelectrics are discussed in this paper from a structural point of view. The structural definition helps also in clarifying the relationship between ferroelectrics and antiferroelectrics, conducive for the design of new antiferroelectrics or tuning of existing ones. Relaxor ferroelectrics are discussed too, in the structural context. The contemporary material (Na1/2Bi1/2)TiO3 is then used to illustrate the subjects discussed, the existing knowledge, and the missing understanding. This paper, on the occasion of the 500(th) volume of the journal Ferroelectrics has the limited scope of (a) reemphasizing the structural aspects of ferroelectricity and (b) serving as a primer on ferroelectric materials for the novice.