The role of Pb in piezoelectrics and possible substitutions for it

The interplay between the various patterns of structural instability that are related to the electromechanical coupling in ferroelectric perovskites is discussed using results of density functional based first principles calculations. We focus on two main issues: (1) the competition between tetragonal and rhombohedral ferroelectric states in A-site driven (tolerance factor, t < 1) perovskites, and (2) the competition of ferroelectricity and antiferroelectric octahedral rotation. The role of Pb is reviewed in terms of cross gap hybridization of Pb 6p states with O 2p orbitals, and an alternate mechanism for reducing the tendency towards tilt instabilities in low tolerance factor perovskites based on frustration is discussed.