Enhancement of Piezoelectricity from Defect Dipoles in Transition Metal Doped Ferroelectrics*

Using a combination of first-principles density functional theory computations and fundamental experiments we are studying transitional metal doped ferroelectrics. We have performed molecular dynamics simulations using first-principles based potentials, and affirm the importance of defect dipoles in the ageing effect. We assume that defect dipoles, consisting of transition metals coupled with oxygen vacancies, align in the poling direction. Greatly enhanced piezoelectric coupling is obtained as the polarization rotates away from the poling direction. The defect dipoles provide a restoring force for the polarization to rotate back on reduction of the applied field. We have studied this as a function of dopant concentration and find that less than 1\% dopants are required to obtain an appreciable enhancement in electromechanical coupling. We are now performing experiments on transition metal doped samples. S. Liu and R. E. Cohen, APL 111, 082903 (2017);
J. B. J. Chapman, R. E. Cohen, A. V. Kimmel, and D. M. Duffy, PRL in press (2017)