Ferroelectric and piezoelectric properties in the presence of compositionally broken inversion symmetry.

We extend our first-principles study of novel ferroelectric perovskite systems in which compositional inversion symmetry is broken [N. Sai, B. Meyer and D. Vanderbilt, Phys. Rev. Lett. 84, 5636 (2000)] by (i) focusing on the piezoelectric response of the triple-cell heterovalent system Ba(Ti-δ,Ti, Ti+δ)O[sub 3], and (ii) studying the strength of the symmetry breaking in the double-cell system (Ba, Sr)(Ti-δ,Ti+δ)O[sub 3] with simultaneous A-site and B-site substitutions. We observe the enhanced piezoelectric response coefficient e[sub 33] when increasing the compositional parameter δ in the triple-cell system. This enhancement is quite drastic for the metastable minimum, but only modest for the stable minimum, corresponding respectively to the minority and majority wells of the ferroelectric double-well structure. In the double-cell system, we find that the increase in strength of symmetry breaking with the chemical or concentration perturbation is dominated by the term linear in δ, in contrast with the δ[sup 3] dependence found in the triple-cell system. A symmetry-based justification of the dominance of the linear term is provided. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]