Enhanced piezoelectricity in ?⁢?⁢O3 ferroelectrics via intrinsic stress-driven flattening of the free-energy profile

An approach to greatly enhance the piezoelectric properties ($\ensuremath{\sim}400$ pC/N) of the tetragonal $\mathrm{BaTi}{\mathrm{O}}_{3}$ polycrystal using a small number of $A$-site acceptor-donor substitutions [D. Xu et al., Acta Mater. 79, 84 (2014)] has been proposed. In this study, $\mathrm{Pb}(\mathrm{ZrTi}){\mathrm{O}}_{3}$ (PZT) based polycrystals with various crystal symmetries (tetragonal, rhombohedral, and so on) were chosen to investigate the piezoelectricity enhancement mechanism. X-ray diffraction results show that doping generates an intrinsic uniaxial compressive stress along the ${[001]}_{\mathrm{pc}}$ direction in the $AB{\mathrm{O}}_{3}$ lattices. Piezoelectric maps in the parameter space of temperature and Ti concentration in the PZT and doped system show a more significant enhancement effect of $\mathrm{L}{\mathrm{i}}^{+}\text{\ensuremath{-}}\mathrm{A}{\mathrm{l}}^{3+}$ codoping in tetragonal PZT than in the rhombohedral phase. Phenomenological thermodynamic analysis indicates that the compressive stress results in more serious flattening of the free-energy profile in tetragonal PZT, compared with that in the rhombohedral phase. The chemical stress obtained by this acceptor-donor codoping can be utilized to optimize the piezoelectric performance on the tetragonal-phase site of the morphotropic phase boundary in the PZT system. The present study provides a promising route to the large piezoelectric effect induced by chemical-stress-driven flattening of the free-energy profile.