Increasing intervention of nonferroelectric distortion and weakening of ferroelectricity at the morphotropic phase boundary in Na0.5Bi0.5TiO3−BaTiO3

Morphotropic phase boundary (MPB) and polymorphic phase boundary ferroelectric solid solutions show enhanced electromechanical response and are sought after as actuator and transducer materials. Compared to others, the lead-free piezoelectric solid solution ${\mathrm{N}}_{0.5}\mathrm{B}{\mathrm{i}}_{0.5}\mathrm{Ti}{\mathrm{O}}_{3}\ensuremath{-}\mathrm{BaTi}{\mathrm{O}}_{3}$ stands out in two important respects: (i) low piezoelectric coefficients $(\ensuremath{\sim}200\phantom{\rule{0.16em}{0ex}}\mathrm{pC}/\mathrm{N})$ and (ii) anomalous decrease in the depolarization temperature at the MPB, the reason for which is still unclear. Here we show that both features are related; this is caused by an increasing intervention of a nonferroelectric structural distortion which compromises the gains the system acquires at the MPB via the interferroelectric instability. The propensity of the intervening nonferroelectric distortion, identified as in-phase octahedral tilt, grows considerably as the MPB is approached causing considerable structural-polar disorder, weakening the overall strength of ferroelectric interaction.