Preponderant influence of disordered P4bm phase on the piezoelectricity of critical compositions of Na0.5Bi0.5TiO3-based ferroelectrics

We investigated the phase boundaries of three ${\mathrm{Na}}_{0.5}{\mathrm{Bi}}_{0.5}{\mathrm{TiO}}_{3}$-(NBT) based piezoelectric systems NBT-${\mathrm{BaTiO}}_{3}$ (NBT-BT), NBT-${\mathrm{K}}_{0.5}{\mathrm{Bi}}_{0.5}{\mathrm{TiO}}_{3}$ (NBT-KBT) and NBT-${\mathrm{SrTiO}}_{3}$ (NBT-ST), all exhibiting similar piezoelectric responses (${d}_{33}\ensuremath{\sim}180\phantom{\rule{0.16em}{0ex}}pC/N$). We found that, apart from a qualitative difference (NBT-KBT/NBT-BT exhibits R3c-P4mm interferroelectric instability and NBT-ST does not), the critical compositions of all the three systems exhibit onset of a disordered P4bm phase. Our study suggests that the preponderant effect of this distortion overrides the interferroelectric instability, limiting the piezoelectric response of NBT-BT/NBT-KBT like that in NBT-ST.