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A-site off-stoichiometry has been an efficient method to enhance the electromechanical properties of lead-free Na0.5Bi0.5TiO3(NBT)-based piezoceramics. In this work, we have reported the effect of Na/Bi off-stoichiometry on the microstructural, structural, and electromechanical properties of the tetragonal 0.80Na0.5Bi0.5TiO3–0.20BaTiO3 (NBT–20BT) ceramic. The maximum piezoresponse ~ 115 pC/N is obtained for 4 mol% Na-deficient (20BT–Na46) and 2 mol% Bi-excess (20BT–Bi52) compositions. It is a 25% increment over the piezoresponse of the stoichiometric composition (20BT–Na50 ~ 90 pC/N). The enhancement in piezoresponse has been attributed to an optimized presence of polar-structural heterogeneity/disorder in the poled ceramic matrix and an optimized distortion of the long-range tetragonal ferroelectric phase. It is established that A-site off-stoichiometry is influencing the polar-structural heterogeneity/disorder and tetragonal lattice distortion via the grain size.

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.

Lead free (0.94-x)Na0.5Bi0.5TiO3–0.06BaTiO3-xK0.5Na0.5NbO3 ceramics are under the spotlight owing to their multifunctional nature. We have systematically investigated the microstructural, structural, dielectric, piezoelectric and energy storage properties of the system in the range 0 ≤ x ≤ 0.08. There is a systematic reduction in grain size with increasing K0.5Na0.5NbO3(KNN) concentration. The enhancement in disorder with K0.5Na0.5NbO3(KNN) doping is evident from the increase in slope of the dielectric dispersion plots. The unpoled compositions have a cubic like structure in the whole range i.e. from x = 0 to x = 0.08. However, the application of electric field(poling) results in a structural transformation to lower symmetry upto x = 0.04. Beyond that the disorder dominates over the applied electric field and the structure is cubic like even in the poled state. The energy storage density increases from 0.08 J/cm3 for x = 0 to 1.19 J/cm3 for x = 0.05 and saturates thereafter. Discharge efficiency also follows a similar trend. The piezoresponse decreases with KNN doping from 151 pC/N for x = 0 to 4 pC/N for x = 0.08. It is observed that energy storage density and piezoresponse are complimentary properties which increase at the expense of each other.