Structural insights into electric field induced polarization and strain responses in K_0.5Na_0.5NbO₃ modified morphotropic phase boundary compositions of Na_0.5Bi_0.5TiO₃-based lead-free piezoelectrics

K0.5Na0.5NbO3 (KNN)-modified morphotropic phase boundary (MPB) compositions of the two Na0.5Bi0.5TiO3-based lead-free piezoelectrics, namely, 0.94Na(0.5)Bi(0.5)TiO(3-0.06)BaTiO(3) (NBT-6BT) and 0.80Na(0.5)Bi(0.5)TiO(3-0.20)K(0.5)Bi(0.5)TiO(3) (NBT-20KBT) are model systems exhibiting large (>0.4%) electric-field-driven strain. There is a general perception that (i) increasing KNN concentration monotonically weakens the direct piezoelectric response (d(33)), and (ii) maximum electrostrain occurs when KNN pushes the system in the fully ergodic relaxor state. We have examined these issues using various complementary techniques involv-ing electrostrain, piezoelectric coefficient (d(33)), ferroelectric switching-current measurements, and field-driven structural studies on the global and local scales using laboratory and synchrotron x-ray diffraction, neutron powder diffraction, and Eu+3 photoluminescence techniques. Our investigations revealed the following important features: (i) In the low-concentration regime, KNN induces a tetragonal ferroelectric distortion, which improves the weak signal piezoresponse. (ii) Beyond a threshold concentration, in-phase octahedral tilt sets in and weakens the long-range ferroelectric order to partially stabilize an ergodic state. (iii) The maximum electrostrain (-0.6%) is achieved in the mixed (nonergodic + ergodic) state. (iv) The mixed state invariably exhibits a less-known phenomenon of field-driven ferroelectric-to-relaxor transformation during bipolar field cycling. (v) The enhanced electrostrain in the mixed state is associated with the electric field increasing the correlation lengths of the short-ranged tetragonal and rhombohedral ferroelectric regions without overall transformation of one phase to the other. We summarize the findings of this work in a comprehensive electric field composition (E-x) phase diagram. The findings reported here are likely to be true for other NBT-based MPB systems.