On the nature of the phase boundary causing enhanced electromechanical response in the Na0.5Bi0.5TiO3-SrTiO3 piezoelectric system
2Centre for Nanoscience and Engineering, Indian Institute of Science, Bangalore, Karnataka, India
Na0.5Bi0.5TiO3 (NBT)-based ferroelectric systems are among the few Pb-free piezoelectric alternatives and have received considerable attention over the years. Most studies have focussed their attention on morphotropic phase boundary (MPB) solid solutions like Na0.5Bi0.5TiO3 -BaTiO3 (NBT-BT) and Na0.5Bi0.5TiO3 - K0.5Bi0.5TiO3 (NBT-KBT). Interestingly, the critical composition of the Na0.5Bi0.5TiO3 -SrTiO3 (NBT-ST) system shows almost similar piezoelectric response (d33 ~ 180 pC/N) as the MPB compositions of NBT-BT and NBT-KBT. We performed a comparative structural investigation of all three systems NBT-ST, NBT-BT and NBT-KBT using a combination of x-ray, neutron powder diffraction, and Eu+3 photoluminescence techniques. Our study revealed while NBT-BT and NBT-KBT exhibit rhombohedral (R3c)- tetragonal (P4mm) structural instability at their respective MPBs, the critical composition of NBT-ST does not show such an instability. The common features associated with the critical compositions of all the three systems was found to be the onset of a disordered P4bm phase which comprise of short-range ordered in-phase tilted octahedra. Based on our results, we argue that this distortion is the primary factor which determines (rather limits) the piezoelectric response of the three systems NBT-BT, NBT-KBT and NBT-ST.
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