Role of in-phase octahedral tilt influencing structure and properties in Na0.5Bi0.5TiO3 - based piezoceramics
Das Adhikary, Gobinda
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High performance perovskite (ABO3) based piezoceramics are used as actuators, pressure sensors, and transducers in wide-ranging applications spanning sectors like health, space, defence and automobiles. For over four decades, Pb(ZrxTi1-x)O3, commonly known as PZT, has been the preferred choice for such applications because this system exhibits high electromechanical response due to interferroelectric instability at its morphotropic phase boundary (MPB), which separates ferroelectric tetragonal (space group P4mm) phase from a ferroelectric rhombohedral (space group R3m) phase. Increased environmental concerns and governmental directives in the past two decades have sought to replace toxic Pb-based piezoelectrics with Pb-free alternatives for the commercial use. In this context the lead-free Na0.5Bi0.5TiO3 (NBT)-based piezoelectrics made a remarkable impact as it demonstrated seemingly MPB characteristics. The parent compound NBT exhibits local in-phase octahedral tilt embedded in ferroelectric (R3c) matrix. Population of in-phase tilt increases towards the MPB of NBT-based systems (especially Na0.5Bi0.5TiO3 -K0.5Bi0.5TiO3 (NBT-KBT) and Na0.5Bi0.5TiO3 -BaTiO3 (NBT-BT)), where both KBT and BT are tetragonal (P4mm) ferroelectrics and makes the systems complicated in contrast to conventional MPB of PZT. In this thesis work, we have demonstrated enormous impact of in-phase octahedral tilt on structure and properties of NBT-based piezoceramics. We have shown first time weakening of ferroelectricity at the MPB of NBT-KBT and NBT-BT due to increasing intervention of in-phase tilt. We also have addressed some unsolved issues related to depoling process of NBT-based system. So far it was believed in perovskite based ferroelectrics that normal ferroelectric to relaxor transition occurs in gradual manner du to increase in local random fields. First time we have shown an abrupt ferroelectric to relaxor crossover within tetragonal regime of NBT-KBT system caused by onset of in-phase tilt disorder. Simultaneously large piezoelectric response (~208pC/N), large electrostrain (~0.6%) and high depolarization temperature (~180 oC) have been observed at single tetragonal composition of NBT-KBT system without any doping among NBT-based systems due to enhanced reversible domain switching, restoring force for reversibility is provided by retained in-phase tilt as disorder.