| dc.description.abstract | Ferroelectric materials with a perovskite (ABO₃) structure exhibiting enhanced electromechanical properties are widely used in actuators, pressure sensors, electro-optic devices, and transducers across sectors such as healthcare, space, defence, and automotive industries. Among these, lead zirconate titanate (Pb(ZrₓTi₁₋ₓ)O₃ or PZT) has been the material of choice for over four decades due to its superior electromechanical performance, particularly near the morphotropic phase boundary (MPB) that separates the ferroelectric tetragonal (P4mm) and rhombohedral (R3m) phase fields. However, the toxicity of lead and its volatility during processing pose significant environmental and health risks, prompting global legislation to encourage the development of lead-free alternatives. This has driven intensive research into lead-free piezoelectrics, including (K,Na)NbO₃ (KNN), BiFeO₃ (BF), BaTiO₃ (BT), Na₀.₅Bi₀.₅TiO₃ (NBT) and K₀.₅Bi₀.₅TiO₃ (KBT) -based systems. KBT-based ferroelectrics have attracted attention for their distinctive properties, including high tetragonality, elevated depolarization temperature (~260°C) and the ability to form morphotropic phase boundary (MPB) lead-free solid solution with rhombohedral NBT. KBT has received limited attention in the literature, primarily due to challenges in achieving high densification during synthesis, which arise from the volatile nature of K, Na, and Bi. In this thesis, we have explored KBT-based solid solutions, and a comprehensive investigation has been carried out to understand the structure–property correlations in KBT and its related solid solutions. These structure–property correlations are central to the design and optimization of advanced ferroelectric materials for applications. This thesis primarily focuses on lead-free KBT and its solid solutions—including KBT–NBT (K₀.₅Bi₀.₅TiO₃-Na₀.₅Bi₀.₅TiO₃), KBT–BMT (K₀.₅Bi₀.₅TiO₃-Bi(Mg0.5Ti0.5)O3), KBT–KBZN ((K₁/₂Bi₁/₂TiO3-K₁/₂Bi₁/₂(Zn₁/₃Nb₂/₃)O₃), and KBT–KBMN (K₁/₂Bi₁/₂TiO3-K₁/₂Bi₁/₂(Mg₁/₃Nb₂/₃)O₃) to investigate the interplay between crystal structure and functional properties in ferroelectric materials. In the xKBT–(1–x)NBT system, the property anomaly observed at x = 0.40 has been investigated in depth by probing the local structure using transmission electron microscopy, complemented by X-ray and neutron diffraction techniques. Conclusive evidence of a rhombohedral phase persisting up to x = 0.40 well beyond the conventional MPB—has been established. Exploiting the tetragonality of KBT; the thermal expansion behaviour of KBT and its derivatives: KBT-NBT and KBT-BMT is studied in detail. This study reveals a strong correlation between tetragonality and zero thermal expansion, underscoring the role of structural control in achieving thermal stability. Furthermore, starting with the assumption KBT as lead-free analogue of PT. The parallels with classical lead-based relaxor systems such as Pb(Mg₁/₃Nb₂/₃)O₃–PbTiO₃ (PMN–PT) and KBMN–KBT and KBZN–KBT are critically re-evaluated, demonstrating that the enhanced electromechanical response in KBMN–KBT and KBZN–KBT arises from fundamentally different mechanisms. Collectively, these findings contribute to a better understanding of structure–property relationships in lead-free ferroelectric systems, with implications for designing next-generation functional materials. | en_US |
