Role of structural factors in the composition-driven phase transition to the antiferroelectric Pbam phase in rare-earth doped BiFeO3

As a consequence of man-made environmental pollution accelerating climate change, significant harm is being caused to the environment, and there are significant risks to humanity if current trends continue. This highlights the urgent need for environmentally friendly alternative materials for use in electrical and energy storage devices to help reduce pollution generated by currently used materials. Antiferroelectric (AFE) materials are a promising novel class of energy-storage dielectric materials due to their characteristic double hysteresis polarisation-electric field hysteresis loops, which enable higher energy density in storage devices compared to other materials. However, practical applications are limited by an incomplete understanding of the mechanisms driving AFE formation and challenges in optimising material performance. Bismuth ferrite (BFO) is particularly attractive in addressing these gaps. The literature reports indicate a composition-driven phase transition to a PbZrO3 -like AFE Pbam phase. By investigating the mechanism driving the formation of this AFE phase, insight into the factors controlling AFE behaviour. This thesis investigates the effect of samarium (Sm) substitution into BFO to determine the composition range for AFE Pbam phase formation and how different synthesis methods influence phase formation. Compositions from x=0.1 to x=0.2 in Bi1-xSmxFeO3 were synthesised in 5 % increments using two different methods, producing powder and pellet samples. X-ray powder diffraction and Raman spectroscopy were then used for structural analysis, while electrical and magnetic properties were evaluated via Polarisation-Electric (P-E) field hysteresis loops and Superconducting Quantum Interference Device (SQUID) measurements. The data confirmed the presence of the composition-driven phase transition from the ferroelectric (FE) R3c phase to the AFE Pbam to the paraelectric Pbnm phase in both sample types. The composition at which these transitions occurred differed between synthesis methods, suggesting that the synthesis method may influence Pbam phase formation. The role of size effects in Pbam formation was also explored. A proposed phase diagram of average A-site radius versus composition was used to guide co-doping with La and Sm to target three average radii, corresponding to different phases: 1.1676 Å, corresponding to the Imma(00ɣ)s00 phase, 1.1636 Å, corresponding to a phase mixture of Pbam and R3c, and 1.15797 Å, corresponding to the Pbam phase. X-ray diffraction and Raman spectroscopy confirmed the formation of the predicted mixed Pbam/Pbnm phases and a single-phase Pbam structure, though the Imma(00ɣ)s00 phase was absent. This suggests that size effects may influence AFE Pbam formation but does not fully account for phase behaviour. Overall, this work demonstrates that there are significant gaps in the understanding of AFE formation but highlights the potential role of size effects and synthesis method in controlling phase stability, providing guidance for future optimisation of AFE materials.