Synthesis and structure-property relationships in rare earth doped bismuth ferrite

Abstract

There has been significant interest in BiFeO₃ over the past decade. This interest has focused on the magnetic and electrical properties, which in the long term may prove useful in device applications. This thesis focuses on the synthesis, electrical characterisation, and structural origin of the electrical properties of rare earth doped bismuth ferrite. Two systems have been studied: BiFeO₃ doped with lanthanum and neodymium (Bi₁₋ₓREₓFeO₃ RE= La, Nd). Specific examples have been highlighted focusing on a detailed structural analysis of a lanthanum doped bismuth ferrite, Bi₀.₅La₀.₅FeO₃, and a neodymium analogue, Bi₀.₇Nd₀.₃FeO₃. Both adopt an orthorhombic GdFeO₃-type structure (space group: Pnma) with G-type antiferromagnetism. Structural variations were investigated by Rietveld refinement of temperature dependent powder neutron diffraction using a combination of both conventional “bond angle/bond length” and symmetry-mode analysis. The latter was particularly useful as it allowed the effects of A-site displacements and octahedral tilts/distortions to be considered separately. This in-depth structural analysis was complemented with ac-immittance spectroscopy using the multi-formulism approach of combined impedance and modulus data to correlate structural changes with the bulk electrical properties. This approach was essential due to the complex nature of the electrical response with contributions from different electroactive regions. The structural variations occur due to a changing balance between magnetic properties and other bonding contributions in the respective systems. This results in changes in the magnitude of the octahedral tilts, and A-site displacements giving rise to phenomena such as negative thermal expansion and invariant lattice parameters i.e., the invar effect. More specifically, analysis of Bi₀.₅La₀.₅FeO₃ highlights a structural link between changes in the relative dielectric permittivity and changes in the FeO₆ octahedral tilt magnitudes, accompanied by a structural distortion of the octahedra with corresponding A-site displacement along the c-axis; this behaviour is unusual due to an increasing in-phase tilt mode with increasing temperature. The anomalous orthorhombic distortion is driven by magnetostriction at the onset of antiferromagnetic ordering resulting in an Invar effect along the magnetic c-axis and anisotropic displacement of the A-site Bi³⁺ and La³⁺ along the a-axis. This contrasts with the neodymium analogue Bi₀.₇Nd₀.₃FeO₃ in which a combination of increasing A-site displacements in the ac-plane and decrease in both in-phase and anti-phase tilts combine with superexchange giving rise to negative thermal expansion at low temperature. The A-site displacements correlate with the orthorhombic strain. By carefully changing the synthesis conditions, a significant change in bulk conductivity was observed for a number for Bi₁₋ₓLaₓFeO₃ compositions. A series of Bi₀.₆La0.₄FeO₃ samples are discussed, where changes in the second step of the synthesis result in significantly different bulk conductivities. This behaviour is also observed in other compositions e.g. Bi₀.₇₅La₀.₂₅FeO₃. Changes in the electrical behaviour as a function of temperature are discussed in terms of phase composition and concentration gradients of defects. Activation energies associated with the conduction process(es) in Bi₁₋ₓLaₓFeO₃ samples, regardless of composition, fall within one of two broad regimes, circa. 0.5 eV or 1.0 eV, associated with polaron hopping or migration of charge via oxygen vacancies, respectively. The use of symmetry-mode analysis, in combination with conventional crystallographic analysis and electrical analysis using multi-formulism approach, presents a new paradigm for investigation of structure-property relationships in rare earth doped BiFeO₃.