| Summary: | Bismuth ferrite (BiFeO3) is an inorganic chemical compound with a distorted rhombohedral (R3c) perovskite structure. It is a base for promising PbO-free piezoelectric and ferroelectric materials and also exhibits multiferroic properties at room temperature. However, the major drawback of the material is its high leakage current particularly under high electric field. It is postulated that the high leakage currents arise from the presence of oxygen vacancies, VO formed by the reduction of Fe3+ to Fe2+ during sintering. Recently, RE doping has been shown by Karimi et al. (2009) to induce a transition from a ferroelectric (FE) to antiferroelectric (AFE) structure similar in character to that observed in Pb(Zr,Ti)O3 (PZT). The main goal of this work therefore, was to fabricate compositions in the solid solution Bi1-xNdxFeO3 but additionally utilise Ti4+ doping on the B-site to control conductivity with the intention of achieving greater understanding of the intrinsic ferroelectric and dielectric properties. Initially, Bi0.85Nd0.15FeyTi1-yO3 0 ≤ y ≤ 0.1 compositions were fabricated. For y = 0, a mixture of FE (R3c) and AFE (Pbam) phases were present in XRD spectra. For 0.01 ≤ y ≤ 0.03, all XRD peaks could be indexed according to the PbZrO3 (PZ)-like AFE phase. For higher values of y, the absence of unique PZ-like reflections and broad rather than split peaks in XRD spectra suggested that long range antipolar order was destroyed. The substitution of 1% Ti for Fe resulted in a large decrease in the room temperature bulk conductivity from ~ 1 mS cm-1 to < 1 μS cm-1 and a large increase in activation energy (Ea) for conduction from 0.29 eV to > 1.0 eV. Increasing the Ti concentration further had little effect on either conductivity or Ea. As a result of the decrease in conductivity, large fields (5kV/cm) could be applied to ceramic samples which resulted in linear dielectric behaviour typical for an AFE structure below the field required to switch to the FE state. Subsequently, phase transitions as a function of Nd concentration were investigated in 3% Ti doped Bi1-xNdxFeO3 ceramics. Paraelectric (PE) to FE transitions were observed for compositions with x ≤ 0.125 which manifested themselves as peaks in permittivity. In contrast, PE to AFE transitions for 0.15 ≤ x ≤ 0.20 gave rise to a step-like change in the permittivity, with x = 0.25 exhibiting no sharp anomalies and remaining PE until room temperature. The large volume changes at the PE to FE/AFE transitions reported by Levin and co-workers [Phys. Rev. B, 81, 020103 (2011)] were also observed by dilatometry. It is proposed that the large volume change coupled with their 1st order character constrains the transitions in Nd-doped BiFeO3 to occur uniformly throughout the material in an avalanche-like manner. Hence, anomalies in DSC, permittivity and thermal expansion were observed over commensurately narrow temperature intervals. Despite the large volume change and eye-catching anomalies in DSC, the latent heats for the transitions in Ti-doped Bi1-xNdxFeO3 are similar to Pb(Zr,Ti)O3 (1-3 kJ/mol) with each an order of magnitude greater than BaTiO3 (~0.2 kJ/mol). Thin films of Bi0.825Nd0.175Fe0.97Ti0.03O3, which in bulk ceramic has the PZ-like structure, were deposited by Pulsed Laser Deposition on Pt/Si substrates in collaboration with the group of Professor Trolier-Mckinstry at Pennsylvania State University, PA, USA. Thin films deposited at 625 oC and 650 oC were predominantly PZ-like but contained regions composed of the R3c phase. Polarisation vs. field measurements revealed large coercive fields (~400 kV/cm) and high remanent polarisation (100 C/cm2) which are speculated to at least in part arise from AFE-FE switching in the system.
|
|---|
