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Abstract

Bismuth ferrite, BiFeO3 (BFO), has been recognized as a promising lead-free material for high-temperature piezoelectric applications due to its high Curie temperature of 825 °C. However, the practical use of BFO has been limited in particular by its elevated electrical conductivity. In addition, BFO shows the characteristics of ferroelectric hardening, reflected in pinched and biased polarization hysteresis loops and aging behavior. These phenomena have so far been explained by the presence of different types of point defects, including bismuth vacancies (𝑉𝐵𝑖′′′), oxygen vacancies (𝑉𝑂••), Fe2+ (𝐹𝑒𝐹𝑒′) and Fe4+ (𝐹𝑒𝐹𝑒•), as well as defect complexes, such as 𝑉𝐵𝑖′′′−𝑉𝑂••, 𝐹𝑒𝐹𝑒′−𝑉𝑂•• and 𝑉𝐵𝑖′′′−𝐹𝑒𝐹𝑒• (defects are given in Kröger-Vink notation). Despite being of key importance for designing the functional properties of BFO-based materials for high-temperature applications (>250 °C), the true origin and the complex inter-relationships between the electrical conductivity, point defects and hardening in BFO remains unclear and calls for a systematic study.
The objective of this PhD study was to controllably introduce point defects in BFO and investigate their influence on the electrical conductivity and domain-wall-pinning effects. This was achieved by introducing a Co dopant into the BFO along with post-annealing in different oxygen partial pressures.
All the Bi(Fe1-xCox)O3 ceramics contained comparable amounts of secondary phases (<1 wt%). It was found that the conductivity and hardening, the latter observed from polarization-electric-field hysteresis loops and the mechanical quality factor, increase with increasing Co concentration. The effect was attributed to the increased concentration of 𝑉𝑂•• and 𝐹𝑒𝐹𝑒•, hence it was concluded that Co acts as acceptor (𝐶𝑜𝐹𝑒′).
The domain switching was impeded after annealing the ceramics in N2, which was primarily attributed to the increased concentration of 𝑉𝑂•• and possibly 𝐶𝑜𝐹𝑒′. In-situ measurements of the electrical conductivity and Seebeck coefficient demonstrated the p-type conductivity; however, the fast redox kinetics upon isothermal switching between O2 and N2 atmospheres gave an indication of the local redox reactions. This was further confirmed by conductive atomic force microscopy (c-AFM) analyses, which revealed that the conductivity of the domain walls (DWs) and grain boundaries is on average lower after annealing in N2, thus demonstrating their p-type character. Furthermore, scanning transmission electron microscopy analyses confirmed the presence of 𝑉𝐵𝑖′′′ and 𝐹𝑒𝐹𝑒• at the DWs in as-sintered samples and a reduced 𝐹𝑒𝐹𝑒• concentration after N2 annealing. In addition, the electron paramagnetic resonance confirmed the presence of 𝐶𝑜𝐹𝑒′−𝑉𝑂•• defect complexes. The tendency of 𝑉𝑂•• to be close to Co was further demonstrated by first-principle calculations, giving strong indications that Co acts as an acceptor, i.e., 𝐶𝑜𝐹𝑒′.
Finally, the study showed that 𝐶𝑜𝐹𝑒′−𝑉𝑂•• defect complexes play the key role in the hardening behavior under switching conditions. On the other hand, the sub-switching converse piezoelectric response, in particular the large piezoelectric nonlinearity and hysteresis measured at low (Hz and sub-Hz) driving field frequencies, was found to be dominated by the p-type conductivity and thus 𝐹𝑒𝐹𝑒• defects. The results thus point to a complex role of point defects where different types of defects can control the macroscopic response depending on the driving field conditions (amplitude and frequency).