Lead-based relaxor ferroelectrics, such as Pb(Mg_1/3Nb_2/3)O3–PbTiO₃ (PMN-PT), have been studied extensively due to their exceptional electromechanical properties [1]. However, the mechanisms behind their ultrahigh piezoelectric properties are still debated. In recent years, there has been some evidence that the disorder, typical of relaxor ferroelectric ceramics, can enhance both lattice and domain-wall contributions to dielectric and piezoelectric response [2]. Despite these studies, the focus has shifted towards lead-free materials, and PMN-PT and other lead-based relaxor ferroelectrics will have to be replaced. One such promising lead-free relaxor material is the BFO-BT system with the morphotropic composition (~67% BFO), which has attractive piezoelectric properties (>110 pC/N) and a high Curie temperature (>400°C) [3]. However, the BFO-BT system has issues with i) heterogeneous composition characterized by core-shell structures and secondary phases and ii) high leakage current resulting from the evaporation of Bi₂O₃ and change in the valence state of Fe cations. To overcome these problems, MnO₂ is often added to increase the resistivity of the ceramic while the quenching method is often used to improve chemical homogeneity. However, it has been demonstrated in other systems, such BiFeO₃ and Pb(Zr,Ti)O₃, that quenched samples contain kinetically locked disordered defects which have the tendency to return to their equilibrium ordered positions over time, leading to time-changing electrical properties [4]. Therefore, it is clear that an alternative processing technique is needed. For these reasons, this work focuses on the influence of the mechanochemical activation (MA) on the reaction pathways and chemical homogeneity of BFO-BT ceramics. The MA proves to be a simple and effective way to produce high-quality ceramics with high densities and minimal amount of secondary phases. The effect of MA and MnO₂ addition on the microstructure of BFO-BT will be described and discussed. Moreover, some preliminary results on point defects of BFO-BT and BFO-BT +0.1 wt.% of MnO₂ will be presented by evaluating the electrical conductivity and Seebeck coefficient measured in situ under different temperatures and oxygen partial pressures to understand the redox processes and defect states in the ceramic.

References
[1] F. Li et al., Adv. Funct. Mater. 28, 1801504, 2018.
[2] M. Otonicar et al., Adv. Funct. Mater. 30, 2006823, 2020.
[3] G. Ferrero et al., J. Eur. Ceram. Soc. 43, 350-361, 2023. 
[4] T. Rojac et al., J. Appl. Phys. 108, 074107, 2010.