Chemical control of octahedral tilting and off-axis A cation displacement allows ferroelectric switching in a bismuth-based perovskite

Bi(Fe_2/8Mg_3/8Ti_3/8)O₃ (BFTM) is a member of a small class of pure Bi³⁺ A site perovskites which are stable without recourse to high pressure synthesis. BFTM is polar in the R3c space group, but ferroelectric switching of the polarisation is not attainable in bulk ceramics. Formation of solid solutions with BaTiO₃ produces enhanced functional behaviour. The composition 0.75Bi(Fe_2/8Mg_3/8Ti_3/8)O₃–0.25BaTiO₃ displays ferroelectric hysteresis loops and piezoelectric response (high field d₃₃ of 85 pC N⁻¹ at 0.1 Hz and low field d₃₃ of 16 pC N⁻¹). This change in functional behaviour is associated with significant changes in the average structure, where the rhombohedral distortion is reduced and transformed to a pseudo-cubic R3m space group, as substitution of the larger Ba²⁺ cation suppresses tilting of the BO₆ octahedra. Polar Bi displacements are refined solely along the pseudocubic <111>_p direction of the perovskite subcell, with the off-axis displacements characteristic of BFTM being suppressed. The local structure deviates from the average structure in a similar way to PZT as shown by diffuse scattering in selected area electron diffraction, suggesting correlated <111>_p displacements along directions other than the average rhombohedral unique axis. The switchable polarisation measured by PUND measurements is considerably smaller than the remanence measured in P(E) loops.