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

摘要

Bi(Fe_(2/8)Mg_(3/8)Ti_(3/8))O3 (BFTM) is a member of a small class of pure Bi~(3+) 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 sohd solutions with BaTiO3 produces enhanced functional behaviour. The composition 0.75Bi(Fe_(2/8)Mg_(3/8) Ti_(3/8))O3-0.25BaTiO3 displays ferroelectric hysteresis loops and piezoelectric response (high field d_(33) of 85 pC N~(-1) at 0.1 Hz and low field d_(33) 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 Ba~(2+) 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 selectedarea electron diffraction, suggesting correlated <111>_p displacements along directions other than the average rhombohedral unique axis. The switchable polarisation measured by FUND measurements is considerably smaller than the remanence measured in P(E) loops.