Structural insight into the multifunctionality of non-stoichiometric BNT ferroelectrics

Fascinatingly high saturation polarization and electric-field induced strain make bismuth sodium titanium (BNT) promising alternatives. Interestingly, significantly improved oxide-ion conductive capacity and ultrahigh asymmetric strain can be stimulated respectively, both of which show great sensitivity to the non-stoichiometry brought by either nominal acceptor dopant or intrinsic Bi volatilization. The weak bonded Bi-O covalency of the ferroelectrics plays an unexpected role in the multifunctional presentations. The highly polarized Bi ions configured with lone pair electrons contribute to the off-centering of the coordination environment and varied bond lengths. The inconspicuous structural change concerning the multifunctionality raises difficulty and necessity in recognizing the origin on both average and local. Herein, the structural evolution and defect form on the lattice level are elaborated including the oxygen-octahedral tilting, cations displacements, and their chemical environment by comparing with nominal oxygen-deficient composition. The impedance, polarization, and strain responses are discussed in detail to reveal the local polar distortions and average disorder for the non-cubic polytypes. The less symmetry of the spatial configurations and larger cations displacement are identified. Combined with the oxygen vacancy and defect dipole dynamics in the ferroelectric/strain/conductive performances, this work will arouse the interest of Bi-based ferroelectrics in the search for their multifunctional applications.