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, both of which show great sensitivity to the non-stoichiometry introduced by either nominal acceptor doping or intrinsic Bi volatilization. The weakly bonded Bi–O covalency in the ferroelectrics plays an unexpected role in the functional diversity. The highly polarized Bi ions configured with lone pair electrons contribute to the off-centering of the coordination environment and varying bond lengths. The inconspicuous structural changes concerning the multifunctionality raise difficulty and necessity in recognizing the origin at both average and local levels. Herein, the structural evolution and defect formation at the lattice level are elaborated including oxygen-octahedral tilting, cation displacements, and their chemical environment by comparing with the nominal oxygen-deficient composition. The impedance, polarization, and strain responses are discussed in detail to reveal the local polar distortions and average disorder in the non-cubic polytypes. The lower symmetry of the spatial configurations and larger cation displacements are identified. Combined with the oxygen vacancy and defect dipole dynamics in the ferroelectric/strain/conductive performances, this work will arouse interest in Bi-based ferroelectrics in the search for their multifunctional applications.