Effect of isovalent lanthanide cations compensation for volatilized A-site bismuth in Aurivillius ferroelectric bismuth titanate

Bismuth titanate ferroelectric ceramics (BiT) compensated(doped) by 3 mol% of Bi³⁺ or Ln³⁺ (BiT–Bi, BiT–Ln; Ln = La, Nd, Sm) were synthesized by conventional solid-state reaction, then the effects of compensation towards the lattice structure as well as the ferroelectric properties were studied. Rietveld refinement of the structure confirmed the pure Aurivillius phase with space group B2cb in all the constituents, however the trace amount of Bi₂O₃ and Ln₂O₃ could also act as sintering aids during sintering. The 6s² lone pair electrons of Bi³⁺ in perovskites localize into a lobe shape and the lobes major axis prefer to align along the a-axis, resulting the Bi³⁺ in perovskites showing a smaller effective radius compared with Ln³⁺ and causing higher lattice distortion. In addition, the paraelectric–ferroelectric phase transition temperature (Curie temperature, T_c), piezoelectric coefficient (d₃₃), spontaneous polarization (P_s), a.c. conductivity [σ(ω)] and activation energy of dielectric relaxation (E_a) increase with decreasing A-site cation radius. Furthermore, ferroelectric polarization and electromechanical properties of the BiT–Bi were enhanced dramatically due to the oxygen vacancies suppression by the compensation, which was confirmed by electric modulus approach.