Relationship between average and local crystal structure and the ferroelectric properties of a Sr-Bi-Ta-Si-O ferroelectric material

We investigated the relationship between the average and local crystal structures and the ferroelectric properties of Bi₂SiO₅, Bi₄Si₃O₁₂, or Bi₂O₃ added Sr_1-xBi_2+xTa₂O₉ (x=0, 0.2) produced by a solid-state reaction. By measuring the P-E hysteresis, we found that Sr_1-xBi_2+xTa₂O₉ (x=0, 0.2) has higher P_r and E_c than SrBi₂Ta₂O₉ (SBT). P_r increased and E_c decreased by adding Bi₂SiO₅, Bi₄Si₃O₁₂, or Bi₂O₃ to Sr_1-xBi_2+xTa₂O₉. The average crystal structures were determined by the Rietveld method. On the other hand, the local structure is important, because the ferroelectric property is related to the distortion, and ferroelectric complex oxides have domains. We also investigated the local crystal structure using atomic pair distribution function (PDF) analysis. Based on the results, the bond angle variance, σ², of each TaO₆ octahedron increased by substituting Si for the Ta site. The tilting angle, α_a,α_b, of each TaO₆ octahedron increased relative to that of the average structure, and the symmetry of the TaO₆ octahedron in the local structure deteriorated in comparison to that of the average structure. This distortion and symmetry of TaO₆ contributes to the remanent polarization.