Ferroelectric phase transitions in SrBi₂Nb₂O₉ and Bi₅Ti₃FeO₁₅:   A powder neutron diffraction study

Structural phase transitions in the Aurivillius phase ferroelectrics SrBi₂Nb₂O₉ and Bi₅Ti₃FeO₁₅, containing two and four perovskite layers, respectively, have been studied using powder neutron diffraction. At temperatures below the ferroelectric Curie temperature both materials crystallize in the polar orthorhombic space group A2₁am. On passing through T_C, both phases appear to transform directly to a tetragonal paraelectric phase, space group I4/mmm. This behavior contrasts with that of the analogs Sr_0.85Bi_2.1Ta₂O₉ and SrBi₄Ti₄O₁₅, both of which have been shown unambiguously to transform via an intermediate paraelectric orthorhombic phase, space group Amam.