Ferroelectric phase transitions inSrBi2Nb2O9andBi5Ti3FeO15: A powder neutron diffraction study

Abstract

Structural phase transitions in the Aurivillius phase ferroelectrics ${\mathrm{SrBi}}_{2}{\mathrm{Nb}}_{2}{\mathrm{O}}_{9}$ and ${\mathrm{Bi}}_{5}{\mathrm{Ti}}_{3}{\mathrm{FeO}}_{15},$ 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 ${A2}_{1}\mathrm{am}.$ 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 ${\mathrm{Sr}}_{0.85}{\mathrm{Bi}}_{2.1}{\mathrm{Ta}}_{2}{\mathrm{O}}_{9}$ and ${\mathrm{SrBi}}_{4}{\mathrm{Ti}}_{4}{\mathrm{O}}_{15},$ both of which have been shown unambiguously to transform via an intermediate paraelectric orthorhombic phase, space group Amam.