The ferroelectric phase transition in the Aurivillius phase ${\mathrm{Bi}}_3{\mathrm{Ti}}_{1.5}{\mathrm{W}}_{0.5}{\mathrm{O}}_9$ was studied by high-resolution neutron powder diffraction. Below the ferroelectric Curie temperature $T_c=735°\mathrm{C}$, ${\mathrm{Bi}}_3{\mathrm{Ti}}_{1.5}{\mathrm{W}}_{0.5}{\mathrm{O}}_9$ adopts the polar orthorhombic space group $A{2}_{1}am$. A direct transition to the nonpolar tetragonal space group $I4∕mmm$ is observed on passing through $T_c$, with no evidence of an intermediate nonpolar orthorhombic $Amam$ phase. Above $700°\mathrm{C}$, ${\mathrm{Bi}}_3{\mathrm{Ti}}_{1.5}{\mathrm{W}}_{0.5}{\mathrm{O}}_9$ is metrically “tetragonal” (i.e., $a=b$, within experimental precision), however, the presence of weak supercell reflections indicates that the material remains crystallographically orthorhombic below $T_c$.

• Received 19 June 2004

DOI:https://doi.org/10.1103/PhysRevB.71.024119