Dielectric properties and structure of Bi4NbO8Cl and Bi4TaO8Cl

The layered intergrowth phases Bi4MO8Cl (M = Nb, Ta), structurally related to the well-known Aurivillius phase ferroelectrics, have been studied using SHG and a.c. impedance techniques. Both undergo a polar to centrosymmetric phase ferroelectric-type transition (T-c = 765 and 640 K for M = Nb and Ta , respectively). The crystal structure of the low temperature phase of Bi4N bO8Cl has been re-analysed, and a tentative structure for Bi4TaO8Cl determi ned, from powder neutron diffraction data. The basic structural features ar e confirmed, but significant details differ from the previously reported X- ray structure of Bi4NbO8Cl.