USE OF IN-SITU XRD TO DEVELOP CONDUCTING CERAMICS WITH THE AURIVILLIUS CRYSTAL STRUCTURE
In-situ X-ray diffraction was used to study structural transformations in several n=3 Aurivilliustype phases: Bi4Ti3O12, Bi2La2Ti3O12, Bi2Sr2Nb2TiO12, Bi2Sr2Nb2AlO11.5 and Bi2Sr2Nb2GaO11.5. Order-disorder transformations have been linked to transitions to fast ion conduction in certain oxygen-deficient phases, such as Bi 2Sr2Nb2AlO11.5 and Bi2Sr2Nb2GaO11.5. Bi2La2Ti3O12 and Bi2Sr2Nb2TiO12 were synthesized and indexed as body-centered tetragonal with 4/mmm Laue symmetry. At room temperature, Bi4Ti3O12 has an orthorhombic unit cell with space group symmetry B2ab. As previously documented, it transforms to tetragonal above its Curie temperature of 675 �C. Indexing of XRD data collected for this study indicates the high temperature phase is body-centered tetragonal with 4/mmm Laue symmetry, similar to the structures of Bi2La2Ti3O12 and Bi2Sr2Nb2TiO12. None of these phases undergo additional phase transformations up to 1100 �C. Both Bi 2Sr2Nb2AlO11.5 and Bi2Sr2Nb2GaO11.5 have oxygendeficient perovskite-layers, with either primitive tetragonal or orthorhombic unit cells at room temperature. Both materials undergo multiple phase transformations. Above 875 �C, the unit cells are similar to those of the non-oxygen deficient n=3 Aurivillius-type phases, being bodycentered tetragonal with 4/mmm Laue symmetry. This behavior agrees with the expectations for oxygen-vacancy order-disorder transformations. The intermediate phases are not yet fully understood: they may be associated with partial disordering of oxygen vacancies or with a ferroelectric transition.
Recent developments in oxide ion conductors: Aurivillius phases
Recent advances in the study of Aurivillius phases (Bi2An-1BnO3n+3, n = 1−5) as oxide ion conductors are presented. The structures of modified Aurivillius phases containing extrinsic oxygen vacancies as well as Aurivillius phases containing intrinsic oxygen vacancies are surveyed. A detailed discussion of the conductivity behavior of these Aurivillius phases is given.
Electrical and Optical Properties of Ferroelectric Bi4Ti3O12 Single Crystals
The symmetry of ferroelectric Bi4Ti3O12 is shown to be monoclinic point group m. The spontaneous polarization PS is approx 50 μC/cm2 at 25°C and lies in the monoclinic a‐c plane at an angle of less than 5 deg from the plane of the crystal sheets. A complete determination of the optical indicatrix as a function of temperature is given. Switching for fields along the c axis occurs by a unique ``rocking'' of the large PS vector and leads to interesting electrical‐optical behavior. The domain structure is quite complex, and observed wall orientations are compared with those predicted theoretically.