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.
Ferroelectricity in Bi 4 Ti 3 O 12 and Its Solid Solutions
On the basis of dielectric studies on polycrystalline specimens, Bi 4 Ti 3 O 12 has been established as a ferroelectric with a Curie temperature of 675°C. The symmetry is orthorhombic with a = 5.411 A, c = 32.83 A, and b/a = 1.007 at 25°C. A high-temperature x-ray study revealed a symmetry change to tetragonal at 675°C. The polar axis is probably the orthorhombic b axis. Bi 4 Ti 3 O 12 is a member of oxides with the general formula (Bi 2 O 2 ) 2+ (Me x-1 R x O 3x+1 ) 2- . According to Aurivillius, the crystal structure of Bi 4 Ti 3 O 12 comprises a stacking of Bi 2 O 2 and perovskite-like Bi 2 Ti 3 O 10 layers along the pseudotetragonal c axis. Multiple ion substitutions of (Bi 3+ Ti 4+ ) in (Bi 2 O 2 ) 2+ (Bi 2 Ti 3 O 10 ) 2- by (Me 2+ Nb 5+ ) where Me 2+ =Ba, Pb, or Sr lead to a steep decrease of the Curie temperature.
Dielectric behavior of Bi4Ti3O12 at low temperature
Abstract The dielectric permittivity, spontaneous polarization and coercive field at 60 Hz have been measured for fields applied along the ‘c’ axis of unpoled bismuth titanate crystals over the temperature range from 350°K to 5°K. In contrast to the behavior of normal ferroelectrics, P s decreases with temperature; E c at first decreases slowly to a minimum at about 100°K, then increases rapidly on further cooling. The permittivity e c only decreases slightly over the whole measured range.