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.
Only the Third Heaven?: 2 Corinthians 12.1-10 and Heavenly Ascent
CHAPTER 1 General Introduction PART 1: Texts of Heavenly Ascent CHAPTER 2 Introduction: Heavenly Ascent Texts: A History of Scholarship CHAPTER 3 Jewish Apocalyptic Texts Pre-70 AD CHAPTER 4 Jewish Apocalyptic Texts Post-70 AD. CHAPTER 5 The New Testament CHAPTER 6 Post-Biblical Early Christian Apocalypses CHAPTER 7 The Nag Hammadi Codices CHAPTER 8 The Hekhalot Literature CHAPTER 9 Conclusions to Part One PART 2: 2 Corinthians 12:1-10 and Heavenly Ascent CHAPTER 10 2 Corinthians 12: 1-10 as a Heavenly Ascent CHAPTER 11 2 Corinthians 12.1-10 as a Subversion of the Heavenly Ascent CHAPTER 12 General Conclusions.