An investigation of the relationship between the structure and properties of some stacked perovskites

Abstract

This thesis discusses a comprehensive study of the structure and properties of selected stacked perovskites using non-ambient powder and single-crystal x-ray diffraction, powder neutron diffraction, birefringence microscopy and dielectric spectroscopy. The major portion of this work focuses on materials forming in Aurivillius phases, with the general chemical formula [Bi2O2]2+[An−1BnO3n+1]2−, and containing n perovskite layers sandwiched between fluorite-like interstices. Comparisons are made between the structure and properties of the isomorphous materials SrBi2Nb2O9 and BaBi2Nb2O9, and the ferroelectric phase transition in SrBi2Nb2O9 is compared to that in SrBi2Ta2O9. The latter shows two distinct phase transitions upon cooling from the high temperature phase: a ferroelastic transition to space group Amam at 848 K, followed by a ferroelectric transition to A21am at 608 K. By contrast, the transition in SrBi2Nb2O9 does not go via the intermediate Amam phase, as is demonstrated using both structural and property measurements. It is also shown that in a material that is both ferroelectric and ferroelastic, the individual contributions to the birefringence of strain and polarization can be isolated using birefringence microsocopy by applying the principles of Landau theory to extract the two contributions. The structure and properties of a number of other materials within the general class of ferroelectrics are investigated using similar techniques, in particular, single-crystal x-ray diffraction is used to reinvestigate the room-temperature structure of BaTiO3 following prominent reports of monoclinicity of this phase. Quantitative investigation of the (101) ferroelastic twinning in these crystals was also undertaken. In addition, evidence for the existence of a ferroelectric phase transition in CsBiNb2O7 is derived from the results of powder neutron diffraction as a function of temperature. The effect of substituting fluorine on to the anion site on both the structure and properties of ferroelectric stacked perovskites is also investigated, and is shown to have a radical effect on the the ferroelectric properties of the material, as well as on their phase formation