Investigations into the structure and properties of ordered perovskites, layered perovskites, and defect pyrochlores

by 1975- Knapp, Meghan C.

The work described in this thesis explores the effects of chemical substitution on the structures and properties of perovskites, layered perovskites, and defect pyrochlores. Layered perovskites, particularly of the variety K2NiF4, the n = 1 Ruddlesden-Popper structure, were studied to determine the factors that drive octahedral tilting distortions. It was determined that these structures, which are more inherently strained than perovskites, are influenced by the bonding environment around the anions and the A-cation as well as the electrostatic interactions between layers. The effects of cation ordering on the symmetry of Ruddlesden-Popper structures are also presented. Dion-Jacobson structures were also analyzed, and it was found that the trends that govern the behavior of Ruddlesden-Popper structures were not applicable. When n = 1 for Dion-Jacobson structures, the weak inter-layer interactions make the parent structure prone to tilting and plane slippage. This stoichiometry has several competing structures, many of which are observed for AMO4 compounds with highly covalent M-O interactions. Stoichiometric perovskites with multiple A-cations rarely exhibit layered ordering of the A-cations. Double perovskites having two A-cations and two M-cations with the formula AA'MM'O6 (A= Na, K, Li, A' = La, M = Mg, Sc M' = W, Nb, Sb, Te, or when M = M', M = Ti, Zr) were studied to determine the driving force for layered ordering of A-site cations. ii It was determined that such ordering is cooperative with the displacement of d0 transition metals from the M-cation site, which allows for relief of the bonding strain on the intralayer oxygen ions. This represents a novel way to propagate cation displacements, i.e. via the ordering of the A-cation which works synergistically with the M-site cation displacements. Such displacements can produce desirable dielectric properties, and these properties can be further enhanced by the use of an A-cation with a stereo-chemically active lone pair. As such, analogous compounds were prepared where A' = Bi3+. It was found that when perovskites were formed, no layered ordering of the A-cations was produced. When M' was a main group element, namely Sb5+, the perovskite phase and the defect pyrochlore phase were observed to be competitive. The dielectric properties of these materials were tested and it was found that the bismuth structure containing Nb5+ had the highest dielectric constant. iii