Layered Perovskites as Tailored Functional Materials

Abstract

The projects described in this thesis set out to investigate structure-property relationships in perovskite-type oxides containing Bi3+ and d0 transition metal cations. The investigation began by characterising the long-range average crystal structures of the perovskite solid-solution K0.5Bi0.5Ti1-xZrxO3, 0 ≤ x ≤ 1. The high-temperature tetragonal-cubic phase transition for x = 0 was tracked using in situ synchrotron X-ray diffraction, which showed relaxor behaviour. X-ray absorption spectroscopy showed that increasing x pushes Zr cations to higher symmetry environments while leaving the symmetry of the Ti cations unaffected. The investigation was extended to the n = 4 Ruddlesden-Popper layered perovskite K2.5Bi2.5Ti4O13, which transforms from a body-centered unit cell to a primitive unit cell upon hydration. Ab initio geometry optimisation calculations and molecular dynamics simulations determined the water molecules to be in a vertical “pillared” orientation between the perovskite-type layers where water rotations increased in radius with increasing temperature. Quasielastic neutron scattering experiments showed two H-motions at 300 K; a relatively slow rotation about the stacking axis, and a faster oscillation parallel to the stacking axis. . The final study is of Sillén-Aurivillius phases, Bi3A2Nb2O11X, A = Pb/Sr and X = Cl/Br. Rietveld refinements against X-ray and neutron powder diffraction data showed a significant contraction parallel to the stacking axis upon replacing Pb with Sr. This was attributed to the decrease in the concentration of stereochemically active 6s2 lone pairs of Bi3+/Pb2+ in the [BiAO2]2+ layer, leading to a less rigid structure. X-ray absorption spectroscopy showed that these substitutions had indirectly increased the symmetry of the Nb cations. From this series of projects, we have shown that cation size effects, intercalation, structural dynamics and lattice matching play an important role in the chemical design of new layered oxides.