Novel oxides with interesting ionic, electronic and/or magnetic properties.

Abstract

This work summarises the outcome of research performed at the Universities of Liverpool and Oslo during the course of an EU-Marie Curie EST fellowship and funded through the NOVELOX program under the 6th framework (FP6) of the European Union. It focuses on perovskite related materials that possess the Ruddlesden-Popper structure and are composed of Pr-Sr-Co-Fe-O atoms. After an introduction chapter that aims at presenting a general overview of perovskites and related materials through their structures and technological applications, a brief description of the experimental methods used throughout this work is given in Chapter 2. The third chapter summarises non published but relevant results as well as summaries of published, submitted or under review manuscripts. It begins by describing the attempts at lanthanide substitution within the LnSr3Co1.5Fe1.5O10-δ system and the subsequent choice of Ln=Pr. It is then followed by an account of the effect of oxygen deficiencies in PrSr3Co1.5Fe1.5O10-δ on the oxidation state of the transition metals and their local magnetic environment and behaviour as examined respectively by XANES and Mössbauer spectroscopy. These confirm the reduced state of the transition metal cations as well as the presence of magnetic ordering. Thereafter results summarising the structural evolution of the compound upon heating under inert conditions (Paper I) and the subsequent hydration of the thus obtained phases (Paper II) are given. It is shown that the onset of oxygen mobility occurs at relatively low temperatures (c.a. 200°C) and a conduction mechanism is proposed. For the reduced phases, hydration as well as carbonation is shown to occur as a function of the hydration mechanism used. In addition, the topotactic de-hydration through a hydroxide phase, as studied by in-situ synchrotron radiation experiments, is also discussed. The chapter terminates by summarizing the impact of Co for Fe substitution on the magnetic properties and structure of PrSr3Co(Fe1-x Cox)3O10-δ (x=0.0 to 0.6, Paper III). As the Co content increases, the transition from a complex anti-ferromagnetic structure, to one where ferromagnetic interactions are dominant is observed. Finally, a discussion on the possibilities that this work offers followed by the list of publications and references terminate this thesis. Complete reproductions of the publications are to be found in the Annexe of this work.