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

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

Tuning of Water and Hydroxide Content of Intercalated Ruddlesden–Popper-type Oxides in the PrSr₃Co_1.5Fe_1.5O_10−δ System

A series of hydration experiments of the Ruddlesden–Popper phase PrSr₃Co_1.5Fe_1.5O_10−δ with varying levels of oxygen nonstoichiometry were performed with the goal to clarify phase formation and underlying mechanisms and driving forces. The hydration reaction is most intense for partly reduced samples with a vacancy concentration corresponding to δ ≈ 1. Fully oxidized samples show little or no tendency toward hydration. Presence of oxygen vacancies acts as a prerequisite for hydration. Probably, the basicity of the materials owing to A-site cations is another contributing factor to the hydration ability. Under CO₂ free conditions pure hydrates and oxide hydroxides are formed. In CO₂-containing atmosphere, additional carbonate anions are easily incorporated into the hydrate, probably at the expense of hydroxyl groups. The <i>I-</i>centered PrSr₃Co_1.5Fe_1.5O₈(OH)₂·1H₂O achieves a highly expanded <i>c</i>-axis upon the topochemical insertion reactions. In situ powder synchrotron X-ray diffraction (SXRD) shows that the hydrate converts to an oxide hydroxide, PrSr₃Co_1.5Fe_1.5O₈(OH)₂, at 70 °C with a primitive orthorhombic unit cell. Upon heating above 170 °C, an <i>I-</i>centered product is formed for which further dehydroxylation occurs at around 400–500 °C. Rietveld refinement of SXRD data shows that the absorbed water molecules fill the tetrahedral voids of the [AO]^RS rock salt layer of the monoclinic hydrate

Tuning of Water and Hydroxide Content of Intercalated Ruddlesden–Popper-type Oxides in the PrSr₃Co_1.5Fe_1.5O_10−δ System

A series of hydration experiments of the Ruddlesden–Popper phase PrSr₃Co_1.5Fe_1.5O_10−δ with varying levels of oxygen nonstoichiometry were performed with the goal to clarify phase formation and underlying mechanisms and driving forces. The hydration reaction is most intense for partly reduced samples with a vacancy concentration corresponding to δ ≈ 1. Fully oxidized samples show little or no tendency toward hydration. Presence of oxygen vacancies acts as a prerequisite for hydration. Probably, the basicity of the materials owing to A-site cations is another contributing factor to the hydration ability. Under CO₂ free conditions pure hydrates and oxide hydroxides are formed. In CO₂-containing atmosphere, additional carbonate anions are easily incorporated into the hydrate, probably at the expense of hydroxyl groups. The <i>I-</i>centered PrSr₃Co_1.5Fe_1.5O₈(OH)₂·1H₂O achieves a highly expanded <i>c</i>-axis upon the topochemical insertion reactions. In situ powder synchrotron X-ray diffraction (SXRD) shows that the hydrate converts to an oxide hydroxide, PrSr₃Co_1.5Fe_1.5O₈(OH)₂, at 70 °C with a primitive orthorhombic unit cell. Upon heating above 170 °C, an <i>I-</i>centered product is formed for which further dehydroxylation occurs at around 400–500 °C. Rietveld refinement of SXRD data shows that the absorbed water molecules fill the tetrahedral voids of the [AO]^RS rock salt layer of the monoclinic hydrate

Tuning of Water and Hydroxide Content of Intercalated Ruddlesden–Popper-type Oxides in the PrSr₃Co_1.5Fe_1.5O_10−δ System

A series of hydration experiments of the Ruddlesden–Popper phase PrSr₃Co_1.5Fe_1.5O_10−δ with varying levels of oxygen nonstoichiometry were performed with the goal to clarify phase formation and underlying mechanisms and driving forces. The hydration reaction is most intense for partly reduced samples with a vacancy concentration corresponding to δ ≈ 1. Fully oxidized samples show little or no tendency toward hydration. Presence of oxygen vacancies acts as a prerequisite for hydration. Probably, the basicity of the materials owing to A-site cations is another contributing factor to the hydration ability. Under CO₂ free conditions pure hydrates and oxide hydroxides are formed. In CO₂-containing atmosphere, additional carbonate anions are easily incorporated into the hydrate, probably at the expense of hydroxyl groups. The <i>I-</i>centered PrSr₃Co_1.5Fe_1.5O₈(OH)₂·1H₂O achieves a highly expanded <i>c</i>-axis upon the topochemical insertion reactions. In situ powder synchrotron X-ray diffraction (SXRD) shows that the hydrate converts to an oxide hydroxide, PrSr₃Co_1.5Fe_1.5O₈(OH)₂, at 70 °C with a primitive orthorhombic unit cell. Upon heating above 170 °C, an <i>I-</i>centered product is formed for which further dehydroxylation occurs at around 400–500 °C. Rietveld refinement of SXRD data shows that the absorbed water molecules fill the tetrahedral voids of the [AO]^RS rock salt layer of the monoclinic hydrate

Interstitial oxide ion conductivity in the layered tetrahedral network melilite structure

High-conductivity oxide ion electrolytes are needed to reduce the operating temperature of solid-oxide fuel cells. Oxide mobility in solids is associated with defects. Although anion vacancies are the charge carriers in most cases, excess (interstitial) oxide anions give high conductivities in isolated polyhedral anion structures such as the apatites. The development of new families of interstitial oxide conductors with less restrictive structural constraints requires an understanding of the mechanisms enabling both incorporation and mobility of the excess oxide. Here, we show how the two-dimensionally connected tetrahedral gallium oxide network in the melilite structure La1.54 Sr0.46 Ga3 O7.27 stabilizes oxygen interstitials by local relaxation around them, affording an oxide ion conductivity of 0.02-0.1Scm1 over the 600-900°C temperature range. Polyhedral frameworks with central elements exhibiting variable coordination number can have the flexibility needed to accommodate mobile interstitial oxide ions if non-bridging oxides are present to favour cooperative network distortions. © 2008 Nature Publishing Group