En raison de la résistance élevée à l’irradiation de certaines compositions, les oxydes de structure pyrochlore A₂B₂O ₇ sont considérés comme des matrices potentielles d’immobilisation des actinides produits en réacteur nucléaire. Dans ce contexte, l’étude de la stabilité structurale sous irradiation de ces oxydes est particulièrement importante. Ce travail concerne donc l’étude de l’influence de la composition des pyrochlores et de leur microstructure sur leur comportement vis-à-vis de l’irradiation.Dans ce but, quatre compositions d’oxydes pyrochlores de formule générale A₂B₂O ₇ avec A = Gd, Nd et B = Zr, Ti ont tout d’abord été élaborées. Différentes voies de synthèse (réaction à l’état solide, chimie douce par la méthode Pechini) de poudres pyrochlores ont été explorées afin de maîtriser in fine la taille de grains des céramiques densifiées par frittage classique ou par frittage SPS. Les résultats obtenus par chimie douce sont prometteurs : des poudres fines (<100 nm) de structure pyrochlore ont été élaborées. Enfin, selon les conditions de frittage SPS choisies, des céramiques denses ont été obtenues, avec une taille de grains de 700 nm pour Y₂Ti₂O ₇ et de 300 nm pour Nd₂Zr₂O ₇ , ce qui, à notre connaissance, correspond aux plus petites tailles de grains pour ce type de composé.Afin d’étudier le comportement sous irradiation, les oxydes élaborés de compositions Gd₂Ti₂O ₇ , Y₂Ti₂O ₇ , Gd₂Zr₂O ₇ et Nd₂Zr₂O ₇ , ont été irradiés avec des ions lourds de basse énergie (Au 4MeV délivrés par l’installation JANNuS à Orsay) pour déterminer les modifications structurales produites par les effets nucléaires et par des ions lourds d’une centaine de MeV (ligne IRRSUD) ou de l’ordre du GeV (ligne SME) du GANIL à Caen afin d’étudier les effets induits par les excitations électroniques. Les transformations induites par irradiation ont été caractérisées par Diffraction des Rayons X (parfois in situ avec le dispositif ALIX du GANIL), Microscopie Electronique à Balayage et en Transmission (MEB, MET) et spectroscopie Raman. La spectroscopie d'absorption (XANES et EXAFS effectuées au synchrotron SOLEIL) a été utilisée pour caractériser l'ordre local dans la phase amorphe induite par irradiation dans les titanates. Dans le domaine du ralentissement nucléaire, les résultats principaux sont que les titanates s’amorphisent sous irradiation alors que seule une transition pyrochlore-fluorine est observée pour les zirconates. Dans le domaine du ralentissement électronique, les titanates s’amorphisent par un mécanisme d’impact direct et Gd₂Zr₂O ₇ subit uniquement une transition pyrochlore-fluorine. Par contre, contrairement à son comportement observé sous irradiation avec des ions de basse énergie, Nd₂Zr₂O ₇ s’amorphise avec des ions lourds de haute énergie, avec une cinétique d’endommagement complexe qui met en jeu des transitions pyrochlore-amorphe, pyrochlore-fluorine et fluorine-amorphe. La structure interne des traces induites par excitations électroniques a été étudiée par MET. Les analyses par spectroscopie Raman en coupe transverse des échantillons irradiés ont permis la détermination de l’épaisseur de la phase amorphe ainsi que du pouvoir d’arrêt électronique seuil pour la formation des traces. Les conductivités thermiques des échantillons irradiés sont plus faibles que celles obtenues dans les composés non irradiés. Enfin, l’étude de la restauration thermique des composés irradiés avec des ions lourds de haute énergie montre que les transformations de phases ainsi que les températures auxquelles se produisent ces transformations dépendent de la composition. / Due to the high resistance to irradiation of some compositions, pyrochlore-type oxides with A₂B₂O ₇ structure are considered as potential matrices for the immobilization of actinides produced in nuclear reactor. In this context, the study of the structural stability after irradiation of these oxides is particularly important and should be investigated. This work aims at studying the effects of both the composition and the microstructure on the behavior of pyrochlores upon irradiation. For this purpose, four compositions of pyrochlore oxides of general formula A₂B₂O ₇ (with A = Gd, Nd and B = Ti, Zr) were first elaborated. Powders were prepared using different routes (solid state reaction, soft chemistry with Pechini process) in order to control the grain size of the ceramics densified either by conventional sintering or by SPS. The results obtained by soft chemistry are promising: fine powders (<100 nm) with the pyrochlore structure were prepared. Finally, according to the selected SPS conditions, dense ceramics were obtained with a grain size of 700 nm for Y₂Ti₂O ₇ and 300 nm for Nd₂Zr₂O ₇ , which, to our knowledge, corresponds to the smaller grain size for these compositions. Pyrochlore oxides with the compositions Gd₂Ti₂O ₇ , Y₂Ti₂O ₇ , Gd₂Zr₂O ₇ and Nd₂Zr₂O ₇ were irradiated with low energy heavy ions (4 MeV Au ions delivered by the JANNuS platform in Orsay) to determine the structural modifications produced by nuclear collisions. The same ceramics were also irradiated with swift heavy ions (hundreds of MeV on the IRRSUD beamline or of the order of GeV on the SME beamline) at the GANIL accelerator in Caen to study the effects induced by electronic excitations. The transformations induced by irradiation were characterized by XRD (in situ with the ALIX set up of the GANIL or ex situ after irradiation), scanning and transmission electron microscopy (SEM, TEM) and Raman spectroscopy. Fine absorption spectroscopy (XANES and EXAFS performed at the synchrotron Soleil) was implemented in order to characterize the local order in the amorphous phase induced by irradiation in the titanates. In the ballistic process regime, the main results are the amorphization of the titanates, while only an anion-deficient fluorite phase is formed for zirconates. The phase transformations induced by electronic excitation show that titanates become amorphous by a direct impact mechanism whereas a pyrochlore-fluorite transition occurs for Gd₂Zr₂O ₇ . However, in contrast to the behavior observed upon irradiation with low energy ions, Nd₂Zr₂O ₇ becomes amorphous upon high energy ion irradiations, with a complex damage kinetics involving pyrochlore-amorphous, pyrochlore-fluorite and fluorite-amorphous phase transitions. The internal structure of tracks induced by electronic excitation was studied by TEM. Raman spectroscopy analyses performed on cross-sectioned irradiated samples allowed the determination of the amorphous phase thickness and the electronic stopping power threshold for the formation of tracks.The thermal conductivities of the irradiated samples are lower than those obtained for unirradiated compounds. Finally, a specific study concerning the thermal recovery of irradiated pyrochlores with swift heavy ions shows that the phase transitions and the temperature at which these transformations occur depend on the composition.

Materials chemistry is essentially concerned with the design/synthesis of new solids endowed with functional properties that could be of relevance to today’s materials technology. Among the large variety of solid materials that attract attention, metal oxides continue to contribute significantly to current materials chemistry. A wide variety of oxide materials (based on rocksalt, spinel, corundum, perovskite, garnet, pyrochlore and other structures) and their properties have been investigated over the years. Most of these oxides are derived from the transition metals. Transition metal oxides with structures derived from metal-oxygen (MO6) octahedra, in particular, display an array of exotic properties with potential or proven technological application. While it is traditionally believed that the partially filled d shell (dn : 0 < n < 10) of the transition metal atoms plays a crucial role in deciding the electronic properties, the significance of d0 metal atoms for the properties (and structure) of transition metal oxides is not fully recognized. Magnetism (SrRuO3, Fe3O4), metallicity (ReO3, LaNiO3), colossal magnetoresistance (La1-xCaxMnO3) and superconductivity (La2xSrxCuO4, Sr2RuO4) are some of the properties that can be traced to the presence of partially filled d shell, while properties like ferroelectricity (BaTiO3), piezoelectricity (PbZr1-xTixO3) and nonlinear optical response (LiNbO3) could be traced to the presence of transition metals (TiIV, ZrIV, NbV) with d0 electronic configuration. The empty d orbitals on the metal atoms constitute the low lying unoccupied states (LUMO) that mix with the highest occupied states (HOMO) of the ligand atoms (oxygen) through special chemical bonding effects (second order Jahn-Teller effect, SOJT). This mixing results, among others, in out-of-centre distortion(s) of the MO6 octahedra and this distortion is at the heart of several properties mentioned above. Among the transition metal oxide structures based on MO6 octahedra, three structures are noteworthy: the perovskite, the pyrochlore and the rutile. The AMO3 perovskite structure consists of a three-dimensional framework of corner sharing MO6 octahedra in which the A cation occupies the dodecahedral site surrounded by twelve oxide ions. The perovskite structure can accommodate a large variety of substitutions at both the A and the M sites as well as vacancies at the A/O sites, giving a large number of derivatives. Several variants of the perovskite structure are also known, for instance, the layered perovskites and ordered perovskites. Many nonperovskite structures are also known for the composition AMO3 : hexagonal YMnO3 is an alternative structure for AMO3 composition where manganese exists as MnO5 trigonal bipyramids. The A2M2O7 pyrochlore structure is also based on a corner-connected network of MO6 octahedra which interpenetrates an A2O network. The rutile (TiO2) is a well-known structure consisting of chains of edge-sharing MO6 octahedra, which are connected through corners to adjacent chains. A large number of oxide materials based on the above three structure types have been reported : for example, perovskite [Ba3ZnTa2O9 (microwave telecommunication ceramic), Pb3MgNb2O9 (relaxor ferroelectric), Bi4Ti3O12 (high temperature ferroelectric)], pyrochlore [Nd2Mo2O7 (metallic ferromagnet), AOs2O6 for A = K, Rb, Cs (superconductor)] and rutile [TiO2 (photocatalyst), CrO2 (metallic ferromagnet), VO2 (insulator-metal transition)]. Considering the current interest in oxide materials of these three structure types which continue to generate new variants and novel properties, we undertook the present research project to synthesize new derivatives of these structure types, and characterize their structures and relevant electronic properties. In doing so, we recognized that synthesis based on an understanding of the reactivity of the constituents and crystal chemistry of the expected products plays a crucial role in this effort. Accordingly, we tailored several new compositions of AMO3, A2M2O7 and MO2 stoichiometries and adopted appropriate methodologies for their synthesis. We have characterized the structures and properties of the solid products by means of state-of-the-art methods available to us. There are two main approaches to the synthesis of nonmolecular inorganic solids: conventional ceramic route and chimie douce / soft chemistry routes. In the ceramic route, solid reactants are heated at elevated temperatures for long durations with intermittent mixing/grinding until the reaction is complete. Chimie douce routes, on the other hand, utilize gentle reactions such as dehydration, decomposition, intercalation, ion exchange, and so on to synthesize the desired phases. The ceramic route generally provides access to the thermodynamically controlled product(s), while chimie douce routes allow access to metastable phases (kinetically controlled product(s)). Disadvantages notwithstanding, the ceramic route has been the mainstay of materials chemistry and several important materials continue to be discovered / synthesized by this route. The choice of the synthetic route based on an understanding of the crystal chemical preferences and the reactivities of the constituents involved is often crucial to achieve the desired final products. The present thesis is devoted to the synthesis and investigation of MO6 octahedra-based oxides belonging to the perovskite, pyrochlore and rutile structure types wherein we have explored alternate synthetic strategies (perovskite-based Ba3MM'2O9 telecommunication ceramics and a solution route for the synthesis of ruthenium-based pyrochlores) and probed structure-property relations of perovskite oxides (Ba3MM'M''O9 oxides for various M/M'/M'' atoms) as well as formation of new derivatives of layered Aurivillius phases. In addition, we have also synthesized new noncentrosymmetric oxides possessing the YMnO3 structure. Our investigation of rutile based oxides has resulted in the discovery of a new lead-free relaxor ferroelectric material, FeTiTaO6. Given that the lone pair PbII:6s2 plays a crucial role in the ferroelectric properties of Pb-based perovskite oxides, we have also investigated members of the Pb1-xLix/2Lax/2TiO3 system for their structure and dielectric response. The present thesis describes the results of these investigations in eight chapters. Chapter 1 provides a general introduction to oxides of the perovskite, pyrochlore and rutile structures. In Chapter 2, we describe a new one-pot metathesis strategy for the synthesis of dielectric ceramics Ba3MM'2O9 (M = Mg, Ni, Zn; M' = Nb, Ta). Rietveld refinement of X-ray diffraction data shows near-complete ordering of M-site ions in many cases. The dielectric properties of the products synthesized are found to be in reasonable agreement with reported data. The synthesis of ordered materials at lower temperatures (~1100 °C) than that employed in the conventional ceramic route (~1500 °C) is a significant result of this work. Chapter 3 presents a study of Ba3MIIMIVWO9 (MII = Ca, Zn; MIV = Ti, Zr) perovskite oxides for the purpose of synthesizing new dielectric ceramic materials and to gain understanding of the factors that stabilize 3C vs. 6H structures. In general, a 1:2-ordered 6H perovskite structure is stabilized at high temperatures (1300 °C) for all of the Ba3MIITiWO9 oxides investigated. An intermediate phase possessing a partially ordered 1:1 double perovskite (3C) structure with the cation distribution, Ba2(Zn2/3Ti1/3)(W2/3Ti1/3)O6, is obtained at 1200 °C for Ba3ZnTiWO9. A metastable perovskite, Ba3CaZrWO9, that adopts the 1:1 3C structure has also been synthesized by a low-temperature metathesis route. Besides yielding several new perovskite oxides that may be useful as dielectric ceramics, the investigation provides new insights into the complex interplay of crystal chemistry (tolerance factor) and chemical bonding (anion polarization and d0-induced distortion of metaloxygen octahedra) in the stabilization of 6H versus 3C perovskite structures for the Ba3MIIMIVWO9 series. In Chapter 4, we describe the synthesis and investigation of the structure and dielectric properties of Ba3MIIITiMVO9 (MIII = Fe, Ga, Y, Lu; MV = Nb, Ta, Sb) perovskite oxides. The MV = Nb, Ta oxides adopt disordered/partially ordered 3C perovskite structures, where all the MIII/Ti/MV metal-oxygen octahedra are corner-connected. In contrast, the MV = Sb oxides show a distinct preference for the 6H structure, where SbV/TiIV metal-oxygen octahedra share a common face, forming (Sb,Ti)O9 dimers, that are corner-connected to the MIIIO6 octahedra. Investigation of dielectric properties of MIII = Y/Lu, MV = Nb/Ta oxides reveals a normal low loss dielectric behaviour with ε = 30 – 50 in the temperature range 50 – 350 °C. The MIII = Fe, MV = Nb/Ta members show a dielectric behaviour similar to relaxor ferroelectric materials. Chapter 5 deals with a study of isomorphous substitution of several metal atoms in two Aurivillius structures, Bi5TiNbWO15 and Bi4Ti3O12, in an effort to probe structure-property correlations. These investigations have led to the synthesis of new derivatives, Bi4LnTiMWO15 (Ln, = La, Pr; M = Nb, Ta), as well as Bi4PbNb2WO15 and Bi3LaPbNb2WO15, that largely retain the Aurivillius intergrowth structure of the parent oxide Bi5TiNbWO15, but characteristically tend toward a centrosymmetric / tetragonal structure for the Ln-substituted derivatives. On the other hand, coupled substitution, 2TiIV Æ MV + FeIII in Bi4Ti3O12, yields new Aurivillius phases, Bi4Ti3-2xNbxFexO12 (x = 0.25, 0.50) and Bi4Ti3-2xTaxFexO12 (x = 0.25) that retain the orthorhombic noncentrosymmetric structure of the parent Bi4Ti3O12. Chapter 6 describes the design and synthesis of a new series of noncentrosymmetric oxides, R3Mn1.5CuV0.5O9 (R = Y, Ho, Er, Tm, Yb, Lu) possessing the YMnO3 structure. Investigation of the Lu-Mn-Cu-V-O system revealed the existence of an isostructural solid solution series, Lu3Mn3-3xCu2xVxO9 for 0 < x ≤ 0.75. Magnetic and dielectric properties of the oxides are consistent with a random distribution of Mn3+, Cu2+ and V5+ atoms that preserves the noncentrosymmetric RMnO3 structure. An exploratory investigation of the synthesis, structure and electronic properties of new ruthenium(IV) pyrochlore oxides and their manganese-substituted derivatives is presented in Chapter 7. The richness of the electronic properties of ruthenium-based metal oxides is affirmed by the results which revealed several novel electronic ground states : a metallic and Pauli paramagnetic state for BiPbRu2O6.5 that turns into a semiconducting ferromagnetic spin-glass state at 50 K for BiPbRuMnO6.5 ; a metallic state that likely shows a charge density wave (CDW) instability at 50-225 K for Bi1.50Zn0.50Ru2O6.75, that is suppressed by manganese substitution in Bi1.50Zn0.50Ru1.75Mn0.25O6.50, and a metallic ferromagnetic spin-glass-like state for Pb2Ru1.75Mn0.25O6.15. We describe the investigation of the structure and dielectric properties of rutile-based MTiTaO6 (M = Al, Cr, Fe) in Chapter 8. All the oxides possess disordered rutile structure. FeTiTaO6 shows a strong relaxor ferroelectric effect, while CrTiTaO6 shows a weaker relaxor ferroelectric behaviour. This work is significant for two reasons: the new material is lead-free and it is based on the rutile structure, unlike the conventional relaxors which are mostly derived from the perovskite structure. The work presented in the thesis is carried out by the candidate as a part of the Ph.D. training programme and most of it has been published in the literature. She hopes that the studies reported here will constitute a worthwhile contribution to materials chemistry in general.