Transition metal fluorides represent an important family of complex solids displaying a variety of different properties and interesting phenomena. Despite their remarkable behaviour, these classes of materials have not received much attention and the rationalization of their behaviour is still lacking a systematic approach. This thesis aims to contribute to the field by examining previously unknown or understudied complex fluorides. The compounds were selected for their intriguing physical properties that range from superconductivity to multiferroism. The discovery of superconductivity in the iron pnictides sparked new interest in materials with layered ZrCuSiAs-type structure. Herein the properties of one of these systems, namely SrFeAsF, will be discussed. We have found that it behaves as a poor metal and undergoes a tetragonal (P4/nmm) to orthorhombic (Cmma) structural transition at T = 180 K, accompanied by a spin density wave in magnetic susceptibility and electrical resistivity. Below T < 150 K, the Fe moments order in antiferromagnetic spin-stripes. Electron doping with La3+ is a successful route to obtain superconducting phases, with maximum Tc = 27 K (x = 0.2). The isostructural AeMnPnF series (Ae = Sr, Ba; Pn = P, As, Sb) was also investigated to elucidate the influence of transition metal d-electrons and size effects of Ae and Pn on the physical properties. The isoelectronic replacement of Ae and Pn leads to a significant distortion in the tetragonal building blocks. All d5 Mn fluorides investigated here are insulating antiferromagnets with TN ~ 350 K. Due to the coexistence of electronic and magnetic ordering, the tetragonal tungsten bronze (TTB) materials KxM2+ xM3+ 1-xF3 (x = 0.4 – 0.6; M = transition metal) are potential multiferroics. The type of structural distortion adopted by these systems is strongly dependant on the M2+/M3+ ratio. For instance, our high-resolution diffraction study on K0.5Mn0.5Cr0.5F3 has revealed a small orthorhombic distortion, which indicates full chemical order of Mn2+ and Cr3+ on all crystallographic sites. K0.5Mn0.5Cr0.5F3 remains orthorhombic Ccc2 on cooling through the ferromagnetic transition at TN = 23 K. On heating, the structure is acentric up to T = 373 K, where a change to tetragonal P42/mbc symmetry marks the transition from ferroelectric (polar) to paraelectric (apolar) states. High-pressure diffraction experiments have shown that the Ccc2 structure is robust upon pressurization with anisotropic axial compressibility up to the maximum pressure applied p = 18 GPa. The crystal structure of related mixed-valence TTB fluoride K0.6Cr2+ 0.6Cr3+ 0.4F3 is influenced by the presence of Jahn-Teller active Cr2+. The structural analysis described here revealed the presence of a small polar monoclinic distortion (P112) providing a clear signature of full charge order (CO). On heating, the gradual loss of CO leads to two consecutive structural phase transitions to orthorhombic (Pba2, T = 423 K) and then tetragonal (P42/mbc, T = 823 K) lattices, the latter is the signature of the ferro- to paraelectric transition. Below T = 150 K, increased X-ray exposure time leads to CO-melting and the stabilization of a new, charge-disordered orthorhombic phase (Cmm2), with a phenomenology similar to the CO manganites. In highpressure diffraction experiments, a further transition to tetragonal P4bm symmetry is found at p = 6 GPa. The magnetic susceptibility points towards a complex spin arrangement, with two transitions at TN = 33 K and 6 K. The results presented herein show the richness of the structural, electronic and magnetic phase diagrams of transition metal fluorides and clearly demonstrate that systematic studies on these systems will greatly enhance our current understanding of the underlying mechanisms of important phenomena such as superconductivity and ferroelectricity.

There has been an increase in the interest in fluoride materials over the last decade. This interest has focused on multiferroic materials and kagome lattices, to name but a few areas. This thesis focuses on the synthesis and crystallographic characterisation of selected transition metal fluorides and oxyfluorides. Work is presented on the tetragonal tungsten bronze solid solutions of KₓFeF₃, where x = 0.58 and x ≈ 0.5, and the copper analogue, K₃Cu₃Fe₂F₁₅; the kagome structure of Cs₂ZrCu₃F₁₂; and hydrothermal reactions using vanadium, manganese, or molybdenum as the transition metals in the formation of new fluorides and oxyfluorides. The tetragonal tungsten bronze compounds KₓFeF₃ (x = 0.58 and x ≈ 0.5) are both tetragonal at 500 K. In the variant with the lower K-content, there is a clear phase separation into two tetragonal phases even at this temperature. The K₀.₅₈FeF₃ sample separates into two distinct phases below 340 K to possess one tetragonal and one orthorhombic phase. Then at roughly 300 K, both samples undergo a phase transition where the tetragonal phase in the P4/mbm space group in K₀.₅₈FeF₃ changes to an orthorhombic phase with a larger unit cell; and the tetragonal phase in P4₂bc for the K₀.₅FeF₃ sample changes to the same orthorhombic model, whilst the P4/mbm model remains unchanged. The evolution of the lattice parameters and phase fractions is studied in detail using synchrotron powder X-ray diffraction (sPXRD). The kagome structure investigated, Cs₂ZrCu₃F₁₂, possesses the “ideal” kagome lattice at room temperature, but previous work has suggested that there is a phase transition at 225 K. The two structures are determined by single crystal X-ray diffraction at 300 K and 125 K. Variable temperature sPXRD studies are performed between these two temperature ranges to determine the phase evolution as a function of temperature. The structure changes from a rhombohedral to a monoclinic phase at low temperature. This is the result of the buckling of the kagome layers at the phase transition. The Zr⁴⁺ ion changes from 6 to 7 coordinate and this is seen as the main driving force for the distortion of the kagome layer from its “ideal” planar arrangement. ii The phase transition is first-order as seen from the electrical impedance measurements. The hydrothermal reactions presented reveal seven new materials and their crystal structures. Sr₂V₂F₁₀·H₂O is new and found to be isostructural to Sr₂Fe₂F₁₀·H₂O. BaVO₂F₃ is a cubic material that is potentially piezoelectric. Two hybrid organic inorganic manganese compounds are reported. The ladder structure (C₃N₂H₅)[Mn₂F₆(H₂O)₂] crystallises in a polar space group and shows promise as a candidate for multiferroic studies. The second hybrid material, (C₇NH₁₆)₂[MnF₅(H₂O)]·2H₂O, crystallises in a centrosymmetric space group. The Mo hybrid materials are all centrosymmetric and possess isolated molybdenum-centred monomeric or dimeric octahedral units.

Les travaux présents ici portent sur l’étude et la prospection de matériaux ferroiques. Cette étude consiste en une approche revêtant plusieurs aspects que sont la chimie du solide, la physique du solide et la science des matériaux. Deux parties sont développées, avec deux approches différentes. La première se concentre sur les liens entre composition/structure/propriétés dans des matériaux de la famille des TTB dérives de Ba2NdFeNb4O15 à travers l’étude de solutions solides à base de Li et l’étude de l’impact des différents paramètres de synthèse sur la nature de l’anomalie diélectrique mesurées sur pastilles densifiées. La mise en évidence de modulation structurale dans cette famille de matériaux semble être en lien direct avec l’observation des variations de propriétés diélectriques. L’utilisation de diffraction électronique notamment permet la mise en évidence de ces modulations structurales et leur évolution en température. Dans une seconde partie, l’approche consiste à utiliser le Spark Plasma Sintering (SPS) comme technique de densification pour des matériaux dits “fragiles” mais aussi d’explorer des propriétés diélectriques jusqu’alors inaccessibles, sur matériaux massifs. Le développement de la technique SPS à basse température permet ainsi non seulement de densifier à basse température des matériaux fragiles, mais aussi d’obtenir des phases inaccessibles dans des conditions de températures similaires par traitement thermique conventionnel. De même, l’obtention de céramique moléculaire de très haute densité a pu être réalisée, malgré des températures de décomposition extrêmement faible (100 ˚C). / The present work focuses on the prospection and understanding study of ferroic materials. It consists in a multiple aspect approach, including materials chemistry, materials physics and materials processing. Two parts compose this work, with two different approaches. The first one focuses on the links between composition, structure and properties in materials belonging to the TTB family, more specifically derived from Ba2NdFeNb4O15, through the exploration of Li containing solid solutions, and the impact of synthesis parameters on measured dielectric anomalies on dense samples. The observation of structural modulation in these materials seems to be closely related to the observation of dielectric anomalies variations. Using electron diffraction techniques allowed the evidence of such anomalies and the following of their thermal evolution. In a second part, the approach consists in using Spark Plasma Sintering (SPS) as a densification technique for so called “fragile” materials but also explore dielectric properties impossible to experimentally measure thus far. Developing low temperature SPS technique not only allows to densify ceramics at low temperatures fragile materials, but also to obtain inaccessible phases in similar temperature conditions using conventional thermal treatments. Moreover, sintering of molecular ceramic at very high density was possible, even if its decomposition temperature is extremely low (100 ˚C).

La structure bronze quadratique de tungstène, grâce à sa flexibilité cristallochimique, est une candidate légitime pour le développement de matériaux fonctionnels. L’étude des propriétés diélectriques, pyroélectriques, et ferroélectriques de céramiques de formulation Ba2NdFeNb4-xTaxO15 montre un crossover relaxeur-ferroélectrique-paraélectrique, avec une hystérèse thermique de la transition ferroélectrique. L’étude structurale des monocristaux relaxeurs Ba2LnFeNb4O15 (Ln=La, Pr, Nd, Sm, Eu), obtenus par la méthode du flux, a mis en évidence une structure modulée. L’affinement de la structure de base prouve l’existence de moments dipolaires dans le plan ab (dus à une distorsion des octaèdres [NbO6]). Afin d’établir un lien entre la structure cristalline et les propriétés d’un composé dérivé du multiferroïque MnWO4, la croissance en four à image de Mn0,85Mg0,15WO4 a été entreprise avec succès. / Tetragonal Tungsten Bronze structure, thanks to its compositional flexibility, is a legitimate candidate for the development of functional materials. Study of dielectric, pyroelectric, and ferroelectric properties of Ba2NdFeNb4-xTaxO15 ceramics reveals a relaxor-ferroelectric-paraelectric crossover, with thermal hysteresis of the ferroelectric transition. Structural analysis of Ba2LnFeNb4O15 (Ln=La Pr, Nd, Sm, Eu), relaxor single crystals obtained by flux method, highlighted a modulated structure. Refinement of the basic structure proves the existence of a dipolar moment in the ab plane (induced by a distortion of [NbO6] octahedra). In order to establish a correlation between the crystal structure and the properties of a compound derived from multiferroic MnWO4, the crystal growth in image furnace of Mn0,85Mg0,15WO4 was successfully undertaken.

In this work, in-depth structural and electrical characterisation is used to study a family of “empty” tetragonal tungsten bronzes (TTBs), A2₄A1₂B1₂B2₈O₃₀. An initial investigation into the effect of the A1-cation size on the properties of empty Ba₄R₀.₆₇◻₁.₃₃Nb₁₀O₃₀ TTBs (where R is the A1-cation and R = La, Nd, Sm, Gd, Dy and Y; ◻ = vacancy) was performed. These were determined to be metrically tetragonal by powder x-ray diffraction, with decreasing R cation size inducing increased crystal anisotropy. This tetragonal structural distortion, driven by contraction in the ab-plane, is shown to stabilise c-axis ferroelectricity; a direct correlation between tetragonality and the ferroelectric Curie temperature, T[sub]C, is demonstrated. Further examination of the relaxor ferroelectric (RFE) to ferroelectric (FE) crossover in Ba₄(La₁₋ₓNdₓ)₀.₆₇◻₁.₃₃Nb₁₀O₃₀ TTBs using detailed structural studies employing variable temperature, high resolution neutron, synchrotron X-ray and electron diffraction revealed a common superstructure with 2√2 × √2 × 2 cell with respect to the basic tetragonal aristotype cell. However, they display different degrees of order/disorder which can disrupt polar order (ferroelectricity). La-rich analogues exhibit a disordered regime between the low and high temperature ferroelectric and non-polar phases. Although polar, this disordered regime is non-ferroelectric, however, large polarisation may be established with an applied electric field, but relaxes back to the disordered phase upon removal of the field. Substitution of Nd for La at the A1-site leads to destabilisation of the disordered phase and reintroduces “normal” ferroelectric behaviour. Finally, isovalent substitution of Sr²⁺∙ for Ba²⁺ is shown to lead to the development of relaxor behaviour at higher dopant concentrations in Ba₄₋ₓSrₓDy₀.₆₇◻₁.₃₃Nb₁₀O₃₀, (x = 0, 0.25, 0.5, 1, 2, 3; ◻ = vacancy). With increasing x the unit cell contracts in both the ab- plane and c-axis coinciding with a decrease in T[sub]C and development of relaxor behaviour for x ≥ 2. This observation is rationalised by differing cation occupancies: for x ≤ 1, Sr²⁺ principally occupies the A2-site while for x ≥ 2 significant Sr²⁺ occupation of the A1-site leads to the observed RFE characteristics. The FE to RFE crossover is discussed in the context of a previously proposed TTB crystal chemical framework with the A1-site tolerance factor identified as the dominant influence on relaxor behaviour.

There is great interest in the development of new polar dielectric ceramics and multiferroic materials with new and improved properties. A family of tetragonal tungsten bronze (TTB) relaxors of composition Ba₆M³⁺Nb₉O₃₀ (M³⁺ = Ga³⁺, Sc³⁺ and In³⁺, and also their solid solutions) were studied in an attempt to understand their dielectric properties to enable design of novel polar TTB materials. A combination of electrical measurements (dielectric and impedance spectroscopy) and powder diffraction (X-ray and neutron) studies as a function of temperature was employed for characterising the dynamic dipole response in these materials. The effect of B-site doping on fundamental dipolar relaxation parameters were investigated by independently fitting the dielectric permittivity to the Vogel-Fulcher (VF) model, and the dielectric loss to Universal Dielectric Response (UDR) and Arrhenius models. These studies showed an increase in the characteristic dipole freezing temperature (T[subscript(f)]) with increase B-cation radius. Crystallographic data indicated a corresponding maximum in tetragonal strain at T[subscript(f)], consistent with the slowing and eventual freezing of dipoles. In addition, the B1 crystallographic site was shown to be most active in terms of the dipolar response. A more in-depth analysis of the relaxor behaviour of these materials revealed that, with the stepwise increase in the ionic radius of the M³⁺ cation on the B-site within the Sc-In solid solution series, the Vogel-Fulcher curves (lnf vs. T[subscript(m)]) are displaced to higher temperatures, while the degree of relaxor behaviour (frequency dependence) increases. Unfortunately, additional features appear in the dielectric spectroscopy data, dramatically affecting the Vogel-Fulcher fitting parameters. A parametric study of the reproducibility of acquisition and analysis of dielectric data was therefore carried out. The applicability of the Vogel-Fulcher expression to fit dielectric permittivity data was investigated, from the simple unrestricted (“free”) fit to a wider range of imposed values for the VF relaxation parameters that fit with high accuracy the experimental data. The reproducibility of the dielectric data and the relaxation parameters obtained by VF fitting were shown to be highly sensitive to the thermal history of samples and also the conditions during dielectric data acquisition (i.e., heating/cooling rate). In contrast, UDR analysis of the dielectric loss data provided far more reproducible results, and to an extent was able to partially deconvolute the additional relaxation processes present in these materials. The exact nature of these additional relaxations is not yet fully understood. It was concluded application of the Vogel-Fulcher model should be undertaken with great care. The UDR model may represent a feasible alternative to the evaluation of fundamental relaxation parameters, and a step forward towards the understanding of the dielectric processes in tetragonal tungsten bronzes.

Des recherches très importantes ont été dédiées à l'étude des composés ferroélectriques de type bronze de tungstène quadratique (TTB). Cette thèse concerne la synthèse et la caractérisation de 7 matériaux TTB de formule générale A6M2M'8O30 (A = Sr2+, Ba2+ ou Pb2+, M = Ti4+, Zr4+ ou HF4+; M' = Nb5+ ou Ta5+). Ces composés cristallins de grande taille ∼100 nm ont été synthétisés sous forme de poudre. Les moyennes expérimentales : diffractions des rayons X, spectroscopie Raman, la microscopie électronique MEB et MET, l'optique non linéaire ONL, ont permis d'établir des relations étroites entre structure et propriétés physiques. Les susceptibilités optiques non linéaires de 2eme et 3eme ordres, χ(2), χ(3) de chaque composés ont été déterminées et les valeurs résultantes du χ(3) ne montrent pas les mêmes caractéristiques que χ(2), car le signal de THG est moins dépendant de structure que SHG qui requiert un milieu non-centrosymétrique. Le décalage fréquentiel (Δσ) des modes de vibrations entre les structures à base de strontium(Sr) et celles à base de Barium(Ba) a été constaté afin d'identifier les signatures propres de ces composés. L'interprétation de ces poudres n'était pas facile car les pics sont très délicats. Nos mesures expérimentales montrent clairement que les positions des pics peuvent être modifiées par le changement cationique au sein des matériaux, plus la maille cristalline est rigide, plus on a des pics plus discernables. La différence est nettement visible là ou l'émission est autour de 808 cm-1 pour la structure (Sr), alors que pour le (Br) elle est autour de 780cm-1. Nous attribuons ce Δσ (28cm-1) à la transition de groupe de symétrie spatiale de Pba2 vers P4bm. L'ensemble des résultats décrits dans ce travail ont montrés la grande variété des paramètres sur lesquels il est possible d'agir afin d'optimiser les compositions en vue d'une application spécifique. / Recently, very significant research has been devoted to the study of ferroelectric compounds of tetragonal tungsten bronze (TTB). This thesis deals with the synthesis and characterization of 7 TTB's material with general formula A6M2M'8O30 (A = Sr2+, Ba2+ or Pb2+, M = Ti4+, Zr4+ or HF4+, M '= Nb5+ or Ta5+). These crystalline compounds of large size ~ 100 nm were synthesized in powder form. The experimental Technics: X-ray diffraction, Raman spectroscopy, scanning electron microscopy SEM and TEM and nonlinear optics NLO, allowed us to establish close relationships between structure and physical properties. The nonlinear 2nd and 3rd order optical susceptibility χ(2), χ(3) of all compounds were determined and the resulting values of χ(3) did not show the same characteristics as measures of χ(2) because the THG signal is less dependent on the structure where SHG signal requires a non-centro-symmetric medium. The frequency shift between the vibration modes of structures containing strontium (Sr) and those based on Barium (Ba) has been detailed to identify signatures of these specific compounds. The interpretation of these powders wasn't that easy as these peaks are very delicate but our experimental results clearly show that the positions of the peaks can be modified by the change in cation materials, as long as the crystal lattice is more rigid, more we get discernible Raman modes. This difference is clearly visible where the emission is around 808 cm-1 for the structure Sr, whereas the Br it is centered around 780cm-1. We attribute these Δσ (28cm-1) to the transition of spatial symmetry group from Pba2 to P4bm. Results showed the wide variety of settings in which it is possible to act in order to optimize the compositions for a specific application.

Transparent glasses embedded with TTB structured ferroelectric nano/micro crystals (K3Li2Nb5O15, Ba5Li2Ti2Nb8O30) were fabricated in various tellurite and borate based glass matrices and characterized for their physical properties. Nanocrystals of K3Li2Nb5O15 were successfully grown inside tellurite glass matrix via conventional heat-treatment route. Eventhough, tellurite glasses preferentially crystallize only on the surface, bulk uniform crystallization was achieved in the (100-x) TeO2 - x(1.5K2O-Li2O-2.5Nb2O5) system. Heat capacity studies revealed them to be thermodynamically less fragile than any other tellurite glasses ever reported in the literature. Pyroelectric and ferroelectric effects as well as second harmonic generation were demonstrated for the heat treated (glass nanocrystal composites) samples in this system. The conventional method of melt-quenching of constituent oxides could not yield Ba5Li2Ti2Nb8O30 crystallites. So, Ba5Li2Ti2Nb8O30 microcrystals were successfully formed in tellurite glass matrix by mixing pre-reacted Ba5Li2Ti2Nb8O30 ceramic powders with TeO2. The glass transition temperature was found to be the highest ever reported and this system was kinetically strong based on the fragility parameter. Dielectric studies revealed a frequency and temperature independent nature of the dielectric constant and very low dielectric loss. The SHG measurement which was carried out as a function of temperature demonstrated the incidence of blue second harmonic generation in the microcrystals present in the glass matrix. Ba5Li2Ti2Nb8O30 nanocrystals were successfully crystallized in the transparent glass system (100-x)Li2B4O7 – x(Ba5Li2Ti2Nb8O30). Dielectric constant increased while the dielectric loss decreased with the increase in Ba5Li2Ti2Nb8O30 content. Nuclear magnetic resonance spectroscopic studies were carried out to have an insight into the structure of this system. Transmission studies and refractive index measurements were performed and various optical parameters were calculated. Dielectric and transport properties were studied for the glasses and glass nano/microcrystal composites of all the systems reported in this thesis. Li+ ion was found to be responsible for conduction in all these systems. Evolution of self-organized nanopatterns of K3Li2Nb5O15 crystals has been demonstrated in the glass system (100-x) TeO2 - x(1.5K2O-Li2O-2.5Nb2O5) by excimer laser irradiation. The second harmonic signal observed by the Maker fringe technique has been attributed to the presence of well-aligned nano-sized grating structures in the glass system. Glasses belonging to the systems TeO2-K3Li2Nb5O15, TeO2-Ba5Li2Ti2Nb8O30 and V2Te2O9 undergo spinodal decomposition on exposing to KrF pulsed excimer laser. The spinodally phase separated structures were observed on all the surfaces of the samples. Ring shaped patterns were observed on several locations of the samples at higher frequency of laser pulses probably owing to the shock waves produced by the high intense laser beam. Line shaped patterns were found to originate on the sample surfaces when irradiated for longer periods.