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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.

<p>This thesis is concerned with <i>in-situ</i> time-, temperature- and pressure-resolved synchrotron X-ray powder diffraction investigations of a variety of inorganic compounds with twodimensional layer structures and three-dimensional framework structures. In particular, phase stability, reaction kinetics, thermal expansion and compressibility at non-ambient conditions has been studied for 1) Phosphates with composition <i>M</i><i>IV</i>(HPO₄)₂·<i>n</i>H₂O (<i>M</i><i>IV</i> = Ti, Zr); 2) Pyrophosphates and pyrovanadates with composition<i> M</i><i>IV</i>X₂O₇(<i>M</i><i>IV</i> = Ti, Zr and X = P, V); 3) Molybdates with composition ZrMo₂O₈. The results are compiled in seven published papers and two manuscripts.</p><p>Reaction kinetics for the hydrothermal synthesis of α-Ti(HPO₄)₂·H₂O and intercalation of alkane diamines in α-Zr(HPO₄)₂·H₂O was studied using time-resolved experiments. In the high-temperature transformation of γ-Ti(PO₄)(H₂PO₄)·2H₂O to TiP₂O₇ three intermediate phases, γ'-Ti(PO₄)(H₂PO₄)·(2-x)H₂O, β-Ti(PO₄)(H₂PO₄) and Ti(PO₄)(H₂P₂O₇)_0.5 were found to crystallise at 323, 373 and 748 K, respectively. A new tetragonal three-dimensional phosphate phase called τ-Zr(HPO₄)₂ was prepared, and subsequently its structure was determined and refined using the Rietveld method. In the high-temperature transformation from τ-Zr(HPO₄)₂ to cubic α-ZrP₂O₇two new orthorhombic intermediate phases were found. The first intermediate phase, ρ-Zr(HPO₄)₂, forms at 598 K, and the second phase, β-ZrP₂O₇, at 688 K. Their respective structures were solved using direct methods and refined using the Rietveld method. <i>In-situ</i> high-pressure studies of τ-Zr(HPO₄)₂revealed two new phases, tetragonal ν-Zr(HPO₄)₂and orthorhombic ω-Zr(HPO₄)₂ that crystallise at 1.1 and 8.2 GPa. The structure of ν-Zr(HPO₄)₂ was solved and refined using the Rietveld method.</p><p>The high-pressure properties of the pyrophosphates ZrP₂O₇ and TiP₂O₇, and the pyrovanadate ZrV₂O₇were studied up to 40 GPa. Both pyrophosphates display smooth compression up to the highest pressures, while ZrV₂O₇ has a phase transformation at 1.38 GPa from cubic to pseudo-tetragonal β-ZrV₂O₇ and becomes X-ray amorphous at pressures above 4 GPa.</p><p>In-situ high-pressure studies of trigonal α-ZrMo₂O₈ revealed the existence of two new phases, monoclinic δ-ZrMo₂O₈and triclinic ε-ZrMo₂O₈ that crystallises at 1.1 and 2.5 GPa, respectively. The structure of δ-ZrMo₂O₈was solved by direct methods and refined using the Rietveld method.</p>

Neste trabalho foram desenvolvidas amostras de materiais padrão de referência para difratometria de policristais. Materiais de alta pureza foram tratados mecânica e termicamente até atingirem as características necessárias para serem usados como materiais padrão de referência de alta qualidade, comparáveis àqueles produzidos pelo NIST. As medidas foram feitas inicialmente em vários difratômetros convencionais de laboratório de difração de raios X, com geometria de Bragg -Brentano, difratômetros de nêutrons e, posteriormente, em equipamento de alta resolução com fonte síncrotron. Os parâmetros de rede obtidos foram calculados pelo programa Origin e ajustados pelo Método dos Mínimos Quadrados. Esses ajustes foram comparados com os obtidos pelo Método de Rietveld, usando o programa GSAS através da interface gráfica EXPGUI, mostrando-se bastante satisfatórios. Os materiais obtidos foram alumina-&alpha;, ítria, silício, céria, hexaboreto de lantânio e fluoreto de lítio, que apresentaram qualidade semelhante e, em alguns casos, superiores aos padrões desenvolvidos e comercializados pelo NIST, a custos bem menores. / In this work, samples of standard reference materials for diffractometry of polycrystals were developed. High-purity materials were submitted to mechanical and thermal treatments in order to present the adequate properties to be used as high-quality standard reference materials for powder diffraction, comparable to the internationally recognized produced by the USA National Institute of Standards and Technology NIST, but at lower costs. The characterization of the standard materials was performed by measurements in conventional X-ray diffraction diffractometers, high resolution neutron diffraction and high-resolution synchrotron diffraction. The lattice parameters were calculated by extrapolation of the values obtained from each X-ray reflection against cos2&theta; by the Least-Squares Method. The adjustments were compared to the values obtained by the Rietveld Method, using the program GSAS. The materials thus obtained were the &alpha;-alumina, yttrium oxide, silicon, cerium oxide, lanthanum hexaboride and lithium fluoride. The standard reference materials produced present quality similar or, in some cases, superior to the standard reference materials produced and commercialized by the NIST.

Texture, i.e. preferred orientation, can cause large systematic errors in quantitative analysis of crystalline materials using x-ray powder diffraction (XRPD) data. Various mathematical forms have been proposed for the application of preferred orientation corrections. The most promising of these appears to be the single-parameter March (1932) model proposed by Dollase (1986).Li and O'Connor (1989) applied the March model to determine the level of preferred orientation in various gibbsites using two procedures. The first involved the Rietveld (1969) least squares pattern-fitting method. Each pattern was Rietveld-analysed in two ways, initially assuming random orientation of the crystallites and subsequently with the March model. The second procedure for preferred orientation analysis, described here as the line ratio method, determines preferred orientation factors according to the intensity ratios of carefully selected line pairs.In the thesis the procedures proposed by Li and O'Connor for texture analysis have been evaluated with XRPD data sets for molybdite, calcite and kaolinite. The results indicate that while the March formula improves agreement between the' calculated and measured patterns in Rietveld analysis, other forms of systematic error in the intensity data appear to limit the effectiveness of the March formula in general. It has been found also that the line ratio method improves agreement between the data sets, but less effectively than the Rietveld method. It is proposed that extinction is likely to be the most influential source of systematic error competing with texture.

This thesis is concerned with in-situ time-, temperature- and pressure-resolved synchrotron X-ray powder diffraction investigations of a variety of inorganic compounds with twodimensional layer structures and three-dimensional framework structures. In particular, phase stability, reaction kinetics, thermal expansion and compressibility at non-ambient conditions has been studied for 1) Phosphates with composition MIV(HPO4)2·nH2O (MIV = Ti, Zr); 2) Pyrophosphates and pyrovanadates with composition MIVX2O7 (MIV = Ti, Zr and X = P, V); 3) Molybdates with composition ZrMo2O8. The results are compiled in seven published papers and two manuscripts. Reaction kinetics for the hydrothermal synthesis of α-Ti(HPO4)2·H2O and intercalation of alkane diamines in α-Zr(HPO4)2·H2O was studied using time-resolved experiments. In the high-temperature transformation of γ-Ti(PO4)(H2PO4)·2H2O to TiP2O7 three intermediate phases, γ'-Ti(PO4)(H2PO4)·(2-x)H2O, β-Ti(PO4)(H2PO4) and Ti(PO4)(H2P2O7)0.5 were found to crystallise at 323, 373 and 748 K, respectively. A new tetragonal three-dimensional phosphate phase called τ-Zr(HPO4)2 was prepared, and subsequently its structure was determined and refined using the Rietveld method. In the high-temperature transformation from τ-Zr(HPO4)2 to cubic α-ZrP2O7 two new orthorhombic intermediate phases were found. The first intermediate phase, ρ-Zr(HPO4)2, forms at 598 K, and the second phase, β-ZrP2O7, at 688 K. Their respective structures were solved using direct methods and refined using the Rietveld method. In-situ high-pressure studies of τ-Zr(HPO4)2 revealed two new phases, tetragonal ν-Zr(HPO4)2 and orthorhombic ω-Zr(HPO4)2 that crystallise at 1.1 and 8.2 GPa. The structure of ν-Zr(HPO4)2 was solved and refined using the Rietveld method. The high-pressure properties of the pyrophosphates ZrP2O7 and TiP2O7, and the pyrovanadate ZrV2O7 were studied up to 40 GPa. Both pyrophosphates display smooth compression up to the highest pressures, while ZrV2O7 has a phase transformation at 1.38 GPa from cubic to pseudo-tetragonal β-ZrV2O7 and becomes X-ray amorphous at pressures above 4 GPa. In-situ high-pressure studies of trigonal α-ZrMo2O8 revealed the existence of two new phases, monoclinic δ-ZrMo2O8 and triclinic ε-ZrMo2O8 that crystallises at 1.1 and 2.5 GPa, respectively. The structure of δ-ZrMo2O8 was solved by direct methods and refined using the Rietveld method.

This thesis explores the thermal expansion and high pressure behavior of some materials with the ReO₃ structure type. This structure is simple and has, in principle, all of the features necessary for negative thermal expansion (NTE) arising from the transverse thermal motion of the bridging anions and the coupled rotation of rigid units; however, ReO₃ itself only exhibits mild NTE across a narrow temperature range at low temperatures. ReO₃ is metallic because of a delocalized d-electron, and this may contribute to the lack of NTE in this material. The materials examined in this thesis are all based on d⁰ metal ions so that the observed thermal expansion behavior should arise from vibrational, rather than electronic, effects. In Chapter 2, the thermal expansion of scandium fluoride, ScF₃, is examined using a combination of in situ synchrotron X-ray and neutron variable temperature diffraction. ScF₃ retains the cubic ReO₃ structure across the entire temperature range examined (10-1600 K) and exhibits pronounced negative thermal expansion at low temperatures. The magnitude of NTE in this material is comparable to that of cubic ZrW₂O₈, which is perhaps the most widely studied NTE material, at room temperature and below. This is the first report of NTE in an ReO₃ type structure across a wide temperature range. Chapter 3 presents a comparison between titanium oxyfluoride, TiOF₂, and a vacancy containing titanium hydroxyoxyfluoride, Tiₓ(O/OH/F)₃. TiOF₂ was originally reported to adopt the cubic ReO₃ structure type under ambient conditions, therefore the initial goal for this study was to examine the thermal expansion of this material and determine if it displayed interesting behavior such as NTE. During the course of the study, it was discovered that the original synthetic method resulted in Tiₓ(O/OH/F)₃, which does adopt the cubic ReO₃ structure type. The chemical composition of the hydroxyoxyfluoride is highly dependent upon synthesis conditions and subsequent heat treatments. This material readily pyrohydrolyizes at low temperatures (~350 K). It was also observed that TiOF₂ does not adopt the cubic ReO₃ structure; at room temperature it adopts a rhombohedrally distorted variant of the ReO₃ structure. Positive thermal expansion was observed for TiOF₂ from 120 K through decomposition into TiO₂. At ~400 K, TiOF₂ undergoes a structural phase transition from rhombohedral to cubic symmetry. High pressure diffraction studies revealed a cubic to rhombohedral phase transition for Tiₓ(O/OH/F)₃ between 0.5-1 GPa. No phase transitions were observed for TiOF₂ on compression. In Chapter 4, an in situ variable pressure{temperature diffraction experiment examining the effects of pressure on the coefficients of thermal expansion (CTE) for ScF₃ and TaO₂F is presented. In the manufacture and use of composites, which is a possible application for low and NTE materials, stresses may be experienced. Pressure was observed to have a negligible effect on cubic ScF₃'s CTE; however, for TaO₂F the application of modest pressures, such as those that might be experienced in the manufacture or use of composites, has a major effect on its CTE. This effect is associated with a pressure-induced phase transition from cubic to rhombohedral symmetry upon compression. TaO₂F was prepared from the direct reaction of Ta₂O₅ with TaF₅ and from the digestion of Ta₂O₅ in hot hydro uoric acid. The effects of pressure on the two samples of TaO₂F were qualitatively similar. The slightly different properties for the samples are likely due to differences in their thermal history leading to differing arrangements of oxide and uoride in these disordered materials. In Chapter 5, the local structures of TiOF₂ and TaO₂F are examined using pair distribution functions (PDFs) obtained from X-ray total scattering experiments. In these materials, the anions (O/F) are disordered over the available anion positions. While traditional X-ray diffraction provides detailed information about the average structures of these materials, it is not suffcient to fully understand their thermal expansion. Fits of simple structural models to the low r portions of PDFs for these materials indicate the presence of geometrically distinct M{X{M (M = Ti, Ta; X = O, F) linkages, and a simple analysis of the TaO₂F variable temperature PDFs indicates that these distinct links respond differently to temperature.

Synthesis of polycrystalline, vs. single-crystalline porous materials, such as metal-organic frameworks (MOFs), is usually beneficial due to shorter synthetic time and higher yields. However, the structural characterization of these materials by X-ray powder diffraction can be complicated. Even more difficult, is to track structural changes of MOFs by in situ experiments. Hence, we designed several successful techniques for the structural investigation of porous MOFs. These methods utilize the Structure Envelope (SE) density maps. SEs are surfaces which describing the pore system with the framework. It was shown that these maps can be easily generated from the structure factors of a few (1 to 10) of the most intense low index reflections. Application of SE in Charge Flipping calculations shortens and simplifies structure determination of MOF materials. This method provides excellent MOF models which can be used as a good starting point for their refinement. However, the most interesting results have been found by using Difference Envelope Density (DED) analysis. DED plots are made by taking the difference between observed and calculated SE densities. This allows us to study guest related issues of MOFs such as, location of guest molecules in the pores, tracking activation of MOFs and gas loading, etc. We also have shown that, DED created from routine powder diffraction patterns might provide very important information about MOF structure itself. In fact DED can be used for study of interpenetration, substituents locations and effects conformational changes in the MOF ligands. Generation and analysis of SEs and DEDs are easy and straightforward. It provides the information needed to explain major deviations in structure-property relationship in MOFs. In our opinion, this method might become one of the important and routine techniques for MOFs structural analysis.

The majority of the metallic elements adopt simple, high-symmetry structures at ambient pressure. These consist of a highly-ordered arrangement of atoms, which can be described by a crystal lattice that is periodic in three dimensions. It may be expected that close-packed structures, such as face-centred cubic (fcc) and hexagonal close-packed (hcp), would be favoured at high pressures due to the increase in density. However, many of these elements transform to lowsymmetry, complex structures on compression. In particular, a number have been observed to adopt incommensurately-modulated structures at high pressure. In these structures, atoms are displaced from their average positions by a modulation wave, the wavelength of which is an irrational multiple of the lattice periodicity. Diamond-anvil cells (DACs) can be used to compress materials to over a million times atmospheric pressure. In these devices, a small sample is compressed between the tips of two diamond anvils. The atomic arrangement of materials at extreme pressures can then be probed using the intense x-ray beams provided by synchrotron sources. In this work, the high-pressure crystal structures of the lanthanide elements europium (Eu) and samarium (Sm) have been investigated using angle-dispersive x-ray powder dffraction techniques. The high-pressure structural behaviour of Eu has been found to be remarkably different from that of the other lanthanide elements. Two new high-pressure phases of Eu are reported, both of which have an incommensurately-modulated crystal structure.

In this work, the crystallographic structure and its changes under thermal treatment of different systems consisting of metal oxide nanoparticles is studied. The principal method used throughout the thesis is x-ray powder diffraction enriched with grazing incidence small angle x-ray scattering when the nanoparticles form an ordered structure or with x-ray absorption spectroscopy when additional information on local crystallographic structure is required. For all the systems the preparation conditions were optimized according to the crystallographic data for further applications.