RE-doped BiFe03 (RE = La, Nd, Srn, Gd) compositions have been produced and investigated with a view to establishing a broad overview of their crystal chemistry and domain structure. For less than 10% RE dopant, the perovskite phase in all compositions could be indexed according to the rhombohedral, R3c cell of BiFe03. For La, Nd and Srn doped compositions with more than 15% dopant, a new antipolar phase was stabilised similar in structure to PbZr03 Pbam symmetry with a -V2ao2-V2ac,4ac unit cell where a, is the psuedocubic unit cell. The orthoferrite, Pnma structure was present in all RE-doped BiFe03 compounds with higher dopant concentration. The compositional window over which the PbZrO3-like phase was stable increased with increasing end-member tolerance factor, t (effectively controlled by the RE ionic radius). On heating, the PbZrO3-like phase transformed to the orthoferrite, Pnma structure. Tc for all compositions decreased with decreasing A-site, average ionic polarizabilty and end member tolerance factor. For compositions with R3c symmetry, superstructure and orientational and translational (anti-phase) domains were observed in a manner typical of an anti-phase tilted, ferroelectric perovskite. For the new PbZrO3-like phase orientational domains were observed along with antiphase boundaries associated with quadrupling of the unit cell due to the antipolar displacements of the Bi/RE-ions Neutron powder diffraction was used to determine changes in the nuclear and magnetic structures across the phase transitions that relate the high-temperature non- polar (Pnma) to the low temperature polar (R3c) and anti-polar (Pbam) structures, respectively. The high-temperature Pnma phase has a -√2ac, --√2ac,2ac cell with an aa c + octahedral tilting. The low-temperature polar R3c structure is similar to the B-phase of pure BiFe03 and is rhombohedral with aaa octahedral rotations and cation displacements along the peudocubic [111] direction. Although electron diffraction revealed weak additional doubling along c (denoted by the appearance of ¼ {001} in addition to ~{11O} reflections) of the PbZr03 cell, Neutron and X-ray diffraction could not reliably refine a larger cell. This is attributed to the weak intensity of the 1/4 {OO I} reflections and the short coherence length of the 4ac modulation in the c- direction. The transitions from the high temperature Pnma phase to both low temperature phases are accompanied by a large discontinuous expansion of the lattice volume in the low- temperature structure. The Pnma to R3c transition occurs in the paramagnetic state and generates no detectable changes in the magnetic structure. In contrast, the Pnma to Pbam transition, which occurs in the magnetic state, is accompanied by abrupt 90 ° reorientation of the magnetic dipoles. Coupling between the nuclear and magnetic structures is manifested in a significant magnetization anomaly. The dielectric properties were studied as a function of temperature but no anomalies were recorded that were coincident with the structural phase transitions observed by differential scanning calorimetry. The absence of peaks in permittivity at T c was attributed to high conductivity with space charge polarisation dominating the dielectric response.

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Electron backscatter diffraction (EBSD) is a powerful technique for measuring crystallographic orientation in polycrystalline materials. This thesis explores the potential of EBSD for characterising localised inelastic strain from lattice orientation measurements. A systematic study under uniaxial isothermal loading conditions was performed to examine the influence of microstructure and deformation conditions on strain-induced lattice orientation changes (misorientation). The study was conducted on both service-aged and un-aged Type 316H stainless steels through a series of monotonic tests in tension, compression and in constant load creep. The study demonstrates that the development of misorientation depends on many factors which need consideration before EBSD can be applied for strain assessment. It is shown that the measured evolution of misorientations is a function of microstructure and grain size. A misorientation-based strain assessment method is proposed which is relatively insensitive to microstructure and grain size. In service-aged steel, the measured evolution of misorientations is shown to be independent of the deformation temperature (between 24°C and 550°C) and deformation mode (tension vs. compression) for strain rates down to about . Empirical correlations between the accumulated plastic strain and different misorientation metrics are developed for true strains up to 0.23. However, at 550°C the evolution of measured misorientations is shown to be strain rate dependent below 1O-6S-1 . The potential of EBSD to distinguish plastic strain from creep strain is demonstrated. Misorientation development is shown to occur at a faster rate with increasing strain in plastic than in creep deformation. Similarly, the proportion of twin boundaries in service-aged steel is shown to reduce with increasing strain at a faster rate in creep than in plastic deformation. Two novel methods for creep strain estimation are proposed which utilise the disparities in the misorientation development and twin boundary reduction under the two different deformation regimes. A good correspondence is established between the strain estimates from the proposed methods and those derived from hardness measurements and digital image correlation. The methods are shown to be applicable to real power plant components through successful mapping of plastic and creep strain distributions in weldments after different periods in service.

This thesis describes the development of a scheme for applying wavefunction-based quantum chemistry techniques to crystalline solids: the hierarchical method. The hierarchical method was first proposed by Manby, Gillan and Alfe [1] for cubic crystals. The contribution of electron correlation to the cohesive energy of a crystal is found from calculations on finite clusters of atoms. These clusters are chosen to mimic the structure of the crystal and the edge-effects are subtracted using the hierarchical equation. The quantum chemistry techniques used offer advantages in accuracy over existing solid-state methodologies, such as density functional theory, as they are ab initio and systematically improvable. The hierarchical method was applied to three crystalline solids: lithium hydride, lithium fluoride and neon. For lithium hydride and lithium fluoride, results of high accuracy are reported and comparisons are made with experimental extrapolations as well as contemporary theoretical techniques. In the case of neon, a new hierarchical equation was derived, one that was general to any crystal structure. The assumptions made in the hierarchical method, regarding cluster sizes and the use of symmetry, were successfully tested against a many-body expansion [2]. The systematic improvability of the quantum chemistry techniques employed in the hierarchical method allowed chemical accuracy (±l kcal/mol or ±1.6 mEh) to be approached. For example, the cohesive energy of lithium hydride at the zero point Eok coh was calculated to be -175.3 mEh [3], within 0.4 mEh of the value extrapolated from experimental measurements -174.9 mEh [4]. As well as the cohesive energy Eok coh the hierarchical method can also be used to calculate the equilibrium lattice parameter ao, bulk modulus Bo and surface formation energy a at the zero point. The predicted values for lithium hydride are ao = 4.062 A, Bo = 33.2 GPa, Eok coh = -175.3 mEh [3] and a = 0.43 J m-2 [5]; and for lithium fluoride ao = 4.011 A, Bo = 70.9 GPa, Eok coh = -325.5 mEh and σ = 0.53 J m-2.

Under shock compression it is believed that crystalline materials undergo complex, rapid, micro-structural changes to relieve the large applied shear stresses. The mechanisms involved, such as dislocation flow and deformation twinning, under the generated high strain rates are not fully understood and in situ measurements of defects have proven elusive. This work presents the development of a nanosecond x-ray diffraction technique based on the white-light Laue method, and its first use in studying shock compressed copper. Observations of diffuse scattering are interpreted as stress-dependent lattice rotations due to dislocation glide. The results are compared with MD simulations of shocks in copper which are shown not to agree. Simulations of tantalum, shocked along the [001] axis, are demonstrated to undergo deformation twinning. A novel order parameter for identifying twin variants is developed and the deformation twinning mechanism under shock compression is identified.

Le développement de nouveaux matériaux luminescent pour les LEDs du futur, qui ont pour objectifs de préserver l’environnement et de réduire les dépenses énergétiques, est un thème de recherche d’actualité et un défi extrêmement intéressant pour de nombreux chercheurs. Actuellement, la plupart des LEDs commerciales sont des matériaux dopés aux lanthanides. Cependant, la situation économique et politique actuel est telle que pour l’approvisionnement en lanthanides, les pays européens sont dépendant des pays asiatiques qui ont le monopole sur la production et l’exportation des ceux-ci. Dans ce contexte, mes travaux de thèses effectués en collaboration avec l’UCCS de Lille (France) et l’Université de Siegen en Allemagne ont pour objectifs d’étudier les propriétés photoluminescentes de nouveaux oxydes ou oxohalogénures de Bismuth (sans lanthanides) étudiés préalablement par l’UCCS pour leurs caractéristiques structurales. Dans les phases sélectionnées, la connectivité des tétraèdres oxo-centrés O(BiM)4 est un paramètre clé permettant de jouer sur les propriétés optiques en changeant de polytype ou de composition chimique. De ce fait, un des objectifs de cette thèse est de déterminer les paramètres importants permettant de façonner les propriétés optiques de Bi3+ sur des bases de simples concepts tels que la connectivité du Bi, la stéréo-activité de sa paire d’électrons libres ou les structures électroniques…etc. L’étude des familles BiM2XO6 (M=Mg, Cd, Zn et X= P, As, V) et ABiO2X (A=Ca, Cd, Sr, Ba et X=Cl, Br) qui possèdent une émission intense à température ambiante valident la pertinence de ces travaux. / The development of luminescent materials for future LEDs is currently a hot-topic with respect to energy saving materials and it is an interesting challenge for many researchers to find the luminescent materials of tomorrow. Most of the current commercial LEDs are based on lanthanide activators. The economic and political context leads the European countries to be dependent on the Asian monopole, in terms of the production and exportation of lanthanides. In this context, my PhD work developed in collaboration with the UCCS at Lille, France and the University of Siegen, Germany, deals with the photoluminescence properties of some novel Bi3+ oxo-salts phosphors (lanthanide free) studied so far by the french partner for their structural aspects. In the selected phases, one key-parameter is the degree of connectivity between oxo-centered O(Bi,M)4 tetrahedra, which allows a fine tuning of optical properties from one polytype to another one and tuning the chemical composition. Therefore, one goal of this work concerns the deduction of the important parameters tailoring the Bi3+ optical properties, on the basis of simple concepts, e.g. Bi bonding scheme, Bi3+ lone pair (LP) stereo-activity, electronic structures … etc. The investigation of the BiM2XO6 (M/M´=Mg, Cd, Zn and X=P, As, V) and ABiO2X (A=Ca, Cd, Sr, Ba and X=Cl, Br) families validated the pertinence of this study, leading to bright luminescence at room temperature.

Complete photonic bandgaps (PBGs) are more readily achieved in highly-symmetric photonic crystals (Ph Cs). Aperiodic crystals (quasicrystals) with arbitrarily high orientational order are promising candidates to lower the dielectric contrast necessary to open PBGs. This thesis in- vestigates the connection between the structural and optical properties of four PhC lattices by studying the effects on transmission and fluorescence spectra. In order of increasing structural isotropy these lattices are: hexagonal, Archimedean-like, Stampfli, and a biomimetic 'sunflower'. High structural isotropy is associated with weaker diffraction. The sunflower's Fourier spectrum is defined by a dense ring of weak reciprocal lattice vectors. Its local morphology, which is everywhere unique, continuously transforms between localised 4- or 6-fold symmetry. All other crystals are spatially uniform with pure point spectra. Although structurally similar to the Archimedean, the Starnpfli improves isotropy without sacrificing diffraction efficiency. TM gaps of high-contrast (~c = 8.61) rod-type PhCs are shown to be nearly independent of the lattice geometry by FDTD simulations. The primary gaps are sensitive to random rod sizes, which disrupts the coherent coupling between the individual rod resonances. Transmission spectra for TE polarisation or hole-type Ph Cs are more dependent on Bragg reflection due to weak or non-existent Mie resonances. In small samples, the TM gap is typically wider in less isotropic crystals. Much larger samples demonstrate the importance of structural isotropy and long-range interactions in low ~c PhCs. The sunflower's 21% TM gap is, to date, the widest TM PBG reported for ~c = 1. The Stampfli also supports a TE gap in the same range as its 14% TM gap, thus yielding a 4.6% absolute PBG. Further band diagram calculations on an 'approximant' of the sunflower reveal the presence of intrinsic dipolar and monopolar defect states. Microwave characterisation of rod-type samples (~c = 8.61) showed complete TM PBGs (> 60dB) with gap ratios ranging from 37.28% (hexagonal) to 25.85% (sunflower). Low-contrast samples (~c = 1.6) showed complete TM PBGs (> 30dB) with gap ratios rising from 10.37% (hexagonal) to an ambiguous value of either 10.48% or 20.95% for the sunflower due to the unusual spiral structuring of the transmission spectra. The Stampfli also supports a complete TE gap (> 10dB) that coincides with its 14.19% TM gap for a 3.55% absolute gap that, to the author's knowledge, represents the first conclusive demonstration of an absolute PBG for ~c = 1.6. A larger sunflower sample was shown to have an extremely large experimental (simulated) TM gap of 33.33% (23.16%), erroneously broadened by the non-parallel rods. A new approach to enhance the efficiency of up conversion pumping in RE-doped media is pro- posed based on PBG suppression of emission from intermediate levels. Preliminary results indi- cate that visible emission from hexagonal and sunflower PhC slabs in 0.2 wt% Er:GLSO pumped at 808nm is enhanced by up to 1.6x at 550nm, or up to 4.5x at 525nm. Subsequent analysis appears to rule out suppression of IR emission, and suggests modified thermal properties as the cause.