Heteroanionic hydrides offer great possibilities in the design of functional materials. For ternary rare earth hydride oxide REHO, several modifications were reported with indications for a significant phase width with respect to H and O of the cubic representatives. We obtained DyHO and ErHO as well as the thus far elusive LuHO from solid-state reactions of RE2O3 and REH3 or LuH3 with CaO and investigated their crystal structures by neutron and X-ray powder diffraction. While DyHO, ErHO, and LuHO adopted the cubic anion-ordered half-Heusler LiAlSi structure type (F4¯3m, a(DyHO) = 5.30945(10) Å, a(ErHO) = 5.24615(7) Å, a(LuHO) = 5.171591(13) Å), LuHO additionally formed the orthorhombic anti-LiMgN structure type (Pnma; LuHO: a = 7.3493(7) Å, b = 3.6747(4) Å, c = 5.1985(3) Å; LuDO: a = 7.3116(16) Å, b = 3.6492(8) Å, c = 5.2021(7) Å). A comparison of the cubic compounds’ lattice parameters enabled a significant distinction between REHO and REH1+2xO1−x (x < 0 or x > 0). Furthermore, a computational chemistry study revealed the formation of REHO compounds of the smallest rare earth elements to be disfavored in comparison to the sesquioxides, which is why they may only be obtained by mild synthesis conditions.

The order of OH groups in the crystal structure of SbC2O4OH, a potential precursor in the synthesis of ternary oxides, was debated. Neutron diffraction on the deuteride SbC2O4OD revealed disordered OD groups with half occupation for deuterium atoms on either side of a mirror plane (SbC2O4OD at T = 298(1) K: Pnma, a = 582.07(3) pm, b = 1128.73(5) pm, c = 631.26(4) pm). O–H stretching frequencies are shifted by a factor of 1.35 from 3390 cm−1 in the hydride to 2513 cm−1 in the deuteride as seen in infrared spectra. SbC2O4OH suffers radiation damage in a synchrotron beam, which leaves a dark amorphous residue. Thermal decomposition at 564 K yields antimony oxide, carbon dioxide, carbon oxide, and water in an endothermic reaction. When using SbC2O4OH as a precursor in reactions, however, ternary oxides are only formed at much higher temperatures.

Zirconium alloys are widely used in the nuclear industry because of their high strength, good corrosion resistance and low neutron absorption cross-section. However, zirconium has strong affinity for hydrogen, which may lead to hydrogen concentration build-up over time during a corrosion reaction when exposed to water. Hydrogen stays in solution at higher temperature but precipitates as zirconium hydrides at ambient temperatures. The formation of zirconium hydrides is considered to be a major cause of embrittlement, in particular as a key step in the mechanism of delayed hydride cracking. Despite the fact that zirconium hydrides have been studied for several decades, the basic nature and mechanisms of hydride formation, transformation and exact structure are not yet fully understood. In order to find the answer to some of these problems, the precipitation and dissolution of hydrides in commercial grade Zr powder were monitored in real time with high resolution synchrotron and neutron radiations, and the whole pattern crystal structure analysis, using Rietveld and Pawley refinements, were performed. For the first time all commonly reported zirconium hydride phases and complete reversible transformation between two different Zr-hydride phases were recorded with a single setup and their phase transformation type have been analyzed. In addition, the preparation route of controversial γ-zirconium hydride (ZrH), its crystal structure and formation mechanisms are also discussed. / <p>Note: The papers are not included in the fulltext online.</p><p>Paper II and III in thesis as manuscript, paper II with title "The phase transformation between the δ and ε Zr hydrides"</p>

Crystalline systems formed with transition metal elements tend to exhibit strong magneto-structural coupling that gives rise to unusual but exciting physical phenomena in these materials. In this dissertation, we present our findings from the studies of structural and physical properties of single phase compounds Co2MnSi, Ni16Mn6Si7 and Mn(Ni0.6Si0.4)2. In addition, the stability of a Ni2MnSi composition in a multiphase system is discussed by both theoretical and experimental approaches. All the works have been conducted with a focus on explaining the fundamental behaviors of these systems that have not been adequately addressed by other studies in the literature. We present an experimental and theoretical investigation of the half-metallic Heusler compound, Co2MnSi to address disorder occupancies and magnetic interactions in the material. Contrary to previous studies, our neutron diffraction refinement of the polycrystalline sample reveals almost identical amount of Mn and Co antisite disorders of ~6.5% and ~7.6%, respectively which is also supported explicitly by our first-principles calculations on the system with defects. A reduction of the net moment of Co2MnSi due to an antiferromagnetic interaction introduced by disordered Mn is observed by our theoretical study. The neutron refinements at 298 K, 100 K, and 4 K further supports such reduction of moments. The work also reports the growth of single crystal by the Czochralski method and determination of a Curie temperature of ~1014 K measured by both the electrical resistivity and dilatometry measurement. Studies of a Ni2MnSi Heusler system reveal two new systems i.e., the Ni16Mn6Si7 G-phase and the Mn(Ni0.6Si0.4)2 based Laves phase with complex crystal structures. These systems exhibit strong magneto-structural coupling that could lead to interesting physical behaviors. The lack of thorough understanding of the properties of these materials inspired us to undertake the present studies. We address the geometrically frustrated two-dimensional magnetic structure and spin canted weak ferromagnetic behavior of Ni16Mn6Si7. Our magnetization and specific heat measurements on a Czochralski grown single crystal sample depicts the paramagnetic to antiferromagnetic transition at 197 K, and a second phase change at 50 K. Furthermore, a gradual drop of zero field cooled magnetic susceptibility is observed below 6 K that is associated with the spin freezing effect. The neutron diffraction on the polycrystalline powder samples at the temperatures of interest reveals that the antiferromagnetism is governed by the magnetic ordering of the Mn ions in the octahedral network. Below the Néel temperature of 197 K, the 2/3 of Mn atom moments form a two-dimensional magnetic arrangement, while the 1/3 moments remain geometrically frustrated. The phase transition at 50 K is found to be associated with the reorientation of the 2D moments to a canted antiferromagnetic state and development of ordering of the frustrated paramagnetic ions. Magnetization measurements as a function of temperature and magnetic field in principal directions, permit to determine the anisotropic magnetic behavior of Ni16Mn6Si7 in terms of the magnetic structure obtained by the neutron diffraction measurements. We also report an irreversible smeared spin-flop type transition for the system at a higher magnetic field. The diffuse scattering due to the short-range ordering is a commonly occurring phenomenon in Laves phase materials. The occurrence of such distinct atomic arrangement can considerably influence the physical behavior of the material. Nevertheless, no structural reconstruction of such atomic distribution in Laves phase has ever been reported in the literature. In this work, we present the structural ordering, and the associated physical behavior of an antiferromagnetic Ni-Mn-Si Laves phase with a composition Mn(Ni0.6Si0.4)2. The possibility of unique short-range ordering in the material is first concluded based on our single crystal diffraction analysis. With the high-resolution transmission electron microscopy and electron energy loss spectroscopy analysis, our work resolves the distinct atomic ordering of the Laves phase system. The investigations reveal the origin of the short-range ordering to arise from a unique arrangement between Ni and Si. The study also presents the atomic resolution mapping of the Si atoms which has never been reported by any previous studies. With further electrical conductivity measurement, we find one of the consequences of the unique ordering reflected in a semiconducting like temperature dependence of the compound. The neutron diffraction at 298 K suggests Mn(Ni0.6Si0.4)2 to be a strong antiferromagnetic system, which is further supported by the successive magnetic susceptibility measurement. The Néel temperature is determined to be 550 K. We also address the stability of the hypothetical ferromagnetic Heusler compound Ni2MnSi which has been proposed to be a stable system by numerous theoretical studies. Our first-principles work corroborates those studies with a negative formation enthalpy of -1.46 eV/formula unit. However, after numerous attempts to synthesize the composition, we conclude that a single phase Heusler Ni2MnSi compound cannot form under ambient conditions. Our results show that the system crystallizes as a mixture of the two Ni-Mn-Si compounds, i.e., the Ni16Mn6Si7 type G-phase and Mn(Ni0.6Si0.4)2 based Laves phase. Our work provides a possible explanation for the unstable Ni2MnSi Heusler compound with the calculation of formation enthalpy of the hypothetical Heusler system in terms of the computed energies of the neighboring phases Ni16Mn6Si7 and Mn(Ni0.6Si0.4)2. / Thesis / Doctor of Philosophy (PhD)

Magnetic frustration is the underpinning theme to all of the magnetic oxide systems explored in this dissertation. The materials studied in this thesis belong to two topical families of interest in modern condensed matter physics, namely, the rare-earth titanates R2Ti2O7 and the double perovskites A2BB'O6. Chapter 1 provides the theoretical background necessary to understand the crystalline systems studied in this thesis. Chapter 2 explains the necessity of utilizing neutron scattering and x-ray experiments to tease out the key signatures which were essential to formulating the conclusions made in each study. Chapter 3 outlines the neutron scattering techniques which were employed to investigate the crystal systems. The first objective of this thesis is to understand effect of “stuffing” on the ground state anisotropy of the quantum spin liquid candidate Yb2Ti2O7 via an investigation of the crystal-field excitations in intentionally stuffed samples. The pentultimate study was performed on the monoclinic crystal systems, La2LiRuO6 and La2LiOsO6, to discern the effect of lattice distortions on the spin-orbit induced magnetic ground state of 4d3 and 5d3 double perovskites based on Ru and Os magnetic ions. The final investigation involves an inelastic neutron scattering investigation of magnetic ground states in three d2 double perovskites, Ba2CaOsO6, Ba2MgOsO6 and Ba2ZnOsO6. Here, we make the case for novel octupolar order below their respective transition temperatures T* of 50 K, 49 K and 30 K based on information provided by neutron scattering, heat capacity, muon spin relaxation and synchrotron x-ray diffraction studies. / Thesis / Doctor of Philosophy (PhD)

The thesis reports improvements in the characterization techniques for stress and texture in crystalline materials by x-ray and neutron powder diffraction. Furthermore, advances are made in texture evolution modelling and validation against experimental observations. In the beginning, the fundamental assumption of diffraction strain analysis is numerically examined and verified, namely, that the lattice parameter value determined from fitting the diffraction pattern is equal to the average lattice parameter within the gauge volume. Next, the task of shear strain determination from powder diffraction measurements is addressed. A method is developed and implemented for the complete 2D strain tensor determination from the multi-directional energy-dispersive x-ray diffraction patterns. The method not only offers a way to evaluate the shear strain, but also provides a better overall strain averaging approach. Rotation and translation of sample and/or detectors in powder diffraction mode can effectively increase the pole figure coverage and thus the accuracy of texture determination. However, the movements also introduce uncertainties and aberrations into data analysis due to the changes in the diffraction volume and transmitted intensity. In order to overcome these problems, accurate <strong>single exposure</strong> texture characterization techniques are proposed based on several different powder diffraction setups. Numerical analyses are carried out to prove that any simple texture in cubic polycrystals can be effectively determined using single exposure Debye-Scherrer diffraction pattern analysis. Several experiments are reported on collecting Debye-Scherrer diffraction patterns, multi-directional energy-dispersive x-ray diffraction patterns and multi-directional TOF neutron setup. Efficient data processing procedures of the diffraction patterns for ODF determination are presented. Crystal plasticity finite element models are developed to model the texture evolution in polycrystalline engineering samples during manufacturing. In the present thesis, quantitative measures extracted from orientation distribution function are employed to make precise comparison between the model and experiment. Unlike the simple uni-axial compression and tension considered in the literature, in the present thesis the complex texture evolution during linear friction welding is modelled as a sequence of different shear deformations.

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This thesis is devoted to the numerical and experimental investigation of residual stress and deformation in polycrystalline metallic alloys at different scales. The emphasis in the current study is placed on establishing the connection between the simulation of deformation by the Finite Element (FE) method and experimental characterisation by synchrotron X-Ray Diffraction (XRD). Of particular importance is the interpretation of modelling results and their validation by careful comparison with experimental data. The concept of eigenstrain was used extensively throughout the report to study the residual elastic strain distributions and their sources. A pseudo-thermal strain FE procedure was used systematically to simulate the residual stress states in samples and engineering components of different shape and dimensionality. The case of 1-D strain variation was considered using the example of a plastically bent bar. The direct and inverse problems of eigenstrain analysis were solved, and validated experimentally by the use of XRD and EDM slitting methods. A novel 2-D discrete inverse eigenstrain algorithm was proposed and implemented to reconstruct the residual stress distribution in a worn rail head. The link between the residual stress and deformation history was studied via thermo-mechanical modelling of the Linear Friction Welding (LFW) process. To advance the understanding of polycrystalline deformation behaviour across the scales, a crystal plasticity model was employed to simulate the elastic-plastic deformation behaviour of Ti-6Al-4V alloy. A post-processor was developed to extract the average elastic strains for orientation-specific grain groups and to compare them with XRD data. A “peak constructor” post-processor was developed that utilised the knowledge of both the elastic strain and dislocation density. In a further development step, a strain gradient crystal plasticity formulation was adopted to account for the local dislocation evolution. Intra-granular deformation analysis was carried out and micro-beam Laue experimental diffraction technique was used for validation. Thus, local lattice arrangement was studied at the microscopic, intragranular scale. Special attention was paid to the phenomenon of Laue spot “streaking”, indicative of the local lattice misorientation caused by dislocation activity during deformation. The results presented in this thesis contributed to the fundamental understanding of the residual stress and deformation in polycrystalline metallic alloys and lead to more than 20 publications in peer-reviewed journals and conference proceedings, which are listed in the Appendix.

The currently operating Generation II Advanced Gas-Cooled Reactors (AGR) in the nuclear power stations in the UK, mainly built in the 1960s and 1970s, are approaching their designed life. Besides the development of the new generation of reactors, the government is also seeking to extend the life of some AGRs. Creep and failure properties of Type 316H austenitic stainless steels used in some components of AGR at elevated temperature are under investigation in EDF Energy Ltd. However, the current empirical creep models used and examined in EDF Energy have deficiency and demonstrate poor agreement with the experimental data in the operational complex thermal/mechanical conditions. The overall objective of the present research is to improve our general understanding of the creep behaviour of Type 316H stainless steels under various conditions by undertaking theoretical studies and developing a physically based multiscale state variable model taking into account the evolution of different microstructural elements and a range of different internal mechanisms in order to make realistic life prediction. A detailed review shows that different microstructural elements are responsible for the internal deformation mechanisms for engineering alloys such as 316H stainless steels. These include the strengthening effects, associated with forest dislocation junctions, solute atoms and precipitates, and softening effects, associated with recovery of dislocation structure and coarsening of precipitates. All the mechanisms involve interactions between dislocations and different types of obstacles. Thus change in the microstructural state will lead to the change in materials' internal state and influence the mechanical/creep property. Based on these understandings, a multiscale self-consistent model for a polycrystalline material is established, consisting of continuum, crystal plasticity framework and dislocation link length model that allows the detailed dislocation distribution structure and its evolution during deformation to be incorporated. The model captures the interaction between individual slip planes (self- and latent hardening) and between individual grains and the surrounding matrix (plastic mismatch, leading to the residual stress). The state variables associated with all the microstructure elements are identified as the mean spacing between each type of obstacles. The evolution of these state variables are described in a number of physical processes, including the dislocation multiplication and climb-controlled network coarsening and the phase transformation (nucleation, growth and coarsening of different phases). The enhancements to the deformation kinetics at elevated temperature are also presented. Further, several simulations are carried out to validate the established model and further evaluate and interpret various available data measured for 316H stainless steels. Specimens are divided into two groups, respectively ex-service plus laboratory aged (EXLA) with a considerable population of precipitates and solution treated (ST) where precipitates are not present. For the EXLA specimens, the model is used to evaluate the microscopic lattice response, either parallel or perpendicular to the loading direction, subjected to uniaxial tensile and/or compressive loading at ambient temperature, and macroscopic Bauschinger effect, taking into account the effect of pre-loading and pre-crept history. For the ST specimens, the model is used to evaluate the phase transformation in the specimen head volume subjected to pure thermal ageing, and multiple secondary stages observed during uniaxial tensile creep in the specimen gauge volume at various temperatures and stresses. The results and analysis in this thesis improve the fundamental understanding of the relationship between the evolution of microstructure and the creep behaviour of the material. They are also beneficial to the assessment of materials' internal state and further investigation of deformation mechanism for a broader range of temperature and stress.