Over the years `short-range order' (SRO), whereby the local atomic arrangement differs from that of a random distribution, has been used to explain physical phenomena such as thermodynamic discontinuities, increased strength, anomalous electrical resistivity and magnetic variations in a host of alloys. However, due mainly to experimental difficulties and the complexity of the calculations required for the analysis of diffuse scattering, such work has been largely abandoned and hence quantification and assessment of SRO is notably sparse in the literature. The recent development of reverse Monte-Carlo (RMC) methods for the analysis of total scattering data has opened a promising route for the assessment of a material's local environment and has already provided important insights into a host of complex chemical systems, including liquids, network glasses, nano-materials, functional oxides and metal organic frameworks. The work presented in this thesis focuses on the development of a new methodology for the analysis of local structural effects in metallic systems using total scattering, and the first systematic application to the study of alloys. The simulation of total scattering data from a range of model structures is used to show that the information content of total scattering functions, in particular the pair distribution function (PDF), is sufficiently high to allow the assessment of different types and degrees of short-range order. This is supported by a demonstration of how such orders can be quantified from large box models, produced by fitting total scattering data using the RMC algorithm, with the mathematical analyses outlined. This culminates in a proposed methodology for the analysis of SRO in alloys. Having developed this analytical methodology it is subsequently applied to a number of interesting alloy systems. To demonstrate the efficacy of this methodology it was first applied to the study of a sample of Cu$_{3}$Au - the classically cited case example of a system demonstrating SRO prior to an ordering transition. This experiment provides new insight into this well characterised transition, and also demonstrates the significance of data processing errors on the generation of artefacts in large box modelling. The technique is also applied to the study of the industrially important family of nickel superalloys, assessing ordering in the gamma-phase alloy Ni-Cr and the sublattice orderings occurring in L1$_{2}$ alloys. Next, the use of the technique for the analysis of local strains exhibited in a lattice is presented. A series of models is used to demonstrate how the PDF is expected to change under variations in local strain caused by increased concentration of atomic substitution and variation in atomic radii. This is subsequently used to study the characteristic high-entropy alloy (HEA) CrMnFeCoNi. Through analysis of the PDF, it is demonstrated that the level of local strain exhibited in this alloy is not significantly different from those of other related compositionally simpler alloys. This result is highly significant as it challenges one of the core principles of the field - that the lattices of HEAs are necessarily highly strained. Finally, the energetics of ordering reactions are briefly considered and used to justify some of the observed transformations presented in the earlier work. Together, the body of work in this thesis shows how the total scattering technique can be used to provide valuable insight into a host of interesting local phenomena occurring in alloy systems. It is hoped that this will open up a new field of study into these effects, and ultimately guide the creation of new alloys based on these structure-property relationships.

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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)

Plant roots are critical weathering agents in deep soils, yet the impact of resulting mineral transformations on the vast deep soil carbon (C) reservoir are largely unknown. Root-driven weathering of primary minerals may cause the formation of reactive secondary minerals, which protect mineral-organic associations (MOAs) for centuries or millennia. Conversely, root-driven weathering may also transform secondary minerals, potentially enhancing the bioavailability of C previously protected in MOAs. Here we examined the impact of root-driven weathering on MOAs and their capacity to store C over pedogenic time scales. I compared soil that experienced root-driven weathering, resulting in the formation of discrete rhizosphere zones in deep soil horizons (100-160 cm) of the Santa Cruz Marine Terrace chronosequence (65 ka-226 ka), with adjacent soil that experienced no root growth. Using a combination of radiocarbon, mass spectrometry, Mössbauer spectroscopy, and X-ray spectromicroscopy approaches, we characterized MOA transformations in relation to changes in C content, turnover and chemistry across four soils ranging in age (65 ka-226 ka). We found that the onset of root-driven weathering (65-90 ka) increased the amount of C associated with poorly crystalline iron (Fe) and aluminum (Al) phases, particularly highly-disordered nano-goethite. The increase in C coincided with greater overall C concentrations, longer C residence times, and a greater abundance of microbially-derived C. Continued root-driven weathering (137-226 ka) did not significantly change the amount of C associated with crystalline Fe and Al phases, but resulted in a decline in the amount of C associated with poorly crystalline metal phases. This decline in C associated to poorly crystalline phases coincided with a decrease in C concentrations and potential turnover rates, and a shift toward plant-derived C. In contrast, soil not affected by root-driven weathering showed low amounts of C bound to poorly crystalline Fe and Al phases regardless of soil age and, correspondingly, lower C concentrations and estimated residence times. My results demonstrate that root-driven formation and disruption of poorly crystalline Fe and Al phases directly controls both C accrual and loss in deep soil. Hence root impacts on soil C storage are dependent on soil weathering stage, a consideration critical for predictions of the vulnerability of deep soil C to global change.

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