We present a detailed study of the local atomic and magnetic structure of the type-I multiferroic perovskite system (Sr,Ba)(Mn,Ti)O3 using x-ray and neutron pair distribution function (PDF) analysis, polarized neutron scattering, and muon spin relaxation (μSR) techniques. The atomic PDF analysis reveals widespread nanoscale tetragonal distortions of the crystal structure even in the paraelectric phase with average cubic symmetry, corresponding to incipient ferroelectricity in the local structure. Magnetic PDF analysis, polarized neutron scattering, and μSR likewise confirm the presence of short-range antiferromagnetic correlations in the paramagnetic state, which grow in magnitude as the temperature approaches the magnetic transition. We show that these short-range magnetic correlations coincide with a reduction of the tetragonal (i.e. ferroelectric) distortion in the average structure, suggesting that short-range magnetism can play an important role in magnetoelectric and/or magnetostructural phenomena even without genuine long-range magnetic order. The reduction of the tetragonal distortion scales linearly with the local magnetic order parameter. These findings provide greater insight into the multiferroic properties of (Sr,Ba)(Mn,Ti)O3 and demonstrate the importance of investigating the local atomic and magnetic structure to gain a deeper understanding of the intertwined degrees of freedom in multiferroics.

The oxygen evolution reaction (OER) is kinetically slow and hence a significant efficiency loss in electricity-driven water electrolysis. Understanding the relationships between architecture, composition, and activity in high-performing catalyst systems are critical for the development of better catalysts. This dissertation discusses areas both fundamental and applied that seek to better understand how to accurately measure catalyst activity as well as ways to design higher performing catalysts. Chapter I introduces the work that has been done in the field to date. Chapter II compares various methods of determining the electrochemically active surface area of a film. It further discusses how pulsed and continuous electrodepostition techniques effect film morphology and behavior, and shows that using a simple electrodeposition can create high loading films with architectures that outperform those deposited onto inert substrates. The reversibility of the films, a measure of the films transport efficiency, is introduced and shown to correlate strongly with performance. Chapter III uses high energy x-ray scattering to probe the nanocrystalline domains of the largely amorphous NiFe oxyhydroxide catalysts, and shows that significant similarities in the local structure are not responsible for the change in performance for the films synthesized under different conditions. Bond lengths for oxidized and reduced catalysts are determined, and show no significant phase segregation occurs. Chapter IV seeks to optimize the deposition conditions introduced in Chapter II and to provide a physical representation of how tuning each of the parameters affects film morphology. The deposition current density is shown to be the most important factor affecting film performance at a given loading. Chapter V highlights the different design considerations for films being used in a photoelectrochemical cell, and how in situ techniques can provide information that may otherwise be unobtainable. Chapter VI serves as a summary and provides future directions. This dissertation contains previously published coauthored material.

The antiferromagnetic semiconductor MnTe has recently attracted significant interest as a potential high-performance thermoelectric material. Its promising thermoelectric properties are due in large part to short-range magnetic correlations in the paramagnetic state, which enhance the thermopower through the paramagnon drag effect. Using magnetic pair distribution function (mPDF) analysis of neutron total scattering data, we present a detailed, real-space picture of the short-range magnetic correlation in MnTe, offering a deeper view into the paramagnon drag effect and the nature of the correlated paramagnetic state. We confirm the presence of nanometer-scale antiferromagnetic correlations far into the paramagnetic state, show the evolution of the local magnetic order parameter across the N\'eel temperature T_N=307 K, and discover a spatially anisotropic magnetic correlation length. By combing our mPDF analysis with traditional atomic PDF analysis, we also gain detailed knowledge of the magnetostructural response in MnTe. We observed a spontaneous volume contraction of nearly 1\%, the largest spontaneous magnetovolume effect reported so far for any antiferromagnetic system. The lattice strain scales linearly with the local magnetic order parameter, in contrast to the quadratic scaling observed for the conventional magnetostriction properties of this technologically relevant material. Using neutron and X-ray PDF analysis, we also investigated the local distortion on MnTe and Mn-based systems, MnS and MnO as a function of temperature. Such local distortion on MnTe increases with the rise in temperature and becomes more pronounced at 500 K.

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.

Iron oxide nanoparticles (NPs) are promising materials for use in many applications, including new cancer treatments and in cleaning water, because they exhibit size-dependent magnetic and absorptive properties. NP properties are caused by structural attributes of the NPs, like surface disorder and cation vacancies. However, NP synthetic methods also impact structure, therefore properties, of NPs. Furthermore, the synthetic method is often changed in order to change the core diameter of NPs. Determining if properties are caused by the dimensions of the NP is impossible if there are also structural features present in the NP caused by the synthetic method, like grain boundaries or polycrystalline shells. In Chapter II of this dissertation, we show a new continuous growth synthesis of spinel iron oxide where the diameter of NPs is changed by the amount of precursor added to the reaction, meaning the only structural feature changing between the NPs is size. Continuous growth, therefore, can be used to probe the impact that size has on NP structure and properties. We report that saturation magnetization of NPs produced from continuous growth is size-dependent and higher in magnitude than NPs of the same core diameter made by other syntheses. In chapter III of this dissertation we determine nanoscale structure by Pair Distribution Function (PDF) analysis of Total X-ray Scattering data of NPs isolated from the reaction with core diameters between 3-10 nm. In Chapter IV of this dissertation we monitored the growth of NPs in situ with Total X-ray Scattering to gain insight on the structures of NPs while forming. In situ measurements of Total X-ray Scattering data gave insights into how precursor oxidation state influences the structures formed during formation of NPs, with more oxidized precursor giving a more oxidized product and a reduced precursor yielding a more reduced product even though the NPs formed by either method are indistinguishable by ex situ analysis. This dissertation includes previously published and unpublished co-authored material. / 2021-04-30

The spinel structure (general formula AB2O4) is widely occurring in natural and synthetic materials, and has a marked technological and scientific significance due to its magnetic, electric and multiferroic behaviours. The presence of transition metal cations with multiple oxidation state and the resulting charge, orbital and spin degrees of freedom of the partially occupied d-orbitals lead to uniquely ordered ground states. The coupling of all the three degrees of freedom can result in a structurally distorted ground state where the direct metal-metal interaction forms atomic clusters, or 'orbital molecules'. The Verwey phase of magnetite (Fe3O4), occurring below TV ~ 125 K, is driven by a cooperative bond distortion that forms linear Fe3+-Fe2+-Fe3+ arrangement (trimeron). The effect of non-stoichiometry and chemical modification on this complex structure has been investigated with a variety of samples through microcrystal synchrotron XRD. A mineral sample (Al, Si, Mg and Mn impurities, TV = 119 K) confirms the Verwey phase as the most complex long-range electronic order known to occur naturally; its relevance in space sciences is discussed. Moreover, the structural analysis of two synthetic magnetites (Fe3(1-δ)O4 with 3δ = 0.012 and TV = 102 K, Fe3-xZnxO4 with x = 0.03 and TV = 90 K) univocally confirmed the persistence of the transition, and its first order, at doping level > 1 %, contrary to previous reports. Moreover, the temperature evolution of the trimerons and their persistence above TV was probed through X-ray Pair Distribution Function analysis on pure Fe3O4: the data analysis between 90 K < T < 923 K show that the Verwey phase goes from long-range ordered (T < 125 K) to short-range ordered (T > 850 K). Magnetite can thus only be considered to have a regular cubic spinel structure above the Curie temperature (TC = 858 K). The pyrochlore lattice of B cations in a spinel gives the structure the potential for frustration upon antiferromagnetic ordering. Fe2GeO4 and γ-Fe2SiO4 were synthesised through conventional solid state routes, with the use of high-pressure synthesis for the latter. Magnetometry and heat capacity measurements highlighted two transitions (Tm1 = 8.6 K and Tm2 = 7.2 K, and Tm1 = 11.2 K and Tm2 = 7.5 K respectively). Powder neutron diffraction data between 2 K < T < 25 K showed that both materials stay undistorted below TN. Magnetic Rietveld refinement led to two highly unconventional magnetic structures, with incommensurate propagation vectors and modulation of the moment magnitude. γ-Fe2SiO4 also shows a spin-ice order below Tm2. The results are unique and unusual for transition metal oxides; the models are systematised by proposing a 'frustration wave' model, in which the degree of frustration is a spatial quantity that can be distributed through the structure in order to stabilise the ground state.

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京都大学 / 新制・課程博士 / 博士(人間・環境学) / 甲第23287号 / 人博第1002号 / 新制||人||236(附属図書館) / 2020||人博||1002(吉田南総合図書館) / 京都大学大学院人間・環境学研究科相関環境学専攻 / (主査)教授 内本 喜晴, 教授 田部 勢津久, 准教授 戸﨑 充男 / 学位規則第4条第1項該当 / Doctor of Human and Environmental Studies / Kyoto University / DFAM

The local structure, atomic and magnetic structure correlated over a small length scale, of a material has a strong impact on material properties. Pair distribution function (PDF) analysis is a strong tool to investigate local structure and magnetism of this nature. This work outlines extensions to current one dimensional magnetic pair distribution functions (1D-mPDF) and the fitting of structures with symmetry breaking local atomic distortions. 1DmPDF analysis has been used to study local magnetic structure, but requires a rotational averaging of the correlations so directional information is lost, as in powder diffraction experiments. Three dimensional difference magnetic pair distribution function (3D-∆mPDF) analysis does not require this rotational averaging and preserves directional information. This is a useful tool in analyzing experimental data like single crystal neutron diffraction and studying locally anisoptropic magnetic structures. Here we present a technique and software tools to calculate the 3D-∆mPDF pattern of a given structure and give a brief analysis of the local magnetic structure of MnTe. Another problem in PDF analysis is the modeling of structures with symmetry breaking local atomic distortions. Symmetry-adapted distortion modes have been used for structural refinement in Rietveld refinement for at least 10 years; more more recently, this has also been applied to PDF data. We present here a detailed discussion of the use of symmetry-adapted modes for structural refinement using PDF data. We also outline new open-sourced software tools to apply this technique and show two analyses using symmetry-adapted structural modes.

No description available.