A detailed understanding of fundamental material properties can be obtained through the study of atomic vibrations, performed experimentally with neutron scattering techniques and coupled with the two powerful new computational methodologies I have developed. The first approach involves phonon-based simulations of the pair distribution function - a histogram of localised atomic positions generated experimentally from total scattering data. This is used to reveal ordering behaviour, to validate interatomic models and localised structure, and to give insights into how far dynamic behaviour can be studied using total scattering techniques. Most importantly, the long-standing controversy over dynamic disorder in β-cristobalite is resolved using this technique. Inelastic neutron spectroscopy (INS) allows \emph{direct} study of vibrational modes through their interaction with the neutron beam, and is the experimental basis for the second strand of the new methodology. I have developed new simulation and refinement tools based on the next generation of spectrometers currently being commissioned at the ISIS pulsed neutron source. This allows a detailed powder spectroscopy study of cristobalite and vitreous silica demonstrating that the Bose peak and so-called 'fast sound' features can be derived from standard lattice dynamics in both the crystal and the amorphous counterpart, and allowing discussion of their origins. Given the controversy in the literature, this is a key result. The new methodology also encompasses refinement of interatomic models against powder INS data, with aluminium providing a successful test-case. A more complex example is seen in calcite, with experimental data collected during the commissioning of the new MERLIN spectrometer. Simulated one-phonon coherent INS spectra for the single crystal and powder (the latter including approximations to multi-phonon and multiple scatter) are fully convolved with experimental resolution functions. These are used in the analysis of the experimental data, yielding previously unpublished dispersion curves and soft mode information, as well as allowing the effectiveness of powder refinement of more complex materials to be assessed. Finally, I present further applications with technologically important materials - relaxor ferroelectrics and high temperature pnictide superconductors. The conclusions draw together the different strands of the work, discussing the importance of these new advances together with future developments and scientific applications.

[ES] En esta tesis se realiza un estudio de acidez de zeolita LTA con la relación Si/Al 5 y 40 utilizando la técnica de dispersión inelástica de neutrones (INS) en combinación con otras tecnicas como la modelización computacional y la espectroscopía de resonancia magnética nuclear (RMN). Las zeolitas son aluminosilicatos altamente cristalinos que forman parte de un grupo importante de los materiales funcionales. Las zeolitas son extremadamente útiles como catalizadores para muchas reacciones importantes con moléculas orgánicas. Las más importantes son craqueo, isomerización y síntesis de hidrocarburos. Las propiedades catalíticas de una zeolita dependen principalmente de su acidez y ésta depende de tres factores: el número totál de los sitios ácidos, sus fuerzas individuales y su localización individual, estando estos tres factores relacionados. Los parámetros geométricos definidos por localización de los sitios ácidos (es decir, la longitud y los ángulos de enlaces entre los átomos alrededor del sitio ácido) contribuyen de una manera importante a la fuerza ácida. En la literatura hay muchos trabajos de estudios de acidez en zeolita utilizando la espectrometría infrarroja (IR), sin embargo, con esta técnica es imposible medir las vibraciones de flexión (bending modes, 200-1200 cm-1), las bandas más sensibles al entorno local de los centros ácidos, ya que estas bandas se superponen con las bandas de vibración de la red de zeolite (300-1800 cm-1). En esta tesis se realiza un estudio de acidez utilizando la tecnica de INS, que nos permite medir principalmente los modos de vibración de los átomos de H y también nos permite medir todas las bandas de vibración (incluido el bending). Este estudio muestra que la combinación de la alta calidad de las muestras y la sensibilidad del instrumento utilizado permite detectar con alta precisión los sitios ácidos y obtener la información sobre su posición que a su vez nos perimte obtener la posición de los átomos de Al. Con el fin de interpretar los espectros INS se ha realizado un estudio computacional (cálculos ab-initio) con el objetivo de comparar los espectros cálculados con los resultados experimentales y definir el modelo que reproduce la posición mas probable de los centros ácidos en la muestra. / [CAT] En aquesta tesi es realitza un estudi d'acidesa de zeolita LTA amb la relació Si / Al 5 i 40 utilitzant la tècnica de dispersió inelastica de neutrons (INS) en combinació d'altres tècniques com la modelització computacional i la espectroscòpia de ressonància magnètica nuclear (RMN ). Les zeolites són aluminosilicats altament cristal·lins que formen part d'un grup important dels materials funcionals. Les zeolites són extremadament útils com a catalitzadors per a moltes reaccions importants amb molècules orgàniques. Les més importants són craqueig, isomerització i síntesi d'hidrocarburs. Les propietats catalítiques d'una zeolita depenen principalment de la seva acidesa i aquesta depèn de tres factors: el nombre total dels llocs àcids, les seves forces individuals i la seva localització individual, estant aquests tres factors relacionats. Els paràmetres geomètrics definits per localització dels llocs àcids (és a dir, la longitud i els angles d'enllaços entre els àtoms al voltant del lloc àcid) contribueixen d'una manera important a la força àcida. En literatura hi ha molts treballs d'estudis d'acidesa en zeolita utilitzant l'espectrometria infraroja (IR), però és impossible mesurar les vibracions de flexió (bending modes, 200-1200 cm-1) amb aquesta tècnica ja que aquestes bandes es superposen amb les bandes de vibració de la xarxa de zeolita (300-1800 cm-1). La tècnica de dispersió inelastica de neutrons (Inelastic neutron scattering) ha estat utilitzada per estudiar l'acidesa de la zeolita LTA amb diferents relacions Si / Al (per tant diferents c.ontinguts de H en la zeolita). En aquesta tesi es realitza un estudi d'acidesa utilitzant la tècnica de INS, que ens permet mesurar principalment els modes de vibració dels àtoms d'H i també ens permet mesurar totes les bandes de vibració (inclòs el bending). Aquest estudi mostra que la combinació de l'alta qualitat de les mostres i la sensibilitat del instrument utilitzat permet detectar amb alta precisió els llocs àcids i obtenir la informació sobre la seva posició que en el seu lloc ens perimte obtenir la posició d'Al. Per tal de interpretar els espectres INS s'ha realitzat un estudi computacional (càlculs ab-initio) amb la comparació dels espectres calculats amb els resultats experimentals. / [EN] This thesis is dedicated to the study of acidity of LTA zeolites with Si/Al ratios 5 and 40 by using the technique of inelastic neutron scattering (INS) in combination with other techniques such as computational modeling and nuclear magnetic resonance (NMR) spectroscopy. Zeolites are crystalline and microporous aluminosilicates which form one of the most important groups of functional materials. Zeolites are widely used as solid acid catalysts for the wide range of important processes regarding organic molecules. The most important are cracking, isomerization reaction and synthesis of hydrocarbons. The catalytic properties of a zeolite depend strongly on its acidity, and this in turns depends on: the total number of acid sites, their individual strength, and their individual location. These three factors are strongly correlated. Geometric parameters that are defined by the location of the acid site (i.e., bond angles and lengths around the acid site) make a remarkable contribution to the acid strength. There are several studies, found in literatures, typically done by Infrared (IR) or Nuclear magnetic resonance (NMR) technique and dedicated to acidity of zeolites. However, the hydrogen bending modes (200-1200 cm-1), which are found to be more sensitive to local environment, cannot be observed by this technique, since these bands overlap with strong bands of the vibrations of zeolitic framework (300-1800 cm-1). INS technique used in this study allows to detect the bands of vibrations of hydrogen atoms in zeolites (including bending modes). Moreover, this study shows that the combination of an extremely high quality of the samples and the sensitivity of the instrument allows to detect with high precision the acid sites of both high-silica and low-silica zeolites and obtain information about their position. This in its turn gives us the possibility to obtain the aluminium location in zeolites. In order to fully understand the INS spectra we performed ab-initio calculations that allow to interpret the experimental bands and choose a structural model that reproduces the probable location of acid sites in the sample. / Lemishko, T. (2019). Inelastic neutron scattering study of Brønsted acidity in LTA zeolite [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/119966 / TESIS

Within in this work, I present the successful synthesis of Ba6Yb2Ti4O17 in a powder form. To the best of my knowledge, this is the first instance in which this has been achieved. I also show that single crystal synthesis experiments using the floating zone synthesis (FZS) technique have yet to produce single crystals. The magnetic characterization of Ba6Yb2Ti4O17 powder is also presented. DC susceptibility measurements reveal no spin freezing or ordering down to 0.5K and inverse susceptibility shows antiferromagnetic coupling between spins. A cold neutron scattering experiment was performed at low temperatures with and without an applied magnetic field on Ba6Yb2Ti4O17 powder. This experiment shows the possibility of spinon-like excitations with a band width of approximately 0.3meV, but this is inconclusive due to the signal to noise ratio. The powder averaged g-tensor for Ba6Yb2Ti4O17 was also experimentally estimated to be approximately 2.4. Further work required to complete this study is also discussed. Synthesis of phase pure ErMgGaO4 has yet to be reported in the literature. Here, I show that small amounts of phase pure polycrystalline ErMgGaO4 can be produced using the FZS technique. Through using FZS, 0.85g of phase pure ErMgGaO4 powder was produced and supplemented using 1.26g of ErMgGaO4 powder containing approximately 1% Er3Ga5O12 magnetic impurity. To estimate the effect of this magnetic impurity on the magnetic signatures exhibited by ErMgGaO4, DC susceptibility measurements were performed between 0.5K and 50K. The magnetic impurity in such small quantity was shown to have negligible effect on the bulk ErMgGaO4 magnetic features. Cold neutron scattering conducted on this material revealed a spinon excitation continuum similar to the one observed in YbMgGaO4.[1] Also present at low temperatures was a diffuse elastic magnetic feature that was used to estimate an order parameter for the material. Analysis of this diffuse feature also revealed a Warren line shape, indicative of two dimensional correlations within the Er3+ triangular planes. Analysis of the spinon excitation continuum revealed a gapless spinon continuum overall. Results presented for both Ba6Yb2Ti4O17 and ErMgGaO4 will be discussed and compared to neutron scattering experiments performed on YbMgGaO4 / Thesis / Master of Science (MSc)

A series of neutron scattering studies of unconventional superconductors is presented. These measurements are split into two parts. The first part considers the purely magnetic scattering in low-doped $La_{2−x}Ba_{x}CuO_{4}$. This study is comprehensive and elucidates much of the doping and temperature dependence of the low energy magnetic scattering in this system. It also clearly demonstrates that two dimensional incommensurate magnetic order in this system forms at the expense of three dimensional commensurate magnetic order. The remainder of the thesis is concerned with characterizing and determining the physics underlying pronounced enhancements of the inelastic scattering found to exist at 20 meV at equivalent two dimensional magnetic zone centers in both $La_{2−x}Ba_{x}CuO_{4}$ and $La_{2−x}Sr_{x}CuO_{4}$. Arguments are presented to interpret these features as a result spin-phonon hybridization in 214 cuprate superconductors. The measurements also explore the temperature and doping dependence of these features, determining that the enhancements are largely insensitive to doping and only present parametric response at temperatures relevant for three dimensional magnetic order in this system. In addition, the first evidence for a superconducting spin gap in $La_{2−x}Ba_{x}CuO_{4}$ is presented. The implications of these findings are discussed. / Thesis / Doctor of Philosophy (PhD) / This thesis discusses a series of measurements using brand new state of the art facilities to re-examine the family of high temperature superconductors with the simplest structure. By taking advantage of these new facilities and applying their capabilities to a, relatively speaking, simpler material family, the goal is to try and learn more about what may be the cause of high temperature superconductivity - a magnetism based phenomenon with profound implications for energy efficiency, energy storage, diagnostic healthcare, quantum computing and much more. While many interesting effects were observed, two of the more important findings were observations that unify the physical descriptions of $La_{2−x}Ba_{x}CuO_{4}$ and $La_{2−x}Sr_{x}CuO_{4}$, as is necessary to have a truly universal understanding of high temperature superconductivity, and a detailed characterization we interpret as hybridization. It is clear from this work that any complete solution of superconductivity must also explain this part magnetic and part crystalline structural hybridization phenomenon.

Dealloying is a corrosion process in which one or more elements are selectively removed from an alloy leading to a 3-dimensional porous structure of the more noble element(s). These porous structures have been known to cause stress corrosion cracking in noble metal alloy systems but more recent interest in using the corrosion process to produce porous metals has developed. Applications for these structures range from high surface area electrodes for biomedical sensors to use as skeletal structures for fundamental studies (e.g. low temperature heat exchangers or sensitivity of surface diffusivity to chemical environment). In this work we will review our current understanding of alloy corrosion including our most recent results demonstrating a more accurate method for calculating alloy critical potential based on potential hold experiments. The critical potentials calculated through the potential hold method were â 0.030VMSE, 0.110VMSE, and 0.175VMSE for Cu80Pt20, Cu75Pt25 Cu71Pt29 respectively. We will present the use of porous metals for making surface diffusivity measurements in the Pt systems as a function of chemical environment. A review of the use of small angle neutron scattering to make accurate measurements of pore size is presented and the sensitivity of pore size to electrolyte, electrolyte composition, applied potential and temperature will be explained. The production of porous Pt with pore sizes ranging from 2-200nm is demonstrated. / Ph. D.

No description available.

Doctor of Philosophy / Department of Mechanical and Nuclear Engineering / William L. Dunn / Dense plasma focus (DPF) devices are pulsed power devices capable of producing short-lived, hot and dense plasmas (~10[superscript]19 cm[superscript]-3) through a fast compression of plasma sheath. A DPF device provides intense bursts of electrons and ion beams, X-rays, and 2.5 MeV neutrons when operated with deuterium through the fusion reaction [superscript]2H(d,n)[superscript]3He. The Kansas State University DPF machine was designed and constructed in early 2010. The device was characterized to determine its performance as a neutron source. The device was shown to produce 5.0x10[superscript]7 neutrons/pulse using a tungsten-copper anode. Such machines have the advantages of being non-radioactive, movable, and producing short pulses (typically tens of nanoseconds), which allows rapid interrogation. The signature-based radiation-scanning (SBRS) method has been used to distinguish targets that contain explosives or explosive surrogates from targets that contain materials called “inert,” meaning they are not explosive-like. Different targets were placed in front of the DPF source at a distance of 45 cm. Four BC-418 plastic scintillators were used to measure the direct neutron yield and the neutrons scattered from various targets; the neutron source and the detectors were shielded with layers of lead, stainless steel, and borated polyethylene to shield against the X-rays and neutrons. One of the plastic scintillators was set at 70[supercript]o and two were set at 110[superscript]o from the line of the neutron beam; a bare [superscript]3He tube was used for detecting scattered thermal neutrons. Twelve metal cans of one-gallon each containing four explosive surrogates and eight inert materials were used as targets. Nine materials in five-gallon cans including three explosive surrogates were also used. The SBRS method indicated a capability to distinguish the explosive surrogates in both experiments, although the five gallon targets gave more accurate results. The MCNP code was used to validate the experimental work and to simulate real explosives. The simulations indicated the possibility to use the time of flight (TOF) technique in future experimental work, and were able to distinguish all the real explosives from the inert materials.

Second-order phase transitions involve critical fluctuations just below and above the transition temperature. Macroscopically, they manifest in the power-law behaviour of many physical properties such as the susceptibility and the specific heat. The power-laws are predicted to be universal, i.e. the same exponents are expected for a certain class of transitions irrespective of the microscopic details of the system. The underlying commonality of such transitions is the divergence of the correlation length ξ and the correlation time ξ_τ of the critical fluctuations at the transition temperature. Both ξ and ξ_τ can be directly observed by neutron scattering experiments, making them an ideal tool for the study of critical phenomena. At classical phase transitions, the critical fluctuations will be thermal in nature. However, if a second-order transition occurs at T = 0, thermal fluctuations are frozen, and the transition is driven by quantum fluctuations instead. This is called a quantum critical point. The quantum nature of the fluctuations influences observable properties, also at finite temperatures, and causes unusual behaviour in the vicinity of the quantum critical point or the existence of exotic phases, e.g. unconventional superconductivity. Heavy-fermion compounds are a class of materials that is well suited for the study of quantum criticality. They frequently show second-order transitions into a magnetically ordered state at very low temperatures, which can easily be tuned to T = 0 by the application of pressure, magnetic fields or element substitution. In this thesis, fluctuations near a quantum critical point are investigated for three heavy-fermion systems. CeCu2Si2 shows unconventional superconductivity close to an antiferromagnetic quantum critical point. Results from single-crystal neutron spectroscopy and thermodynamic measurements are discussed and some details are also given about the synthesis of large single crystals. The focus of the study is the comparison of the inelastic response of magnetic and superconducting samples, which are found to be very similar for ΔE > 0.2 meV. CePdAl has an antiferromagnetic state with partial magnetic frustration. The ordering temperature can be suppressed by Ni substitution towards a quantum critical point. Single-crystal neutron diffraction experiments of three members of the substitution series were analysed. They revealed several unusual effects of the frustrated state in the pure sample, and show that magnetic order and frustration persist in the substituted samples. YbNi4P2 is a rare example of a compound with ferromagnetic quantum criticality, which has only been studied in the last few years. The aim of the powder neutron spectroscopy experiments presented here was to obtain an overview of the relevant energy scales, i.e. the crystal electric field, local magnetic fluctuations and ferromagnetic fluctuations. Simulations using the program McPhase were performed for a thorough understanding of the crystal electric field.

In this thesis results of x-ray scattering and neutron scattering experiments on several strongly correlated transition metal oxides are presented. The prototypical charge ordered cuprate La1.48Nd0.4Sr0.12CuO4 was investigated using polarised neutron scattering. The results show that several proposed schemes for the magnetic order in this class of materials may be ruled out, however the data are consistent with one-dimensional stripe-like magnetic order. X-ray diffraction was used to show that the charge order is insensitive to an applied magnetic field, but might be affected by the existence of superconductivity. The magnetic excitations were also studied, and at low energies a gap in the magnetic fluctuations was observed and there is tentative evidence that this is related to magnetic anisotropy. The spin state transition in LaCoO3 was investigated using neutron inelastic scattering, and excitations reminiscent of those observed in ferromagnets above their critical temperatures were observed. The debate surrounding the nature of the excited spin state, S=1 or S=2, could not be resolved, however. The nature of the spin excitations in La0.82Sr0.18CoO3 was investigated using polarised neutrons and it was found that at low energies the excitations take the form of spin-waves. At higher energies this mode becomes heavily damped, and several possible damping mechanisms for this are discussed. Finally, the multiferroic material DyMn2O5 was studied using x-ray resonant scattering. A complex, temperature dependent, magnetic structure was found using a Dy resonance, which reflects an underlying order of the Mn ions. The measurements were in agreement with a theory of multiferroics based on acentric spin-density waves.

Mixed anion systems, such as oxynitrides and oxyfluorides, are an emerging class of interesting materials. The lower stability of mixed anion systems in comparison to oxide materials has had the consequence that this area of materials research is relatively less well explored. However, the development of new synthesis techniques has resulted in the preparation of many new mixed anion systems and so a detailed understanding of their structure and how this relates to their electronic and magnetic properties is necessary. Within this Thesis, several oxide, oxynitride and oxyfluoride systems are investigated with a particular focus on the magnetic behaviour of materials based on geometrically frustrated pyrochlore and kagome lattices. The Lu2Mo2O7 pyrochlore contains a geometrically frustrated network of vertex sharing Mo4+ (d2 S = 1) tetrahedra. Here, the solid state synthesis of Lu2Mo2O7−x is reported along with a discussion of the coexistence of two cubic pyrochlore phases that has been discovered in samples synthesised at 1600 ◦C. Powder neutron diffraction and thermogravimetric analysis have revealed that this two-phase behaviour originates from a miscibility gap between stoichiometric Lu2Mo2O7 and oxygen deficient Lu2Mo2O6.6. Magnetic susceptibility and muon spin relaxation measurements support the formation of a geometrically frustrated spin glass ground state in Lu2Mo2O7 with a spin freezing temperature Tf ∼ 16 K. Low temperature neutron diffraction has confirmed the absence of long range magnetic order and magnetic diffuse neutron scattering data have indicated the presence of competing nearest and next nearest neighbour antiferromagnetic exchange interactions in the spin glass state. The magnetic heat capacity of Lu2Mo2O7 follows a T2-dependence at the low temperatures, indicating that Lu2Mo2O7 is another rare example of an unconventional, topological spin glass, which is stable in the absence of significant chemical disorder. The magnetic properties of the oxygen deficient pyrochlore phase Lu2Mo2O6.6 are qualitatively similar to those of Lu2Mo2O7, but an increase in the spin freezing temperature Tf ∼ 20 K suggests that oxygen-vacancy disorder in Lu2Mo2O6.6 favours the onset of a glassy state at higher temperatures and enhances the degree of frustration. Oxynitride pyrochlores with the ideal composition R2Mo2O5N2 (R = rare earth) contain Mo5+ d1 S = 1 2 cations on the frustrated pyrochlore lattice and are thus ideal candidates to support exotic magnetic ground states. Here, the synthesis of oxynitride pyrochlores of the Lu2Mo2O7 system by thermal ammonolysis is discussed alongside powder neutron diffraction and susceptibility data that show no evidence for long range magnetic order and an absence of spin freezing down to at least 2 K despite the persistence of strong antiferromagnetic exchange (θ = −120 K). A comparison of the magnetic diffuse neutron scattering between the spin glass state of Lu2Mo2O7 and the oxynitride is given, which suggests that the majority of the magnetic scattering in the oxynitride system is inelastic. In addition, low temperature magnetic heat capacity shows an absence of magnetic phase transitions and a continuous density of states through a T-linear dependence down to 500 mK. [NH4]2[C7H14N][V7O6F18], diammonium quinuclidinium vanadium(III,IV) oxyfluoride or DQVOF, is a kagome bilayer system with a geometrically frustrated two-dimensional kagome network of V4+ d1 S = 1 2 cations and V3+ d2 S = 1 cations between the kagome layers. Here, low temperature magnetisation and heat capacity data are presented, which demonstrate that the interplane V3+ d2 cations are well decoupled from the kagome layers at low temperatures such that DQVOF is a good experimental realisation of a S = 1 2 kagome antiferromagnet. Despite significant antiferromagnetic exchange (θ = −60 K) within the kagome planes, muon spin relaxation data have confirmed the absence of spin freezing and the persistence of internal field fluctuations that are intrinsic to the kagome layers down to temperatures of 40 mK. The low temperature heat capacity of the V4+ kagome network follows T-linear behaviour down to the 300 mK, highlighting the absence of a spin gap in the low energy excitation spectrum of DQVOF. The low temperature magnetic study of DQVOF presented here thus strongly supports the formation of a gapless quantum spin liquid phase. In the final results chapter, a discussion of the anion ordering principles in oxynitride systems is given. A high temperature, high resolution neutron diffraction study of the oxynitride perovskite SrTaO2N has revealed that the partial anion order that results in segregated Ta-N zig-zag chains is stable up to 1100 ◦C. Furthermore, these anion ordering principles are extended to the d1 perovskite oxynitrides RVO2−xN1+x (R = La, Nd, Pr) in a variable temperature neutron diffraction study, which confirms that the anion chain ordering discovered in d0 SrTaO2N is robust to electron doping. The R = La analogue also provides an interesting example of a rhombohedral oxynitride perovskite phase which coexists with an orthorhombic phase over the 4−300 K temperature range of the neutron diffraction study.