A comprehensive investigation was reported into the synthesis, characterization and photoluminescence properties of calcium metaniobates and associated alkali earth alloy systems. Previous studies have shown that calcium metaniobate exhibits a strong self-activated blue luminescence at room temperature in stark contrast to the pyroniobates which are known to exhibit a temperature dependent luminescence that quenches above 100 K. The mechanism of this behavior has been studied by measuring the spectral characteristics of the photoluminescence and photoluminescence excitation spectra on the crystalline and morphological properties of the powders as determined from x-ray diffraction and scanning electron microscopy. By correlating the synthesis parameters with the physical, chemical and optical properties of calcium metaniobate, the optimum conditions for efficient blue-visible emission and chemical stability under vacuum ultraviolate (VUV) radiation has been determined. These materials have the potential to replace Barium Magnesium Aluminate, which is currently used as the blue phosphor in plasma displays.

In order to truly understand the process-property behavior of polymer systems it is essential to identify the three dimensional structure of the materials fabricated. For heterogeneous polymer systems such as nanoparticle filled systems, determination of the three dimensional optical properties are particularly difficult. Such information is essential, however, if the behavior of these systems are to be understood and formalized. The purpose of the present research was to develop methods for measuring the optical characteristics of heterogeneous polymer systems nondestructively, in order to characterize their three dimensional behavior. The thesis contains three parts: Part A: Study of an Oriented Uniformly Distributed System: Stretched Isotactic Polypropylene- nano Carbon Black Films (IPP-CB). Three nondestructive optical methods: optical waveguide coupling, Fourier Transform Infrared (FTIR) spectroscopy and x-ray diffraction, were used to investigate the effect of the carbon black on the phase behavior and orientation of the films. It was found that the carbon black has little effect on the crystal form and crystallinity, but has a significant effect on the three dimensional orientation behavior of the polypropylene in the IPP-CB systems. Part B: Study of a non-Uniformly Distributed System: Compression Molded Poly (Methyl Methacrylate) with Nano Indium Tin Oxide (PMMA-ITO) The PMMA-ITO sample is an un-oriented and non-uniformly mixed system which has a grain structure. A unique Break Point Waveguide Method was developed to deal with this problem. It was found that both the refractive index and the extinction coefficient increased with ITO concentration and the samples were three dimensionally random. Part C: Development of Computational Improvements in System Operations Four methods were developed to improve the accuracy of the waveguide methods. They are the Bootstrap Method, the Two-Line Method, the Big Area Method and the Modified Knee method. In conclusion, the three dimensional optical characteristics of two different kinds of heterogeneous polymer systems, oriented uniformly distributed IPP-CB films and non-uniformly distributed PMMA-ITO composites, are obtained and their structures evaluated. Further, several new methods were developed to improve the accuracy of the current optical waveguide methods.

Synthesis and Characterization of Some Low and Negative Thermal Expansion Materials Tamas Varga 370 pages Directed by Dr. Angus P. Wilkinson The high-pressure behavior of several negative thermal expansion materials was studied by different methods. In-situ high-pressure x-ray and neutron diffraction studies on several compounds of the orthorhombic Sc2W3O12 structure revealed an unusual bulk modulus collapse at the orthorhombic to monoclinic phase transition. In some members of the A2M3O12 family, a second phase transition and/or pressure-induced amorphization were also seen at higher pressure. The mechanism for volume contraction on compression is different from that on heating. A combined in-situ high pressure x-ray diffraction and absorption spectroscopic study has been carried out for the first time. The pressure-induced amorphization in cubic ZrW2O8 and ZrMo2O8 was studied by following the changes in the local coordination environments of the metals. A significant change in the average tungsten coordination was found in ZrW2O8, and a less pronounced change in the molybdenum coordination in ZrMo2O8 on amorphization. A kinetically frustrated phase transition to a high-pressure crystalline phase or a kinetically hindered decomposition, are likely driving forces of the amorphization. A complementary ex-situ study confirmed the greater distortion of the framework tetrahedra in ZrW2O8, and revealed a similar distortion of the octahedra in both compounds. The possibility of stabilizing the low thermal expansion high-temperature structure in AM2O7 compounds to lower temperatures through stuffing of ZrP2O7 was explored. Although the phase transition temperature was suppressed in MIxZr1-xMIIIxP2O7 compositions, the chemical modification employed was not successful in stabilizing the high-temperature structure to around room temperature. An attempt has been made to control the thermal expansion properties in materials of the (MIII0.5MV0.5)P2O7-type through the choice of the metal cations and through manipulating the ordering of the cations by different heat treatment conditions. Although controlled heat treatment resulted in only short-range cation ordering, the choice of the MIII cation had a marked effect on the thermal expansion behavior of the materials. Different grades of fluorinert were examined as pressure-transmitting media for high-pressure diffraction studies. All of the fluorinerts studied became nonhydrostatic at relatively low pressures (~1 GPa).

Optical properties of Tl4In3GaSe8, Tl4InGa3Se8, Tl4In3GaS8, Tl2InGaS4 and Tl4InGa3S8 chain and layered crystals were studied by means of photoluminescence (PL) and transmission-reflection experiments. Several emission bands were observed in the PL spectra within the 475-800 nm wavelength region. The results of the temperature- and excitation intensity-dependent PL measurements in 15-300 K and 0.13&times / 10-3-110.34 W cm-2 ranges, respectively, suggested that the observed bands were originated from the recombination of electrons with the holes by realization of donor-acceptor or free-to-bound type transitions. Transmission-reflection measurements in the wavelength range of 400-1100 nm revealed the values of indirect and direct band gap energies of the crystals studied. By the temperature-dependent transmission measurements in 10-300 K range, the rates of change of the indirect band gap of the samples with temperature were found to be negative. The oscillator and dispersion energies, and zero-frequency refractive indices were determined by the analysis of the refractive index dispersion data using the Wemple&ndash / DiDomenico single-effective-oscillator model. Furthermore, the structural parameters of all crystals were defined by the analysis of X-ray powder diffraction data. The determination of the compositional parameters of the studied crystals was done by energy dispersive spectral analysis experiments.

Trapping centres in as-grown TlInS2 layered single crystals have been studied by using a thermally stimulated current (TSC) technique. TSC measurements have been performed in the temperature range of 10-300 K with various heating rates. Experimental evidence has been found for the presence of five trapping centres with activation energies 12, 14, 400, 570 and 650 meV. Their capture cross-sections and concentrations were also determined. It is concluded that in these centres retrapping is negligible as confirmed by the good agreement between the experimental results and the theoretical predictions of the model that assumes slow retrapping. An exponential distribution of traps was revealed from the analysis of the TSC data obtained at different light excitation temperatures. The transmission and reflection spectra of TlInS2 crystals were measured over the spectral region of 400-1100 nm to determine the absorption coefficient and refractive index. The analysis of the room temperature absorption data revealed the coexistence of the indirect and direct transitions. The absorption edge was observed to shift toward the lower energy values as temperature increases from 10 to 300 K. The oscillator and the dispersion energies, and the zero-frequency refractive index were also reported. Furthermore, the chemical composition of TlInS2 crystals was determined from energy dispersive spectroscopic analysis. The parameters of monoclinic unit cell were found by studying the x-ray powder diffraction.

This study focuses on various aspects of nanocrystals embedded in a dielectric matrix. In the first part of this work, a new approach with the use of Fourier Transform Infrared spectroscopy (FTIR) in the nanocrystal analysis was developed and presented. Si and Ge nanocrystals embedded in SiO2 matrix were mainly studied. This new approach is based on the analysis of structural variations of SiO2 matrix during the formation of semiconductor nanocrystlas. It is shown that the chemical and structural variations of the host matrix are directly related to the precipitation of nanocrystals in it. This correlation provides valuable information about the presences of nanocrystals in the matrix. In the second part of this work, fabrication of SiC films with and without Si nanocrystals inclusions was studied. With this aim, stoichiometric SiC and Si rich SiC thin films were fabricated by using magnetron co-sputtering and Plasma Enhanced Chemical Vapor Deposition (PECVD) techniques. For SiC films, the structural and optical analyses were performed. For Si rich SiC films, the formation conditions of Si nanocrystals were investigated. Post annealing studies were carried out to track the evolution of the SiC matrix and formation of Si nanocrystals at different temperatures. Chemical and structural properties of the SiC host matrix were investigated with FTIR spectroscopy. Optimum conditions for the fabrication of stoichiometric SiC layers were determined. The crystallography of the nanocrystals was investigated by X-Ray Diffraction (XRD). The variation of the atomic concentrations and bond formations were investigated with X-Ray Photoelectron Spectroscopy (XPS). Raman spectroscopy and Transmission Electron Microscopy (TEM) were used to verify the formation of Si nanocrystals. We have shown that both single and multilayer Si nanocrystals can be fabricated in the amorphous SiC matrix for applications such as light emitting diodes and solar cells.

Quasi one-dimensional (1D) nanostructures show great promise for many applications, including in solar cells, nanogenerators and chemical sensors, due to the high surface-to-volume ratio and unique properties of nanostructures. The growth of these nanostructures is commonly catalyzed by metal nanoparticles and generally attributed to the vapor-liquid-solid (VLS) mechanism. The purpose of this research is to better understand the role of the catalyst nanoparticles in the growth of 1D nanostructures, in order to allow improved control of the synthesis process. To this end, nanostructures were grown with a variety of compositions, including Au- and Sn-catalyzed ZnO, Au-catalyzed FexOy and Au-catalyzed Si nanostructures. The morphology of the nanostructures was characterized with electron microscopy, and the crystallographic orientation with X-ray texture analysis. The catalyst particles were further characterized with both in-situ and post-growth X-ray diffraction. The types of bonding in the source material and catalyst play a significant role in the diffusion path of the source material to the growth front and in the catalyst particle state during growth. Dissimilar bonding types in the source material and catalyst prevent bulk diffusion of the source material through the catalyst, thereby preventing eutectic melting of the catalyst. These results bring new insight into the catalyzed growth of 1D nanostructures and assist in the informed choice of appropriate catalyst materials, which may aid the utilization of 1D nanostructures in energy-related and other applications.

BaSIC, which stands for Bioclastic and Shape-preserving Inorganic Conversion, is shape-preserving chemical conversion process of biological (or man-made) silica structures for producing complex 3-D microscale structures. This dissertation reports the BaSIC reaction of halide gases (i.e., TiF4, ZrF4, and ZrCl4) with 3-D silica structures, (i.e., diatom frustules, silicified direct-write assembly scaffolds, and Stöber silica spheres) to produce titania and zirconia replicas of the original 3-D structures. The kinetics of reaction of silica with titanium tetrafluoride gas is analyzed by using a novel HTXRD reaction chamber, nitrogen adsorption, and transmission electron microscope (TEM). The crystal structure and the temperature-induced phase transformation (from the room temperature hexagonal R-3c structure to the higher temperature cubic Pm3m structure) of polycrystalline TiOF2 that was synthesized through metathetic reaction of silica with TiF4(g) is reported. Additionally, potential applications of the converted titania diatom frustules (i.e., as a fast micron-sized ethanol sensor, and as a pesticide hydrolyzing agent) are also demonstrated in this work.

Accurate statistical characterization of nanomaterials is crucial for their use in emerging technologies. This work investigates how different structural characteristics of metal nanoparticles influence the line profiles of the corresponding powder diffraction pattern. The effects of crystallite size, shape, lattice dynamics, and surface strain are all systematically studied in terms of their impact on the line profiles. The studied patterns are simulated from atomistic models of nanoparticles via the Debye function. This approach allows for the existing theories of diffraction to be tested, and extended, in an effort to improve the characterization of small crystallites. It also begins to allow for the incorporation of atomistic simulations into the field of diffraction. Molecular dynamics simulations are shown to be effective in generating realistic structural models and dynamics of an atomic system, and are then used to study the observed features in the powder diffraction pattern. Furthermore, the characterization of a sample of shape controlled Pt nanoparticles is carried out through the use of a developed Debye function analysis routine in an effort to determine the predominant particle shape. The results of this modeling are shown to be in good agreement with complementary characterization methods, like transmission electron microscopy and cyclic voltammetry.

The phase diagram of the Mg-Pd system was redetermined in the range from 50 at. % Mg to 100 at. % Mg. It contains several intermediate phases with some of them being complex metallic alloy phases (CMAs), i.e., characterized by (i) giant unit cells with more than hundred atoms, (ii) inherent disorder, (iii) the presence of a cluster substructure. Phase fields and heterogeneous equilibria of the intermediate phases β- Mg6Pd, γ-Mg57Pd13, δ-Mg56.4Pd13.6, ε-Mg306Pd77, ζ-Mg64Pd17, η-Mg3Pd, θ-Mg5Pd2, ι- Mg2Pd and κ-MgPd were determined. The first five phases are CMAs with Mackay clusters as fundamental structural units. The crystal structure of the most magnesium-rich compound β-Mg6Pd was redetermined. It was found to be more complicated than previously thought due to correlated disorder of only two atom sites in the cubic unit cell. The γ-, ε- and ζ-phases form in a small window of temperature (50 oC) and composition (3 at. %) close to 80 at. % Mg. A new structure type was assigned to Mg3Pd (Cu3P, P63cm). The single phase field of the θ-phase is caused by constitutional vacancies. The new ι-phase crystallizes with NiTi2 structure-type. / Das Phasendiagramm des Systems Mg–Pd wurde im Bereich von 50 bis 100 At.-% Mg neu bestimmt. In diesem Phasendiagramm finden sich mehrere intermediäre Phasen, darunter auch komplexe intermetallische Verbindungen (engl.: complex intermetallic alloys, CMAs). CMAs sind charakterisiert durch (i) große Elementarzellen mit mehr als einhundert Atomen, (ii) intrinsischer Fehlordnung und (iii) dem Vorhandensein einer Cluster- Substruktur. Die Phasenfelder und heterogenen Gleichgewichte der intermediären Phasen β-Mg6Pd, γ- Mg57Pd13, δ-Mg56.4Pd13.6, ε-Mg306Pd77, ζ-Mg64Pd17, η-Mg3Pd, θ-Mg5Pd2, ι-Mg2Pd und κ- MgPd wurden bestimmt. Die ersten fünf der eben genannten Phasen sind CMAs mit Mackay Clustern als fundamentales strukturelles Einheit. Alle übrigen Phasen besitzen einen einfacheren kristallografischen Aufbau. Die Kristallstruktur der Mg-reichsten Verbindung β-Mg6Pd wurde neu bestimmt und ist weitaus sich als komplizierter als bisher angenommen. Die Ausdehnung des Einphasenfeldes von β-Mg6Pd lässt sich jedoch sehr einfach mit korrelierter Fehlordnung von lediglich zwei Atomen in der kubischen Elementarzelle verstehen. Die γ-, ε-, und ζ-Phasen bilden sich in einem schmalen Temperatur- (50 °C) und Zusammensetzungsbereich (3 at. %) nahe 80 at. % Mg. Der Verbindung Mg3Pd (Cu3P, P63cm) wurde ein neuer Strukturtyp zugewiesen. Die Ausdehnung des Einphasenfeldes der θ-Phase lässt sich mit dem Einbau konstitutioneller Leerstellen erklären. Die neue ι-Phase kristallisiert im NiTi2 Strukturtyp.