Structure Processing Properties Relationships in Stoichiometric and Nonstoichiometric Oxides

The interrelation among composition, microstructure, and properties of stoichiometric and nonstoichiometric compounds is a major field of research for both scientific and technological reasons. As such, this book focuses on metal oxides, which present a large diversity of electrical, magnetic, optical, optoelectronic, thermal, electrochemical, and catalytic properties, making them suitable for a wide range of applications. By bringing together scientific contributions with special emphasis on the interrelations between materials chemistry, processing, microstructures, and properties of stoichiometric and nonstoichiometric metal oxides, this book highlights the importance of tightly integrating high-throughput experiments (including both synthesis and characterization) and efficient and robust theory for the design of advanced materials

Summaries of FY 1997 Research in the Chemical Sciences

The objective of this program is to expand, through support of basic research, knowledge of various areas of chemistry, physics and chemical engineering with a goal of contributing to new or improved processes for developing and using domestic energy resources in an efficient and environmentally sound manner. Each team of the Division of Chemical Sciences, Fundamental Interactions and Molecular Processes, is divided into programs that cover the various disciplines. Disciplinary areas where research is supported include atomic, molecular, and optical physics; physical, inorganic, and organic chemistry; chemical energy, chemical physics; photochemistry; radiation chemistry; analytical chemistry; separations science; heavy element chemistry; chemical engineering sciences; and advanced battery research. However, traditional disciplinary boundaries should not be considered barriers, and multi-disciplinary efforts are encouraged. In addition, the program supports several major scientific user facilities. The following summaries describe the programs

Hydrothermal Synthesis of Frustrated Lanthanide Pyrochlores and Transition Metal Double Perovskites and Germanates

Magnetically frustrated materials hold promise of unique behavior allowing for the novel study of quantum phenomena. Such materials are poised to become an integral foundation for technological advancement in the post-Silicon Age. Crystalline materials are given special focus where the rigid lattice allows more detailed study of these quantized effects and frustration behavior. As opposed to polycrystalline powders, large single crystals can be preferentially aligned enabling the study of anisotropic behavior. Two cubic structure types have garnered significant interest due to their 3-D tetrahedral arrangement of symmetry-related metal centers with the potential for magnetic frustration: pyrochlores and perovskites. The supercritical hydrothermal crystal growth technique has been applied to a host of refractory oxides to enhance phase purity and minimize crystalline defects often introduced by conventional flux or melt-based based techniques. The supercritical growth in aqueous alkali at temperatures much lower than those of floating zone melts, while maintaining a sealed environment, avoids the possibility for reactant sublimation. In comparison to melt-based techniques, the modest thermal conditions also aid in minimizing atomic site displacements by insertion into different sites than those expected (“stuffing”). Through supercritical hydrothermal conditions, it was possible to obtain the entire series of lanthanide stannate pyrochlores. Stannous oxide, originally applied for in situ ceria reduction, was found beneficial for all lanthanide oxide reactants regardless of oxidation state allowing facile growth of faceted, mm-scale stannate octahedra in only days. This has allowed for the first single-crystal neutron scattering study of the quantum spin liquid candidate Ce2Sn2O7 with similar experiments to follow across the lanthanide series. The sealed, pressurized supercritical hydrothermal technique was also applied in the growth of stoichiometric orthoscandates and site-ordered cubic barium double perovskites. Both systems had structures occur in crystallographic settings previously only possible through thin-film epitaxial growth or application of high pressures and temperatures. Attempts to expand the palette of lanthanide-containing germanium perovskites yielded novel P212121 LnTM(GeO4)(OH) (TM = 3d transition metal) adelite-type structures possessing chiral [TMO4]∞ a-axis chains. These adelites demonstrated the first known inclusion of lanthanides and germanium in the adelite-descloizite supergroup. The synthetic possibilities of the refractory lanthanide oxides and often-overlooked germanium offer great synthetic possibilities poised to further expand the adelite family while also providing a few magnetically frustrated surprises themselves or through the high-symmetry cubic garnet and spinel products encountered during exploratory adelite synthesis under hydrothermal conditions

Molecular Dynamics Modelling of Barium Silicate and Barium Fluorozirconate Glasses

Advancement in science and technology has profoundly depended on new types of glass innovation. The glasses that were studied in this project are binary barium silicate glasses, binary barium fluorozirconate glasses, ZBLAN glasses and ?Eu?^(3+) doped ZBLAN glass (the ZBLAN glasses are based on binary barium fluorozirconate glass). The high atomic number of barium in the barium silicate glasses provides high mass and high electron density providing its applications for heat and X-ray shielding. The phenomena such as phase separation in the barium silicate glass will affect its properties of durability and electrical conductivity. On the other hand, ZBLAN glasses have a broad infrared optical transmission window due to the weaker bonding/interaction of F^- ions. Due to the presence of lanthanum in the composition ZBLAN glass can be easily doped with rare-earth ions such as ?Eu?^(3+) giving it many optical applications such as optical amplifier and fibre lasers. Hence, it's essential to study the structure of these glasses to understand their properties for applications. This thesis used the classical molecular dynamics modelling technique to study the static atomic structure of glass. Generally, fluoride glasses can be formed by totally replacing oxygen atoms in oxide glasses by fluorine atoms. The oxide silicate glasses are common glasses that follow the Zachriasen rules of glass formation but the fluorozirconate glasses do not and lack fixed structural units. The structure analysis was performed at short-range order (e.g. coordination number, bond length and bond angle), medium-range order (e.g. network connectivity) and long- range order (e.g. phase separation). The related crystals were also simulated in similar conditions to the glasses to compare their atomic structure. Normally at short-range order glass structure is similar to its related crystal but the differences between them starts from the position and number of next nearest neighbours and increases thereafter. Additionally, the new methods such as rotational invariants and grid analysis were used to scrutinise structural units and phase separation respectively. The model of barium silicate glass shows good agreement with experimental diffraction data. The typical bond length and coordination number for Ba were 2.97 Å and approximately 7 respectively. The model did not show any phase separation at low Ba content and hence for further investigation very large models of alkaline earth silicate glasses were studied to see how Ba, Ca and Mg are distributed in the glass. The grid analysis was used to see the distributions which show homogeneity for Ba and Ca and inhomogeneity for Mg cation. The structural units of fluorozirconate glasses were carefully studied as they do not follow the Zachriasen glass model. The coordination number for Zr was mixture of 7 and 8. The rotational invariant analysis shows that the structural units of ZrF_n polyhedra for coordination number 7 and 8 were similar to Augmented Triangular Prism and Biaugmented Triangular Prism respectively. However, rotational invariant values for BaF_n polyhedra tend more towards random. The large complex model of ?Eu?^(3+) doped ZBLAN glass was made as it is studied for optical applications. The initial analysis was to observe whether Zr and Ba has similar structural roles as in binary fluorozirconate glass system which they do. Considering the extra elements in ZBLAN glass, Al behaves like a network former and has octahedra structural units whereas La and Na behave like modifiers. In the glass Eu was uniformly distributed with predominantly coordination number of 8 and does not have well defined structural units

Laboratory-directed research and development: FY 1996 progress report

This report summarizes the FY 1996 goals and accomplishments of Laboratory-Directed Research and Development (LDRD) projects. It gives an overview of the LDRD program, summarizes work done on individual research projects, and provides an index to the projects` principal investigators. Projects are grouped by their LDRD component: Individual Projects, Competency Development, and Program Development. Within each component, they are further divided into nine technical disciplines: (1) materials science, (2) engineering and base technologies, (3) plasmas, fluids, and particle beams, (4) chemistry, (5) mathematics and computational sciences, (6) atomic and molecular physics, (7) geoscience, space science, and astrophysics, (8) nuclear and particle physics, and (9) biosciences

Physical Adsorption of Linear Hydrocarbon Quadrupoles on Graphite and MgO (100): Effects of the Compatibility of Surface and Molecular Symmetries

The process of physical adsorption finds a practical role in wide-ranging fields from catalysis, to lubrication, and even optoelectronics. Furthermore, it provides a mechanism to probe the fundamental understanding of intermolecular forces and how symmetries can play a role in the behavior of a system. Linear quadrupoles preferentially adopt square-T configurations when confined in two dimensions. This would lead the system to adopt a four-fold symmetry in the molecular lattice. Two archetypal surfaces often studied in physisorption research are MgO (100), which has a four-fold symmetry of alternating charges, and the basal plane of graphite, which has a six-fold symmetry to its non-polar, weakly corrugated surface. These differing surface symmetries provide two test cases for comparison. In the case of MgO (100), the molecule-molecule and molecule-surface interaction are synergistic, both driving the film towards the same symmetry; whereas for graphite, the six-fold surface symmetry is incompatible with the preferred four-fold interaction symmetry of the molecules. This presents the opportunity for structurally frustrated systems to arise. Acetylene and allene are both simple, linear, rigid hydrocarbons with large quadrupole moments of similar strength. The most distinct variations between these two molecules are size and axial rotational symmetry. These molecules, just like the surface, provide two simple, but contrasting symmetry effects. The simple point group of truly linear molecules of acetylene allow for them to lie completely flat against a surface. The 90-degree dihedral angle between the hydrogen pairs on opposing sides of allene molecules prevent them from easily being able to lie perfectly flat against the surface, creating another opportunity for broken symmetry in the molecule-surface interactions – this instance in the vertical direction rather than the two dimensional adsorption plane. This investigation aims to study the behavioral properties of acetylene and allene films through thermodynamic, structural, and phase behavior analyses when adsorbed on both graphite and MgO. To this end, a combination of volumetric adsorption isotherms, elastic neutron diffraction, and computational modeling have been employed

Dynamic Solvent Models and Exploring the Parameter Space of Hydrogen Fluoride, Hafnium, and Zirconium

Solvation is the interaction of solute and solvent. Every biological interaction hap- pens in a solvent. Most technical procedures occur within solvents and geological processes are too mediated by their solvents. Understanding these effects of solva- tion is therefore critical for the understanding of biology, technology, and geology. While static properties of solvation shells, like coordination numbers and radial dis- tribution functions, are well understood, the dynamic properties of these open sys- tems are rarely studied. Furthermore, an interesting solvation-based phenomenon is the separation of the geochemical twins Zirconium and Hafnium in fluoride-bearing media. In this work, I present a method for evaluating Markov models of solvation shells and investigate ways of combining the solvent models with solute models. Moreover, I attempt to find a suitable two-site and three-site model of hydrogen flu- oride, specialized for its interaction with metal ions. By simulating aqueous and pure HF for several combinations of q, σF , and ϵF and evaluating density, peaks of radial distribution functions as well the solvation free energy of NaF in HF, I hoped to find a suitable model. A three-site model for HF is parameterized by recreating the electrostatic potential of HF with a classical force field, focusing on the location of maximum potential which takes a conical shape around the tip of the ellipsoid and is not located at the poles. A new method is presented which allows the automatic detection of coordination polyhedra based on reference structures and Steinhardt-order parameters. The Lennard-Jones parameter space for tetravalent cations is explored and analyzed in terms of static solvation shell quantities. Finally, the thermal contraction of the solvation shells of Zr4+ and Hf4+ in 1 M HF was investigated using classical MD simulations. The Markov models of solvation shells indicated that solvent dynamics couple close to the solute. Additive combined models yielded slightly higher timescales compared to their individual components. The opposite is true for the multiplicative models which performed just as well or even worse than their components. The parameterization of HF, for the two-site model, yielded two parameter combi- nations that could reproduce three of the five target quantities, the relevant peaks of the F-H and H-H radial distribution functions, as well as the solvation free energy of NaF in HF. After choosing a topology for the three-site model, the Lennard-Jones parameter scans were unable to yield stable simulations of aqueous HF. The project was therefore discontinued and I settled for a recently published HF model. The parameterization of metal cations yielded a very robust result. Static solvation shell properties exist on continuous regions of similar value in the parameter space. These regions appear as a diagonal lines in the log(ϵ M ) − σM parameter space. This behavior is also observed in the coordination polyhedra found by the novel method. The thermal contractions of solvation shells of tetravalent cations could be observed for the four ionic ligands. The contractions are a result of water molecules increas- ing their distance to the central cation. Their missing repulsive Coulomb interaction allows the ionic ligands to move in closer to the central cation, thus causing the thermal contraction. Furthermore, we observed a two-state system for the solvation shells at high temperatures which consists of octahedral and tetrahedral solvation shells interchanging each other. The herein presented results offer new methods for analyzing solvation shells. Firstly by constructing Markov models of these open systems to study their dynamics. Secondly, by automatically determining the coor- dination polyhedron, which is essentially an analysis of the angular distribution of solvent molecules in the solvation shell. The parameterization attempts of HF depict the difficulty of finding parameter com- binations that match all fitting targets, albeit the searched parameter space was rather small. The parameter space for tetravalent cations shows an extremely robust result which can yield the basis for future parameterization attempts. Finally, the peculiar thermal contractions could be explained through classical MD simulations. This shows the power of this method for studying hard ionic systems