In the first section, the electrical conductivity of rhodium phenylene-diisocyanide polymer is reported to be 3.4E-11 S/cm. However, the conductivity also exhibits an inverse exponential decay in air with t = 8 days. This change is attributed to the oxidation of the isocyanide functional group to an isocyanate, leading to degradation in the long-range metal-metal bonding, the dominant conductivity mechanism. Using a more stable carboxylate ligand, the Cu terephthalate (TPA) system is studied and compared against the Mg, Co, Ni, and Zn terephthalates. A synthesis in N,N-dimethylformamide (DMF) is developed and large quantities of the Cu(TPA)DMF can be synthesized in air. The crystal structure of the Cu(TPA) DMF is shown to be in the C2/m spacegroup. Upon desolvation, the Cu(TPA) is shown to have a large surface area of 625 m2/g. The magnetic susceptibility of the Cu(TPA) indicates anti-ferromagnetic coupling between adjacent Cu centers in the same dimer. The thermal stability of the Zn, Ni, Co, and Mg terephthalates is shown to increase with decreasing symmetric carboxylate stretch in the IR. The magnetic susceptibilities of the Co and Ni terephthalates have paramagnetic behavior, with a Weiss temperature of T = -12.9 K and T = 8.8 for Co(TPA) DMF and Ni(TPA)DMF respectively. A heterometallic Zn-Cu terephthalate is synthesized with Cu concentrations ranging from 0 to 100%. Upon the addition of Cu, Zn-rich frameworks increase in surface area, change in thermal stability, and increase their solvent retention from 16% to 25%. Zn is shown to couple with Cu in the same dimer at a high rate, changing the behavior of the dimer from anti-ferromagnetic to paramagnetic. The Weiss temperature suggests weak ferromagnetic interaction.

Thesis (Ph. D.)--Auburn University, 2007. / Abstract. Vita. Includes bibliographical references.

Thesis (Ph.D.)--University of Nebraska-Lincoln, 2009. / Title from title screen (site viewed June 26, 2009). PDF text: xxvii, 209 p. : ill. (some col.) ; 17 Mb. UMI publication number: AAT 3350440. Includes bibliographical references. Also available in microfilm and microfiche formats.

This thesis focuses on the theoretical investigation of the electronic structure and magnetic interactions present in 3d and 4d/5d transition metal compounds. We use many-body quantum chemistry methods that provide a theoretical frame for the rigorous construction and systematic improvement of correlated N-electron wave-functions. In Chapter 3 we compute d-d transitions fully ab initio and assign excitation peaks of experimental spectra measured in spin-Peierls TiPO4 compound. In this material we find that the d1 ground state is composed of an admixture of dz2 and dxz orbital character, which is related to the large positive ionic charge at P sites in the xz plane (defining the shortest Ti-P links) and of Ti nearest-neighbors along the z axis. In addition, the magnitude of the nearest-neighbors Heisenberg magnetic coupling calculated by quantum chemistry methods compares well with resonant inelastic X-ray scattering (RIXS) experimental data. We further demonstrate that the intersite exchange is very sensitive to the Ti-Ti interatomic distance, which is relevant in the context of spin-Peierls physics in TiPO4. In Chapter 4 we have studied the magnetic anisotropy of Fe ions within the Li3N lattice. The calculated magnetic anisotropy splitting of 26.3 meV for Fe2+ d6 ions in D6h symmetry compares favorably to values measured or computed by similar theoretical methods for Fe1+ d7 species with linear coordination. This substantial spin-reversal energy barrier of the Fe2+ ion is associated with a a^1_{1g}e^3_{2g}e^2_{1g} ground-state electron configuration. Our study therefore puts into the spotlight the linearly coordinated Fe2+ d6 ion as candidate for viable single molecule magnet behavior. In Chapter 5 we address the effect of electron-lattice interactions on the magnetic properties of 4d and 5d TM ions with a formally degenerate t^1_{2g} electron configuration in the double-perovskite materials Ba2YMoO6, Ba2LiOsO6 and Ba2NaOsO6. Our analysis indicates that the sizable magnetic moments and g-factors found experimentally are due to both strong TM d -- ligand p hybridization and dynamic Jahn-Teller effects. Our results also point out that cation charge imbalance in the double-perovskite structure allows a fine tuning of the gap between the t2g and eg levels. The mechanism has not been explored so far experimentally but seems to hold much potential in the context of orbital engineering in transition metal compounds. In Chapter 6 we report a study of magnetic exchange interactions in the S=3/2 orthorhombic perovskite NaOsO3. We mapped the ab initio quantum chemistry results onto model Hamiltonians including both isotropic Heisenberg interactions and anisotropic Dzyaloshinskii-Moriya exchange. We found antiferromagnetic nearest-neighbors Heisenberg exchange interactions of J_ac = 24.4 meV and J_b = 20.9 meV, twice larger than the J extracted from the magnon excitation spectra. The quantum chemistry results motivate further experimental measurements or theoretical analysis to clarify the magnitude of the nearest-neighbors Heisenberg couplings. In Chapter 7 we provide valuable insights on the effective magnetic interactions in 5d and 4d oxides with face-sharing oxygen octahedra, BaIrO3 and BaRhO3, for different bond-angles and bond-lengths. The large antiferromagnetic Heisenberg interactions computed here emphasize the subtle interplay among strong spin-orbit interactions, direct intersite orbital overlap and orbital bonding, and couplings to the lattice degrees of freedom in face-sharing compounds. In Chapter 8 we apply a computational scheme for computing intensities as measured in X-ray absorption and RIXS experiments. We take into account the readjustment of the charge distribution in the vicinity of an excited electron for the modeling of RIXS. For L3-edge spectra of Cu2+ 3d9 ions in KCuF3, we discuss the way to consider orbital ordering effects (alternately occupied d_x2-z2 and d_y2-z2 orbitals). For L3-edge spectra of Ni2+ 3d8 ions in La2NiO4, the computed spectra reproduce trends found experimentally for the incoming-photon incident-angle and polarization dependence.

[Truncated abstract] This thesis describes various synthetic approaches toward the synthesis of highly conjugated complexes incorporating multiple transition metal centres. Particular attention is given to the synthesis of mononuclear complexes that allow for the facile assembly of discrete oligo- and poly-nuclear complexes in a controlled, stepwise fashion. Conjugated multi-metallic materials are of interest on account of their unique photophysical and electronic properties, with a particular emphasis on elucidating the nature of intramolecular communication between multiple metal centres. Chapter 1 provides a survey of these topics and current research efforts in the field. Chapter 2 describes the synthesis of Group-VIII allenylidene complexes incorporating a terminal bipyridyl moiety that provides a site for further coordination. The new compound 9-hydroxy-9-ethynyl-4,5-diazafluorene was synthesised, and reaction of this proligand with a coordinatively unsaturated metal fragment yields the allenylidene complexes [MCl(PnP)2=C=C=(4,5-diazafluoren-9-yl)]PF6 (M = Ru, PnP = dppm, dppe, dmpe; M = Os, PnP = dppm) and [CpRu(dppe)=C=C=(4,5-diazafluoren- 9-yl)]PF6. The dmpe-ligated complex is particularly susceptible to decomposition, though it was possible to obtain partial spectroscopic characterisation in addition to a single-crystal X-ray structural determination. The remaining allenylidene complexes are stable compounds readily characterised by standard spectroscopic and electrochemical means, with the bis(diphosphine) complexes characterised by single crystal X-ray structural determinations. ... Reactions of the proligand with [RuCl(PnP)2]+ (PnP = dppm, dppe) led to the isolation of a product spectroscopically consistent with the formation of the target cationic allenylidene complexes, though the complexes were not readily purified and the identity of the accompanying anion was not elucidated. The new compound 4-hydroxy-4- ethynyl-cyclopentadithiophene was also prepared, though the compound was found to be highly unstable and susceptible to rapid decomposition. The derived allenylidene complexes [RuCl(PnP)2=C=C=(4-cyclopentadithiophene)]PF6 (PnP = dppm, dppe) were isolated in a pure form and the complexes stable toward spontaneous decomposition. The thienyl-derived allenylidene complexes were characterised by spectroscopic and electrochemical techniques, with a single-crystal X-ray structural determination undertaken for [RuCl(dppm)2=C=C=(4-cyclopentaditiophene)]PF6. Electrochemical properties are significantly different between the complexes, and also show significant variation between electrodes and solvents. The terminal thienyl substituents are electroactive and show one or two oxidation processes consistent with oligomerisation of the thienyl moiety in dichloromethane solvent, and in acetonitrile solvent cyclic voltammograms are consistent with the deposition of an electroactive film on the electrode surface. The electro-polymerisation of the thienylallenylidene complexes offers a promising new route toward multi-metallic allenylidene complexes.

Freie Leitungselektronen sind bisher in oxidischen Perowskiten mit Elektronenspinresonanz (ESR) selten beobachtet worden. Meist sind solche Elektronen, insbesondere bei tiefen Temperaturen, an Gitterstörungen gebunden. In akzeptorarmem BaTiO3 ist es gelungen, mit ESR Elektronen zu identifizieren, die auch bei tiefen Temperaturen nicht an Defekte gekoppelt sind. Sie entstehen beim Einbau von Nb5+Ti in das Gitter und lokalisieren sich auf Titan-Platz als Ti3+ Jahn-Teller-Polaronen selbst. Die beiden Gebiete Jahn-Teller-Effekt und Polaronen, die sich bisher weitgehend getrennt voneinander entwickelt haben, werden hier in einer experimentellen Untersuchung vereinigt. Beim Jahn-Teller-Effekt wird eine elektronische Bahn-Entartung durch eine spontane Erniedrigung der Punktsymmetrie aufgehoben. Die Polaronenbildung erfolgt unter Bruch der Translationssymmetrie. Dabei wird die Energie des Systems erniedrigt, indem ein Zustand unter die quasi-entarteten Bandzustande abgesenkt wird. Beim System Ti3+ treten beide Effekte gleichzeitig auf: Die nach Lokalisierung eines freien Elektrons an Titan aus dem Leitungsband auftretende Bahn-Entartung des resultierenden T2-Grundzustandes wird durch einen Jahn-Teller-Effekt aufgehoben. Es bildet sich ein sogenanntes Jahn-Teller-Polaron. Als Vorbereitung der experimentellen Untersuchungen an Jahn-Teller- Polaronen dienen ESR-Messungen an verschiedenen reinen Jahn-Teller-Systemen in BaTiO3 (Mo5+, Ni+, Rh2+). Im Gegensatz zu einem Polaron ist das durch eine Störstelle im Kristall hervorgerufene Potential schon vorhanden, bevor dort ein Ladungsträger eingefangen wird. Beim Polaron bewirkt die Anwesenheit des Ladungsträgers selbst die Bildung einer Potentialmulde. Man spricht daher von Selbsteingrabung. Die untersuchten Jahn-Teller-Zentren reagieren auf äußeren uniaxialen Druck mit einer Reorientierung: Die spontan erfolgten Jahn-Teller-Verzerrungen werden entlang der Druckachse ausgerichtet. Auch beim Jahn-Teller-Polaron ist diese Reorientierung zu beobachten. Die Jahn-Teller-Kopplung ist jedoch schwächer als die des isoelektronischen Zentrums Mo5+ (4d1 ). Aufgrund der Äquivalenz aller Titan-Plätze im Kristall ist das ungepaarte Elektron an Ti3+ stärker auf seine Nachbarn delokalisiert als im Fall des Mo5+. Es wird gezeigt, daß eine delokalisierte Wellenfunktion schwächer an das Gitter ankoppelt als eine lokalisierte und daher eine schwächere Jahn-Teller-Kopplung aufweist. Es treten kleine und intermediäre Jahn-Teller-Polaronen gleichzeitig auf. BaTiO3 besitzt die Tendenz, breite Polaronenbänder zu bilden. Polaronen in diesem Material zeigen daher die Tendenz zur Delokalisierung, d. h. sie sind über mehr als einen Gitterplatz ausgedehnt und werden als intermediär bezeichnet. Eine Lokalisierung auf einen Gitterplatz, also ein kleines Polaron, bildet sich nur bei Anwesenheit von lokalen Potentialfluktuationen aus (Anderson-Lokalisierung). Unter uniaxialem Druck erfolgt eine Verringerung dieser Fluktuationen durch die Reorientierung, und ein Übergang vom kleinen zum intermediären Polaron wird beobachtet.

A new approach to the discovery of high absorbing semiconductors for solar cells was taken by working under a set of design principles and taking a systemic methodology. Three transition metal chalcogenides at varying states of development were evaluated within this framework. Iron pyrite (FeS���) is well known to demonstrate excellent absorption, but the coexistence with metallic iron sulfides was found to disrupt its semiconducting properties. Manganese diselenide (MnSe���), a material heavily researched for its magnetic properties, is proposed as a high absorbing alternative to iron pyrite that lacks destructive impurity phases. For the first time, a MnSe��� thin film was synthesized and the optical properties were characterized. Finally, CuTaS���, a known but never characterized material, is also proposed as a high absorbing semiconductor based on the design principles and experimental results. / Graduation date: 2013

Metallohydrolases with dinuclear-zinc active sites perform many important biological hydrolytic reactions on a variety of substrates. In this regard, metallo-β-lactamases (mβ1, class B) represent a unique subset of zine hydrolases that hydrolyze the β-lactam ring in several antibiotics. The antibiotic resistance that results from this hydrolysis is becoming an increased threat for the clinical community. These metalloenzymes can hydrolyze a wide range of β-lactam substrates, such as cephamycins and imipenem that are generally resistant t the serine-containing β-lactamases. Therefore, the clinical application of the entire range of antibiotics is severely compromised in bacteria that produce mβls. Due to the lack of information on the mechanism of mβls, to-date, no clinically known inhibitors is there for mβls. In this present study, we synthesized several mono and dizinc complexes as models for the mβls and investigated the differences in their hydrolytic properties. This study supports the assumption that the second zinc in the dinuclear enzymes does not directly involve in the catalysis, but may orient the substrates for hydrolysis and the basic amino acid residues such as Asp and His may activate the zinc-bound water molecules, fulfilling the role of the second zinc in the mononuclear enzymes. The effect of various side chains on the hydrolysis of some commonly used cephalosporin antibiotics by mβl from B.cereus is described. It is shown that the cephalosporins having heterocyclic thiol side chains are more resistance to mβl-mediated hydrolysis than the antibiotics that do not have such side chains. This is partly due to the inhibition of enzyme activity by the thiol moieties eliminated during the hydrolysis. It is also observed that the heterocyclic side chains in pure form inhibit the lactamase activity of mβl as well as its synthetic mimics. The mode of binding of these heterocyclic side chains to the zinc has been analyzed from the crystal structure of the tetranuclear zinc complexes. The theoretical studies suggest that the eliminated heterocyclic thiols undergo a rapid tautomerism to produce the corresponding thiones. These thiones are found to irreversibly inhibit the LPO-catalyzed iodination reaction. The reaction of various thiones with I2 leads to the formation of thione-iodine complexes similar to that of the most commonly used antithyroid drug methimazole(MMI). These observations suggest that some of the latest generation of antibiotics may show negative effects on thyroid gland upon hydrolysis. Synthetic organophosphorus compounds have been used extensively as pesticides and petroleum additives. These compounds are very toxic to mammals and their widespread use in agriculture leads to serious environmental problems. Therfore, degradation of organophosphorus trimesters and remediation of associated contaminated sites are of worldwide concern. In this regards, the bacterial phsophotriesterase (PTE) enzyme plays an important role in degrading a wide range of organophosphorus esters and the active side of PTE has been shown to be very similar to that of mβl. This identification prompted us to check the hydrolysis of phosphotriesters by the mβl and its mimics. It has been observed that the dinuclear zine(II) complexes that do not allow a strong binding of phosphodiestes would be a better PTE mimics.

Early transition metal oxide clusters have been a focus of study for several years. The production of vanadium oxide cluster anions in a pulsed helium flow reactor provides a relatively precise way of introducing defect sites and controlling the oxidation state of the vanadium atoms. The composition of the clusters can be changed from the V2O5 stoichiometry, where the vanadium atom is in a +5 oxidation state, to more reduced stoichiometries yielding a mixture of oxidation states containing atoms in the +2 oxidation state. The subsequent addition of reactant gases such as H2O and SO2 yields very intense adsorption reactions as well as a demonstration of the robustness of particular defect free clusters. For example, the cluster has been identified as a defect free cluster where all vanadium atoms are in the +5 oxidation state and all oxygen atoms are predicted to be in the 2- state. The cluster has been shown to not adsorb SO2- while clusters in a reduced oxidation state, such as and readily adsorb one or more SO2 molecules. The adsorption process has been shown to be size dependent, with the smallest monovanadium oxide anions being the most reactive.

Transition Metal (TM) silicides are potential materials for different high temperature applications due to their high melting points and chemical stability at elevated temperatures. In the present work, the possible use of Gr (VIB) disilicides: MoSi2 and CrSi2 for high temperature structural application and thermopower generation respectively are investigated. Literature reports on MoSi2 indicate this material to have excellent mechanical and thermal behaviors at temperatures greater than 1273 K. The major problems limiting its use are the low temperature brittleness and oxidation at intermediate temperatures and form the scope of this work. Also, CrSi2 is reported to be a narrow band gap semiconductor. Its feasibility as a thermoelectric material for power generation is investigated. The first chapter briefly summarizes the literature on MoSi2 and CrSi2 relevant to structural and thermoelectric applications respectively. Based on the available literature, the scope of further work is discussed. The second chapter describes the methods of synthesis employed for these materials and the characterization techniques adopted. Some experimental setups like thermal conductivity and hot pressing unit that were fabricated as part of the work are described in detail. The thermal conductivity apparatus is based on the principle of parallel heat flow technique. It allows accurate measurement of K and S in the temperature range 300-700 K. The induction based hot-pressing unit allows compaction of polycrystalline powders to near theoretical densities thereby allowing quantitative evaluation of the physical properties. In the third chapter, an understanding of ductility/brittleness based of electron charge density distribution is attempted. The electron charge density in Tin and simple metals (BCC and FCC) is analyzed using Bader’s Atoms in Molecule (AIM) theory. Also the relevant surface and dislocation energies in these materials are calculated according to the Rice Model. It is found that the electron densities at the critical points correlate in a simple way with the relevant stacking fault and surface energetics. Based on these results, a ductility parameter (DM odel) based on electron charge distribution, to predict the effects of chemical substitutions on ductility/brittleness in materials is proposed. In the fourth chapter, possible elements to impart ductility in MoSi2 are identified based on the DM odel values. Calculations indicate, Nb, Ta, Al, Mg and Ga to be suitable candidates for improving ductility in MoSi2. Also oxidation studies based on present experiments and reported literature data reveal, Al to improve the intermediate temperature (773-873 K) oxidation behavior. Thus to simultaneously improve the low temperature ductility and oxidation resistance, Nb and Al were identified as suitable candidates. In the fifth chapter, the experimental data of Nb and Al co-substituted MoSi2 samples are reported. Oxidation studies carried out by thermogravimetry show improved oxidation resistance in Nb and Al co-substituted samples compared to pure MoSi2 in the temperature range of 773-873 K. Mechanical characterization was carried out for (Mo0.99Nb0.01)(Si0.96Al0.04)2 co-substituted composition. Compression testing at room temperature show plastic deformation at low strain rates (10−3 /sec). Indentation experiments show a reduction in the hardness and stiffness compared to pure MoSi2. There is also an increase in the fracture toughness (K1C ) value with the fracture modes being predominantly transgranular. The sixth chapter describes the structural, thermal and transport properties of CrSi2. Structural refinement was carried out by Rietveld method and the positional, thermal parameters and occupancy were fixed. Thermo-gravimetric analysis shows oxidation resistance in powdered samples upto 1000 K. Thermal expansion (α) studies reveal anisotropy in the α values with an unusual decrease in the average αV values between 500 and 600 K. Measurements of electrical resistivity and seebeck coefficient indicate a degenerate semiconducting behavior. Electronic band structure calculations indicate a narrow indirect band gap (EG) material with EG~0.35 eV. Thermal conductivity (K) measurements show a decrease in K value with increasing temperature. Calculation of the thermoelectric figure of merit (ZT) show a maximum value of 0.18 at 800 K for the temperature range studied. Based on an analysis of the experimental and theoretical results, it is identified that further improvements in ZT of CrSi2 may be possible by reducing the lattice thermal conductivity and optimization of the carrier concentration. In chapter seven, the effect of particle size on ZT of CrSi2 is studied. Nano powders of CrSi2 were prepared by mechanical milling. Contamination is found to be a major problem during milling and the different milling parameters (milling speed, atmosphere, dispersant etc) were optimized to minimize contamination. The milled powders were further hot pressed to achieve high densities in a short duration thereby minimizing the grain growth. It is observed that the lattice thermal conductivity is reduced significantly with decreasing grain size. Measurements of ZT show a maximum value of 0.20 in the milled sample compared to 0.14 in arc melted CrSi2 at 600 K. In chapter eight the effect of chemical substitutions on ZT of CrSi2 is studied. Mn substitutions in Cr site were carried out to study the effect of atomic mass on lattice thermal conductivity (KP ). Al substitutions in Si site were carried out to tune the Fermi level. Results of Mn substitution show a large decrease in KP but also a reduction in the thermoelectric power factor (S2σ). The maximum ZT observed in the Mn substituted samples was 0.12 at 600 K. Al substitution results in an increase in the thermoelectric power factor and a subsequent increase in ZT. The maximum ZT observed was 0.27 at 700 K for 10% substitution of Al in Si site. The work reported in the thesis has been carried out by the candidate as a part of the Ph.D. training programme at Materials Research Centre, Indian Institute of Science, Bangalore, India. He hopes that this work would constitute a worthwhile contribution towards (a) basic understanding of ductility/brittleness in materials and understanding the effects of chemical substitutions, (b) Suitability of chemically substituted MoSi2 to overcome the problems of low temperature brittleness and oxidation. (c) Development of CrSi2 as a high temperature thermoelectric material.