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Formation mechanism and properties of CdS-Ag2S nanorod superlattices

Description: The mechanism of formation of recently fabricated CdS-Ag{sub 2}S nanorod superlattices is considered and their elastic properties are predicted theoretically based on experimental structural data. We consider different possible mechanisms for the spontaneous ordering observed in these 1D nanostructures, such as diffusion-limited growth and ordering due to epitaxial strain. A simplified model suggests that diffusion-limited growth partially contributes to the observed ordering, but cannot account for the full extent of the ordering alone. The elastic properties of bulk Ag{sub 2}S are predicted using a first principles method and are fed into a classical valence force field (VFF) model of the nanostructure. The VFF results show significant repulsion between Ag{sub 2}S segments, strongly suggesting that the interplay between the chemical interface energy and strain due to the lattice mismatch between the two materials drives the spontaneous pattern formation.
Date: August 11, 2008
Creator: Wang, Lin-Wang; Demchenko, Denis O.; Robinson, Richard D.; Sadtler, Bryce; Erdonmez, Can K.; Alivisatos, A. Paul et al.
Partner: UNT Libraries Government Documents Department

Bioinspired Hydrogenase Models: The Mixed-Valence Triiron Complex [Fe₃(CO)₇(μ-edt)₂] and Phosphine Derivatives [Fe₃(CO)₇−ₓ(PPh₃)ₓ(μ-edt)₂] (x = 1, 2) and [Fe₃(CO)₅(κ²‑diphosphine)(μ- edt)₂] as Proton Reduction Catalysts

Description: This article describes the preparation and structural characterization of the mixed-valence triiron complexes.
Date: July 15, 2013
Creator: Rahaman, Ahibur; Ghosh, Shishir; Unwin, David G.; Basak-Modi, Sucharita; Holt, Katherine B.; Kabir, Shariff E. et al.
Partner: UNT College of Arts and Sciences

Dioxygen: What Makes This Triplet Diradical Kinetically Persistent?

Description: This article examines experimental heats of formation and enthalpies obtained from G4 calculations to find that the resonance stabilization of the two unpaired electrons in triplet O₂, relative to the unpaired electrons in two hydroxyl radicals, amounts to 100 kcal/mol.
Date: April 26, 2017
Creator: Borden, Weston T.; Hoffmann, Roald; Stuyver, Thijs & Chen, Bo
Partner: UNT College of Arts and Sciences

The use of bulk states to accelerate the band edge statecalculation of a semiconductor quantum dot

Description: We present a new technique to accelerate the convergence of the folded spectrum method in empirical pseudopotential band edge state calculations for colloidal quantum dots. We use bulk band states of the materials constituent of the quantum dot to construct initial vectors and a preconditioner. We apply these to accelerate the convergence of the folded spectrum method for the interior states at the top of the valence and the bottom of the conduction band. For large CdSe quantum dots, the number of iteration steps until convergence decreases by about a factor of 4 compared to previous calculations.
Date: May 10, 2006
Creator: Vomel, Christof; Tomov, Stanimire Z.; Wang, Lin-Wang; Marques,Osni A. & Dongarra, Jack J.
Partner: UNT Libraries Government Documents Department

Implications of the unusual redox behavior exhibited by the teteropolyanion [EuP{sub 5}W{sub 30}O{sub 110}]{sup 12-}

Description: We report on the novel electrochemical behavior of the heteropolyanion [EuP{sub 5}W{sub 30}O{sub 110}]{sup -12}, with emphasis on the Eu valence in both the oxidized and reduced forms. In aqueous electrolytes, the P-W-O framework is electroactive and can reversibly accept up to 10 electrons before the potential becomes reducing enough for H{sub 2} evolution. Eu L{sub 3}-edge x-ray absorption near edge structure (XANES) results demonstrate that Eu is also electroactive under the same conditions. Implications of this work are discussed re long-standing questions about intermediate- valent materials.
Date: December 1996
Creator: Antonio, M. R. & Soderholm, L.
Partner: UNT Libraries Government Documents Department

Density functional theory study of the structural, electronic, lattice dynamical, and thermodynamic properties of Li4SiO4 and its capability for CO2 capture

Description: The structural, electronic, lattice dynamical, optical, thermodynamic, and CO{sub 2} capture properties of monoclinic and triclinic phases of Li{sub 4}SiO{sub 4} are investigated by combining density functional theory with phonon lattice dynamics calculations. We found that these two phases have some similarities in their bulk and thermodynamic properties. The calculated bulk modulus and the cohesive energies of these two phases are close to each other. Although both of them are insulators, the monoclinic phase of Li{sub 4}SiO{sub 4} has a direct band gap of 5.24 eV while the triclinic Li{sub 4}SiO{sub 4} phase has an indirect band gap of 4.98 eV. In both phases of Li{sub 4}SiO{sub 4}, the s orbital of O mainly contributes to the lower-energy second valence band (VB{sub 2}) and the p orbitals contribute to the fist valence band (VB{sub 1}) and the conduction bands (CBs). The s orbital of Si mainly contributes to the lower portions of the VB1 and VB{sub 2}, and Si p orbitals mainly contribute to the higher portions of the VB{sub 1} and VB{sub 2}. The s and p orbitals of Li contribute to both VBs and to CBs, and Li p orbitals have a higher contribution than the Li s orbital. There is possibly a phonon soft mode existing in triclinic {gamma}-Li{sub 4}SiO{sub 4}; in the monoclinic Li{sub 4}SiO{sub 4}, there are three phonon soft modes, which correspond to the one type of Li disordered over a few sites. Their LO-TO splitting indicates that both phases of Li{sub 4}SiO{sub 4} are polar anisotropic materials. The calculated infrared absorption spectra for LO and TO modes are different for these two phases of Li{sub 4}SiO{sub 4}. The calculated relationships of the chemical potential versus temperature and CO{sub 2} pressure for reaction of Li{sub 4}SiO{sub 4} with CO{sub 2} shows that Li{sub 4}SiO{sub ...
Date: January 1, 2011
Creator: Duan, Yuhua & Parlinski, K.
Partner: UNT Libraries Government Documents Department

THE TWO-DIMENSIONAL VALENCE ELECTRONIC STRUCTURE OF A MONOLYAER OF Ag ON Cu(00l)

Description: The metal overlayer system c(10x2)Ag/Cu(001) was studied at coverages near one monolayer with angle-resolved photoemission. The observed spectroscopic features indicate a two-dimensional d-band electronic structure that can be interpreted using a model with planar, hexagonal symmetry in which crystal field effects dominate over spin-orbit effects.
Date: May 1, 1985
Creator: Tobin, J. G.; Robey, S. W. & Shirley, D. A.
Partner: UNT Libraries Government Documents Department

A universal high energy anomaly in angle resolved photoemissionspectra of high temperature superconductors -- possible evidence ofspinon and holon branches

Description: A universal high energy anomaly in the single particlespectral function is reported in three different families of hightemperature superconductors by using angle-resolved photoemissionspectroscopy. As we follow the dispersing peak of the spectral functionfrom the Fermi energy to the valence band complex, we find dispersionanomalies marked by two distinctive high energy scales, E_1 approx 0.38eV and E_2 approx 0.8 eV. E_1 marks the energy above which the dispersionsplits into two branches. One is a continuation of the near parabolicdispersion, albeit with reduced spectral weight, and reaches the bottomof the band at the Gamma point at approx 0.5 eV. The other is given by apeak in the momentum space, nearly independent of energy between E_1 andE_2. Above E_2, a band-like dispersion re-emerges. We conjecture thatthese two energies mark the disintegration of the low energyquasiparticles into a spinon and holon branch in the high T_c cuprates.
Date: December 19, 2006
Creator: Graf, J.; Gweon, G.-H.; McElroy, K.; Zhou, S.Y.; Jozwiak, C.; Rotenberg, E. et al.
Partner: UNT Libraries Government Documents Department

Synthesis and Electrochemistry of Li3MnO4: Mn in the +5 OxidationState

Description: Computational and experimental work directed at exploringthe electrochemical properties of tetrahedrally coordinated Mn in the +5oxidation state is presented. Specific capacities of nearly 700 mAh/g arepredicted for the redox processes of LixMnO4 complexes based on twotwo-phase reactions. One is topotactic extractionof Li from Li3MnO4 toform LiMnO4 and the second is topotactic insertion of Li into Li3MnO4 toform Li5MnO4. In experiments, it is found that the redox behavior ofLi3MnO4 is complicated by disproportionation of Mn5+ in solution to formMn4+ and Mn7+ and byother irreversible processes; although an initialcapacity of about 275 mAh/g in lithiumcells was achieved. Strategiesbased on structural considerations to improve the electrochemicalproperties of MnO4n- complexes are given.
Date: June 19, 2007
Creator: Saint, Juliette.A.; Doeff, Marca M. & Reed, John
Partner: UNT Libraries Government Documents Department

Conference on Quantum-Mechanical Methods in Valence Theory

Description: Proceedings from the Conference on Quantum-Mechanical Methods in Valence Theory organized into six sections: Atomic and Molecular Problems, The Link with Chemical Valence Concepts, Intermolecular and Nonbonded Interatomic Forces, Transcending the Primitive Approximations, Mathematical Developments, and Integrals. Some papers include additional discussion from other participants. Table of contents starts on page iii.
Date: 1952
Partner: UNT Libraries Government Documents Department

Pentavalent Uranium Chemistry - Synthetic Pursuit Of A Rare Oxidation State

Description: This feature article presents a comprehensive overview of pentavalent uranium systems in non-aqueous solution with a focus on the various synthetic avenues employed to access this unusual and very important oxidation state. Selected characterization data and theoretical aspects are also included. The purpose is to provide a perspective on this rapidly evolving field and identify new possibilities for future developments in pentavalent uranium chemistry.
Date: January 1, 2009
Creator: Graves, Christopher R & Kiplinger, Jaqueline L
Partner: UNT Libraries Government Documents Department

Effect of Mn Substitution for Multiferroic BiFeO3 Probed by High-Resolution Soft-X-Ray Spectroscopy

Description: The electronic structures of BiFeO{sub 3} (BF) and Mn-doped BiFeO{sub 3} (BF(Mn)) have been studied by X-ray absorption spectroscopy (XAS) and soft-X-ray emission spectroscopy (SXES). The BF and BF(Mn) have the mixed valence state of Fe{sup 2+} and Fe{sup 3+}. The valence band is mainly composed of O 2p state hybridized with the majority-spin t{sub 2g} and e{sub g} orbitals of Fe 3d state. The conduction band is composed of the minority-spin t{sub 2g} and e{sub g} orbitals of Fe 3d. The band gaps of BF and BF(Mn) are estimated to be 1.3 eV and 2.7 eV, respectively. The increase of band gap with Mn substitution contributes to the change of bandwidth of valence band.
Date: July 11, 2008
Creator: Higuchi, Tohru; Higuchi, T.; Hattori, T.; Sakamoto, W.; Itoh, N.; Shimura, T. et al.
Partner: UNT Libraries Government Documents Department

Valence band anticrossing in GaBixAs1-x

Description: The optical properties of GaBixAs1-x (0.04< x< 0.08) grown by molecular beam epitaxy have been studied by photomodulated reflectance spectroscopy. The alloys exhibit a strong reduction in the bandgap as well as an increase in the spin-orbit splitting energy with increasing Bi concentration. These observations are explained by a valence band anticrossing model, which shows that a restructuring of the valence band occurs as the result of an anticrossing interaction between the extended states of the GaAs valence band and the resonant T2 states of the Bi atoms.
Date: July 11, 2007
Creator: Alberi, K.; Dubon, O. D.; Walukiewicz, W.; Yu, K. M.; Bertulis, K. & Krotkus, A.
Partner: UNT Libraries Government Documents Department

Redox Dynamics of Mixed Metal (Mn, Cr, and Fe) Ultrafine Particles

Description: The impact of particle composition on metal oxidation state, and on changes in oxidation state with simulated atmospheric aging, are investigated experimentally in flame-generated nanoparticles containing Mn, Cr, and Fe. The results demonstrate that the initial fraction of Cr(VI) within the particles decreases with increasing total metal concentration in the flame. In contrast, the initial Mn oxidation state was only partly controlled by metal loading, suggesting the importance of other factors. Two reaction pathways, one reductive and one oxidative, were found to be operating simultaneously during simulated atmospheric aging. The oxidative pathway depended upon the presence of simulated sunlight and O{sub 3}, whereas the reductive pathway occurred in the presence of simulated sunlight alone. The reductive pathway appears to be rapid but transient, allowing the oxidative pathway to dominate with longer aging times, i.e. greater than {approx}8 hours. The presence of Mn within the particles enhanced the importance of the oxidative pathway, leading to more net Cr oxidation during aging implying that Mn can mediate oxidation by removal of electrons from other particulate metals.
Date: August 1, 2008
Creator: Nico, Peter S.; Kumfer, Benjamin M.; Kennedy, Ian M. & Anastasio, Cort
Partner: UNT Libraries Government Documents Department

Comparison of the oxidation state of Fe in comet 81P/Wild 2 and chondritic-porous interplanetary dust particles

Description: The fragile structure of chondritic-porous interplanetary dust particles (CP-IDPs) and their minimal parent-body alteration have led researchers to believe these particles originate in comets rather than asteroids where aqueous and thermal alterations have occurred. The solar elemental abundances and atmospheric entry speed of CP-IDPs also suggest a cometary origin. With the return of the Stardust samples from Jupiter-family comet 81P/Wild 2, this hypothesis can be tested. We have measured the Fe oxidation state of 15 CP-IDPs and 194 Stardust fragments using a synchrotron-based x-ray microprobe. We analyzed {approx}300 ng of Wild 2 material - three orders of magnitude more material than other analyses comparing Wild 2 and CP-IDPs. The Fe oxidation state of these two samples of material are > 2{sigma} different: the CP-IDPs are more oxidized than the Wild 2 grains. We conclude that comet Wild 2 contains material that formed at a lower oxygen fugacity than the parent-body, or parent bodies, of CP-IDPs. If all Jupiter-family comets are similar, they do not appear to be consistent with the origin of CP-IDPs. However, comets that formed from a different mix of nebular material and are more oxidized than Wild 2 could be the source of CP-IDPs.
Date: July 16, 2010
Creator: Ogliore, Ryan C.; Butterworth, Anna L.; Fakra, Sirine C.; Gainsforth, Zack; Marcus, Matthew A. & Westphal, Andrew J.
Partner: UNT Libraries Government Documents Department

Decamethylytterbocene Complexes of Bipyridines and Diazabutadienes: Multiconfigurational Ground States and Open-Shell Singlet Formation

Description: Partial ytterbium f-orbital occupancy (i.e., intermediate valence) and open-shell singlet formation are established for a variety of bipyridine and diazabutadiene adducts with decamethylytterbocene, (C5Me5)2Yb, abbreviated as Cp*2Yb. Data used to support this claim include ytterbium valence measurements using Yb LIII-edge X-ray absorption near-edge structure spectroscopy, magnetic susceptibility, and complete active space self-consistent field (CASSCF) multiconfigurational calculations, as well as structural measurements compared to density functional theory calculations. The CASSCF calculations indicate that the intermediate valence is the result of a multiconfigurational ground-state wave function that has both an open-shell singlet f13(?*)1, where pi* is the lowest unoccupied molecular orbital of the bipyridine or dpiazabutadiene ligands, and a closed-shell singlet f14 component. A number of other competing theories for the unusual magnetism in these materials are ruled out by the lack of temperature dependence of the measured intermediate valence. These results have implications for understanding chemical bonding not only in organolanthanide complexes but also for f-element chemistry in general, as well as understanding magnetic interactions in nanoparticles and devices.
Date: April 22, 2009
Creator: Booth, Corwin H.; Walter, Marc D.; Kazhdan, Daniel; Hu, Yung-Jin; Lukens, Wayne W.; Bauer, Eric D. et al.
Partner: UNT Libraries Government Documents Department

Crystal-field level inversion in lightly Mn-doped Sr3Ru2O7

Description: Sr3(Ru1-xMnx)2O7, in which 4d-Ru is substituted by the more localized 3d-Mn, is studied by x-ray dichroism and spin-resolved density functional theory. We find that Mn impurities do not exhibit the same 4+ valence of Ru, but act as 3+ acceptors; the extra eg electron occupies the in-plane 3dx2-y2 orbital instead of the expected out-of-plane 3d3z2-r2. We propose that the 3d-4d interplay, via the ligand oxygen orbitals, is responsible for this crystal-field level inversion and the material's transition to an antiferromagnetic, possibly orbitally ordered, low-temperature state.
Date: January 15, 2008
Creator: Hossain, M. A.; Hu, Z.; Haverkort, M. W.; Burnus, T.; Chang, C. F.; Klein, S. et al.
Partner: UNT Libraries Government Documents Department

ESTIMATED UPPER BOUNDS TO THE HALF-LIFE OF THERMAL DECOMPOSITION OF AMMONIA, HYDROGEN, METHANE, AND PROPANE

Description: An estimate was made of the upper bound for the half-time of dissociation at 100 atm for ammonia, methane, and propane at 2500 deg K and hydrogen at 5000 deg K. In each case a unimolecular reactron in the homogeneous gas phase was chosen as most suitable for this purpose. Slater's theory has been used to estimate the necessary frequency factors. The upper bounds to the half- time for dissociation range from 3 x 10/sup -7/ to 6 x 10/sup -6/ sec. Extrapolation of decomposition rate data obtained at --1000 deg C and 1 atm pressure gives smaller values for the half-time of dissociation. (auth)
Date: August 1, 1955
Creator: Herschbach, D.
Partner: UNT Libraries Government Documents Department

Chemical effects of lanthanides and actinides in glasses determined with electron energy loss spectroscopy

Description: Chemical and structural environments of f-electron elements in glasses are the origin of many of the important properties of materials with these elements; thus oxidation state and chemical coordination of lanthanides and actinides in host materials is an important design consideration in optically active glasses, magnetic materials, perovskite superconductors, and nuclear waste materials. We have made use of the line shapes of Ce to determine its oxidation state in alkali borosilicate glasses being developed for immobilization of Pu. Examination of several prototype waste glass compositions with EELS shows that the redox state of Ce doped to 7 wt% could be varied by suitable choice of alkali elements. EELS for a Pu-doped glass illustrate the small actinide N{sub 4}/N{sub 5} intensity ratio and show that the Pu-N{sub 4,5} white line cross section is comparable to that of Gd M{sub 4,5}.
Date: July 1, 1996
Creator: Fortner, J.A.; Buck, E.C.; Ellison, A.J.G. & Bates, J.K.
Partner: UNT Libraries Government Documents Department

Implications of the unusual redox behavior exhibited by the heteropolyanion [EuP{sub 5}W{sub 30}O{sub 110}]{sup 12{minus}}

Description: Materials containing more than one electroactive species may exhibit a variety of unusual properties, including mixed or intermediate valence, and their related phenomena. Intermediate valence arises when localized orbitals, notably valence f-states, interact in a bonding fashion with delocalized, molecular orbitals in the same material. Electrons are viewed to be rapidly hopping between these two very different kinds of orbitals, and it is their relative residence times that are interpreted as a non-integral valence. Whereas there has been considerable efforts expended to understand this unusual behavior, these efforts have not proven successful, largely because the problem is many-bodied and is usually studied on infinite band states, often in chemically complex systems. The authors report here on the novel electrochemical behavior of the heteropolyanion [EuP{sub 5}W{sub 30}O{sub 110}]{sup 12}, and examine the valence of Eu in both the oxidized and reduced form of the anion, as determined from Eu L{sub 3}-edge X-ray absorption near edge structure (XANES). The results demonstrate that, with careful electrochemical control, Eu in this ion may become intermediate valent. Such a result would provide an entirely new and unique approach to longstanding questions related to the phenomenon of intermediate valence.
Date: August 1, 1997
Creator: Antonio, M.R. & Soderholm, L.
Partner: UNT Libraries Government Documents Department