Valent Transition Metal Research Articles

Boron-Based Organometallic Nanostructures: Hydrogen Storage Properties and Structure Stability

Transition-metal (TM) boride and carboride nanostructures are studied as model organometallic materials for hydrogen storage. The dispersed TM atoms function as H2 sorption centers on the surface of the boron or carbon-boron substrate. The flexibility offered in the variety of possible structures permits the study of the effect of the TM-TM distance on the storage capacity. When the TMs are too close to one another, TM-TM bonding reduces the capacity. Even when separated by distances larger than the normal TM-TM bond length, delocalization of TM valence electrons can still lower the hydrogen capacity. An optimal TM-TM distance for the structural motifs studied here is approximately 6 A. Our study also permitted the evaluation of new TM boride nanostructures. We predict a low-energy single-walled scandium triboride (ScB3) nanotube that can bind approximately 6.1 wt % hydrogen with a binding energy of 22 approximately 26 kJ/mol.

Hydrogen binding in coordination compounds of 3(5)-(4-methoxyphenyl)pyrazole

The solid state structures of 3(5)-(4-methoxyphenyl)pyrazole and its coordination compounds with a series of two valent transition metals have been investigated. Since pyrazoles provide not only a nitrogen donor site for the coordination to metal ions, but also an additional N–H function, they are ideal ligands for the formation of hydrogen bound coordination polymers or for the implementation of secondary interactions with other ligands bound to the same central ion, resulting in a rigid ligand environment at the central metal. We chose cobalt, nickel, palladium, copper and zinc as twofold positively charged Lewis acids preferring coordination numbers of four and six to prove the capability of pyrazole to undergo intramolecular hydrogen bonds. In the four-coordinate mode, either tetrahedral (Zn 2+) or square planar coordination geometries (Pd 2+) are possible, providing different geometric restrictions for hydrogen bonding.

Structural observations on La 2(Ni,Co)O 4±δ phases determined from in situ neutron powder diffraction

In situ neutron powder diffraction data have been analysed for a number of compositions in the solid solution series La 2Ni 1− x Co x O 4± δ over the temperature range 150–650 °C. High-quality Rietveld refinements indicate no major structural transitions have occurred in any of the compositions, although close examination of the Ni/Co O bond distances highlight changes in the coordination environment with increasing temperature. Further, the changes in the coordination can be correlated to changes in both the oxide ion and electronic conductivity indicating a direct link between transport properties, bonding and transition metal valence state in these Co containing layered perovskites.

Effects of iron oxide (Fe 2O 3, Fe 3O 4) on hydrogen storage properties of Mg-based composites

When magnesium powder containing small additions of certain multiple valence transition metal oxides (TMOs) is ball milled in hydrogen, the hydrogenated Mg-based product can show remarkable improvements in hydrogen absorption/desorption properties. Using a magnetically controlled Uni-Ball-Mill, small amounts of the iron oxides, Fe 2O 3 and Fe 3O 4, were ball milled with Mg powder in a hydrogen atmosphere (Mg to Fe atomic ratio; 20:1). Milling products as well as samples used in hydrogen absorption/desorption experiments were characterized by scanning electron microscopy (SEM), X-ray diffractometry (XRD), differential scanning calorimetry (DSC) and thermogravimetric (TG) analysis. TG analysis combined with DSC revealed a higher hydrogen storage capacity for the Mg + Fe 2O 3 + H 2-milled product (6 wt% H) compared with 5 wt% H for Mg + Fe 3O 4 + H 2. XRD revealed that during heating, both iron oxides were reduced to pure Fe, a result not previously reported for similar materials milled using different milling devices. For both samples, there was little difference found in the decomposition temperature of the as-prepared MgH 2 and rehydrogenated composites. However, storage capacity degradations were observed for the rehydrogenated composites (4 wt% H storage capacity for MgH 2 + Fe 2O 3 and 4.4 wt% H for MgH 2 + Fe 3O 4). The higher capacity degradation of rehydrogenated MgH 2 + Fe 2O 3 composite is also believed to be a result of the reduction reaction, during which more magnesium was consumed than was consumed by the same amount of Fe 3O 4. The results also were related to the particular ball-milling equipment and low-energy shearing milling mode employed, which promoted the development of a nanostructural hydride product which subsequently changed structure significantly during the first desorption cycle.

Electronic origin of shearing in M2AC (M = Ti,V,Cr,A = Al,Ga)

We have studied shearing in M2AC (space group P63/mmc, prototype Cr2AlC), where M is Ti, V and Cr, and A is Al and Ga, using ab initio calculations.These compounds can be described as interleaved layers of MC and A. As Ti inTi2AlC is substitutedby V, c44 increases by 24.3%. Increasing the transition metal valence electron concentrationfurther, through a substitution of V by Cr, results in 2.2% decrease ofc44. The electronicorigin of this c44 versus valence electron concentration dependence may be understood byanalysing the decomposed band structure: in the vicinity of the Fermilevel, we find two types of dd bonding. One contributes to shearing(t2g+eg symmetry orMC–MC coupling) and theother does not (eg symmetry). We provide evidence that filling of the transition metal dd bonding states with thet2g+eg symmetry may be responsible for the behaviour ofc44. The results presented enable tailoring of the shear properties ofM2AC phases.

Energy levels of point defects in SrTiO3 and related oxides

The energy levels of defects in SrTiO3, PbTiO3 and PbZrO3 have been calculated by the tight-binding Greens function method and compared to experiment. This method is very useful for displaying the chemical trends in defect energy levels. The Ti and O vacancies are found to be shallow. Transition metal impurities at the octahedral ion site are found to give rise to numerous charge states because of the large intra-atomic electron repulsion. The levels are generally in the high spin configuration, except for the high valence transition metals. The levels of Cr4+, Fe4+, and Co4+ lie near midgap. The Ti3+ trapped electron centers become deep at higher Zr contents in Pb(Zr,Ti)O3.

Electron beam induced changes in transition metal oxides.

Electron beam induced changes in maximal valence transition metal oxides V(2)O(5), M(o)O(3) and TiO(2) (anatase) were studied by means of electron energy-loss spectroscopy and electron diffraction in transmission electron microscopy. For V(2)O(5), the observed chemical shifts of the L-edge reveal the reduction of V(5+) to V(2+). The structure changes from orthorhombic V(2)O(5) to cubic VO. MoO(3) can be reduced to a phase with an oxidation state less than that in MoO(2). No notable structural or electronic change in TiO(2) (anatase) is observed. The different behaviours of the studied oxides under an electron beam are discussed with respect to bonding energy and lattice structure.

Structure Changes in Transition Metal Oxides Induced During Electron Microscopy

Many catalytic materials, especially the maximum valence transition metal oxides, are particularly susceptible to electron beam irradiation and thus undergo structural changes. Hence knowledge about the behaviour of catalytic materials under the electron beam is of importance for all TEM investigations of such materials. On the other hand, this effect can be utilised for an in-situ study of the reductive property, phase transition and/or phase stability of various transition metal oxides in an inert, simple ambient high-vacuum. The knowledge so obtained is needed for understanding the reduction mechanism of catalysts in more complicated chemical environments. in the present work, we study the electron beam induced change in MoO3 and TiO2 (anatase) by means of electron energy-loss spectroscopy (EELS), electron diffraction and high-resolution electron microscopy (HREM).Molybdenum trioxide, MoO3, important as catalyst in the selective oxidation of hydrocarbons, forms an orthorhombic crystal layer structure. Fig. 1 shows oxygen AT-edges recorded at various irradiation periods in a Philips 200 FEG electron microscope.

Electron Spectroscopic Data of La1− x Sr x MnO3 and La1− x Sr x CoO3

We report various high-energy electron spectroscopic data on sintered polycrystalline La1−xSrxMnO3 and La1−xSrxCoO3 with x=0.0, 0.1, 0.2, 0.3, and 0.4. The data presented consist of x-ray photoelectron transition metal 2p core level and valence band spectra, x-ray initiated transition metal L3VV spectra, Bremsstrahlung isochromat spectra, and soft x-ray absorption spectra at the oxygen K-edge. Substitution of trivalent La with divalent Sr introduces holes into the system, driving an insulator-to-metal transition along with magnetic transitions. The spectral changes with progressive hole doping reflect the evolution of electronic structure across the phase transition.

Density Functional Modeling of Double Exchange Interactions in Transition Metal Complexes. Calculation of the Ground and Excited State Properties of [Fe2(OH)3(tmtacn)2]2+

Density functional theory has been successfully applied to characterize the electronic structure and the magnetic properties of the mixed valence dinuclear complex [Fe2(OH)3(tmtacn)2]2+ modeled by replacing the tmtacn ligand with three ammonia molecules. Spectroscopic and magnetic properties have been computed in nice agreement with the experimental values. Minimum energy path calculations allowed us to compute the frequencies ν associated with the normal coordinate Q- responsible for the delocalization of the extra electron, and we present here a procedure for the full characterization of mixed valence transition metal dimers.

Composition–Valence Diagrams: A New Representation of Topotactic Reactions in Ternary Transition Metal Oxide Systems. Application to Lithium Intercalation

Topotactic reactions in Li–M–O systems withM= Mn, Ti, V, Fe are described in the framework of composition–valence diagrams, using the Li/Mand the transition metal valence ν(M) as coordinates. We show that this representation is very convenient to depict and compare the various parameters associated with insertion/extraction reactions, i.e., the evolution of ν(M), the extent of intercalationxin LixMOy, the electrochemical potentials, and the cell parameter changes. New directions are suggested for topotactic reaction in oxides, especially in titanium ones. The composition–valence diagram helped in the detection of inconsistencies in the intercalation potentials in LiFe5O8, which are corrected using new voltammetric data. We conclude that lithium intercalation in octahedral sites of iron spinel oxides occurs at constant voltage around 1.6 V, whichever the Li–Fe–O host may be.

Siloxane clusters of higher valence transition metals: Redox properties

Cyclic voltammetric behaviour of polymetallic complexes of hexaphenylyclohexasiloxane-hexaol with Ni, Mn and dodecaphenyl-cyclododecasiloxane-dodecaol with Cu was studied. The complexes react electrochemically as a unit assembly of the complexed metals

EELS fine structure analysis of radiation damage of V2O5

The oxides of maximum valence transition metals have been extensively studied in view of their useful physical and chemical properties. Vanadium pentoxide is one such compound which exhibits important catalytic properties, but is sensitive to electron beam irradiation. Vanadium pentoxide has the orthorhombic symmetry and the structure is built from deformed octahedra with a longer and weak V-O bond along the c direction. Consequently, V2O5 is a layered structure with (001) cleavage plane, and the oxygen atoms located at the cleavage plane can be easily lost during the electron beam irradiation. In this paper we present time resolved parallel detection EELS study of radiation damage of V2O5 with a 200 kV cold field emission analytical electron microscope (HF-2000). Samples of V2O3 and VO2 are also analyzed for comparison. The EELS spectra were obtained with a spectrometer dispersion of 0.1 eV per channel in TEM image mode using probe sizes of 20-50 nm. The beam current recorded by a picoammeter connected to the screen was calibrated by the direct measurement with a Faraday cup for the calculation of electron dosage.

HREM Study of electron-induced surface radiation damage in ReO3

It has been known for a long time that electron irradiation induces damage in maximal valence transition metal oxides such as TiO2, V2O5, and WO3, of which transition metal ions have an empty d-shell. This type of damage is excited by electronic transition and can be explained by the Knoteck-Feibelman mechanism (K-F mechanism). Although the K-F mechanism predicts that no damage should occur in transition metal oxides of which the transition metal ions have a partially filled d-shell, namely submaximal valence transition metal oxides, our recent study on ReO3 shows that submaximal valence transition metal oxides undergo damage during electron irradiation.ReO3 has a nearly cubic structure and contains a single unit in its cell: a = 3.73 Å, and α = 89°34'. TEM specimens were prepared by depositing dry powders onto a holey carbon film supported on a copper grid. Specimens were examined in Hitachi H-9000 and UHV H-9000 electron microscopes both operated at 300 keV accelerating voltage. The electron beam flux was maintained at about 10 A/cm2 during the observation.

The Role of Ballistic, Electronic, and Thermal Processes in Electron Irradiation Damage of Maximum Valence Transition Metal Oxide Surfaces

ABSTRACTSurface-initiated radiation damage processes in a number of the maximum valence transition metal oxides - TiO2, V2O5, WO3, MoO3, Ta2O5, Al2O3, and Nb2O5 - have been investigated by in-situ high resolution electron microscopy under UHV conditions and at incident energies ranging from 3 keV to 300 keV. The relative contributions of ballistic, electronic, and thermal effects are evaluated by a comparison of the observed structural changes and damage rates - made under various experimental conditions of incident electron energy and flux - to theoretical predictions based on thermodynamic and available energy criteria.

"Band-gap theory" of strong ferromagnetism: Application to concentrated crystalline and amorphous Fe- and Co-metalloid alloys

We extend the early work of Stoner, Mott, Friedel, and Terakura and Kanamori, which relates alloy magnetization to solute valence. We describe the conditions under which the simple formula ${\ensuremath{\mu}}_{\mathrm{av}}={\ensuremath{\mu}}_{A}^{0}\ensuremath{-}x(10+{Z}_{B}\ensuremath{-}{Z}_{A})$ can be expected to apply. In particular, we consider Fe- and Co-metalloid alloys. Here ${\ensuremath{\mu}}_{\mathrm{av}}$ is the atom-averaged moment, ${\ensuremath{\mu}}_{A}^{0}$ is the host moment, $x$ is the metalloid-atom fraction, and ${Z}_{B}$ and ${Z}_{A}$ are the valence of metalloid and transition metal, respectively. We show that the validity of this formula rests on the existence of band gaps in the density of states in the spin-up band. Spin-polarized band-structure calculations do indeed show band gaps in moderately concentrated ($x\ensuremath{\sim}0.25$) compounds and indicate that ${\ensuremath{\mu}}_{A}^{0}$ should be somewhat higher for fcc than bcc structures. The theory compares well with data on concentrated amorphous and crystalline alloys of Co with Au, B, Sn, and P, and of Fe with Au, B, Al, Ga, and Si. Our explanation of this large amount of data is far simpler than, and as accurate as, any previous efforts at explanation.

Studies on Thermoluminescence Property of Thoria Ceramics

The phenomenon of thermoluminescence has been frequently employed to obtain information concerning the defects in ionic solids. Present paper reports the results of an investigation on thermoluminescence behaviour of sintered thoria samples with respect to bulk density, sintering atmosphere and the radiation dose.TL glow curves have peaks around 100°, 146°, 215° and 266°C with 215°C as the most intense one. Intensity as well as peak resolution improved with density. All the peaks are found to saturate at about the same γ-radiation dose. Emission centres are assigned to be associated with the variable valence transition metal impurities unintentionally incorporated in ThO2 lattice.

Mechanisms of reactions at square planar metal centres

SQUARE-PLANAR platinum(II) complexes have in the past been found to react by associative mechanisms (see refs 1 and 2). This has been of some comfort to the proponents3 of the ‘16 and 18 electron rules’, which state that all stable transition metal complexes and intermediates in the reactions of such complexes should have either 16 or 18 electrons in the transition metal's valence shell. We point out here that though substitution reactions at platinum(II) occur by an associative route, reactions between ligands coordinated to platinum(II)—here called combination reactions—occur by a dissociative route that violates the ‘16 and 18 electron rules’.

Nonlocal Xα‐Exchange for s‐ and d‐Electrons in Transition Metals

AbstractStarting from Hartree‐Fock expressions a nonlocal exchange potential of the Xα‐type for transition metal valence electrons is calculated, respresented by different values αs and αd. No fitting parameter was introduced. Whereas αs remains nearly constant within one period αd increases nearly linearly with the number of d‐electrons. The dependence on the chosen atomic configuration and the influence of pressure were investigated. For Cu the energy bands mere calculated. They agree well with fitted local calculations.

Kinetic effects of the transition metal valence in ziegler‐type catalyst systems

Kinetic effects of the transition metal valence in ziegler‐type catalyst systems