Probing the electronic structures of ternary perovskite and pyrochlore oxides containing Sn(4+) or Sb(5+)

Experimental and computational studies were performed to understand the electronic structure of ternary perovskites (ASnO(3), A = Ca, Sr, Ba, Cd), pyrochlores (RE(2)Sn(2)O(7), RE = Y, La, Lu; Cd(2)Sb(2)O(7)), and defect pyrochlore oxides (Ag(2)Sb(2)O(6)) containing the main group ions Sn(4+) and Sb(5+). In all compounds, the lowest energy states in the conduction band arise primarily from the antibonding Sn/Sb 5s-O 2p interaction. In the alkaline-earth stannate perovskites (BaSnO(3), SrSnO(3), and CaSnO(3)) the conduction bandwidth decreases strongly in response to the octahedral tilting distortion triggered by the decreasing size of the alkaline-earth cation. This in turn leads to a corresponding increase in the band gap from 3.1 eV in BaSnO(3) to 4.4 eV in CaSnO(3). The band gap of CdSnO(3) is relatively small (3.0 eV) considering the large octahedral tilting distortion. The origin of this apparent anomaly is the mixing between the empty Cd 5s orbitals and the antibonding Sn 5s-O 2p states. This mixing leads to a widening of the conduction band and a corresponding decrease in the band gap. The participation of the normally inert A-site cation in the electronic structure near the Fermi level can be considered an inductive effect, as it utilizes substitution on the A-site to directly modify the electronic structure of the SnO(3)(2)(-) framework. While the pyrochlore structure is more complicated, the energy level and width of the lowest energy conduction band can be analyzed in a manner similar to that utilized on the perovskite structure. The Sn-O-Sn and Sb-O-Sb bonds are highly distorted from linear geometry in pyrochlore, leading to a relatively narrow conduction band and a wide band gap. In Cd(2)Sb(2)O(7) and Ag(2)Sb(2)O(6) the Cd(2+) and Ag(+) ions exhibit a strong inductive effect that widens the conduction band and lowers the band gap significantly, very similar to the effect observed in the perovskite form of CdSnO(3).     

Nanostructured Ti-W mixed-metal oxides: structural and electronic properties

In this article, the structural and electronic properties of Ti-W binary mixed oxide nanoparticles are investigated by using X-ray diffraction, Raman, X-ray absorption spectroscopies (XAS; near edge XANES and extended EXAFS), UV-vis spectroscopy, and density functional calculations. A series of Ti-W binary oxide samples having W content below 20 atom % and with particle size between 8 and 13 nm were prepared by a microemulsion method. The atoms in these nanoparticles adopted the anatase-type structure with a/b lattice constants rather similar to those of the single TiO(2) reference and with a c cell parameter showing a noticeable expansion upon doping. Within the anatase structure, W occupies substitutional positions, while Ti atoms only suffer minor structural perturbations. A change in the W local order at first neighboring distance is observed when comparing samples having a W content below and above 15 atom %. Charge neutrality is mostly achieved by formation of cation vacancies located at the first cation distance of W centers. Upon addition of W to the TiO(2) structure, the Ti charge is not strongly modified, while changes in the W-O interaction appear to drive a modest modification of the W d-electron density throughout the Ti-W series. A combination of these geometrical and electronic effects produced Ti K- and W L(I)/L(III)-edge XANES/EXAFS spectra with distinctive features. UV-vis spectra show a nonlinear decrease of the band gap in the Ti-W solid solutions with a characteristic turning point at a W content of ca. 15 atom %. The relationship between local/long-range order and electronic parameters is discussed on the basis of these experimental results.     

Ruthenium(IV) and Osmium(II) Tetraphenylporphine Complexes: Synthesis and Oxidation Reactions

The Ru(IV) and Os(II) complexes (PhO)₂RuTPP and OsTPP were synthesized from tetraphenylporphine (H₂TPP) and K₂RuO₄ or K₂OsO₄ (taken in the molar ratio of 1 : 30) in boiling phenol. The kinetics of oxidation reactions of these complexes in solutions of HOAc (acetic), H₂SO₄, and HOAc–H₂SO₄ acids was studied. It was found that in the aerated HOAc–H₂SO₄ mixture heated above 340 K, these complexes are oxidized with participation of different reaction sites: the Ru(IV) complex is oxidized at macrocycle to give the -radical-cation (PhO)₂RuPP⁺, while in the Os(II) complex, the metal atom is oxidized to form the Os(III) complex. In the first case, the reaction follows the activation mechanism, whereas in the second case, the activation energy of the reaction is zero.     

Study of the structural,electronic, and magnetic properties of the barium-rich iron(IV) oxides,Ba(2)FeO(4) and Ba(3)FeO(5)

Crystals of Ba(2)FeO(4) and Ba(3)FeO(5), grown from a "self-sealing" KOH-Ba(OH)(2) flux, have been characterized by single-crystal X-ray diffraction, M?ssbauer spectroscopy, and magnetic measurements. Ba(2)FeO(4) forms nonmerohedral twinned crystals with the monoclinic space group P2(1)/n, a = 6.034(2) A, b = 7.647(2) A, c = 10.162(3) A, beta = 92.931(6) degrees, and Z = 4. Ba(3)FeO(5) crystallizes in the orthorhombic space group Pnma, with a = 10.301(1) A, b = 8.151(1) A, c = 7.611(1) A, and Z = 4. While both compounds feature discrete FeO(4)(4-) tetrahedra, the anion found in Ba(2)FeO(4) has shorter Fe-O bonds and is significantly distorted relative to the Ba(3)FeO(5) anion. An iron valence of 4+ was confirmed by magnet susceptibility measurements and by the low-temperature isomer shifts of -0.152 and -0.142 mm/s relative to alpha-iron for Ba(2)FeO(4) and Ba(3)FeO(5), respectively.     

Electrocatalytic Oxidation of Cysteine and Cystine at a Carbon-Paste Electrode Modified with Ruthenium(IV) Oxide

The electrocatalytic activity of ruthenium(IV) oxide incorporated into a carbon-paste electrode was studied in the oxidation of cysteine and cystine. The oxidation potentials of the amino acids decreased and the current peaks of their oxidation increased at a modified electrode as compared to an unmodified one. Procedures for the voltammetric determination of cysteine and cystine with the use of electrodes chemically modified with ruthenium(IV) oxide were proposed.     

Reduction of Ruthenium(IV) to Ruthenium(III) in Aqueous Alcohol Solutions of Hydrochloric Acid under Microwave Radiation

The behavior of a kinetically inert form of ruthenium(IV), the -oxo-bis-[pentachlororuthenate(IV)] ion [Ru₂OCl₁₀]^4–, was studied in aqueous alcohol solutions of hydrochloric acid under microwave radiation and upon the heating of the solutions. The conditions were selected for the quantitative reduction of ruthenium(IV) to ruthenium(III) with alcohols in 0.6–10 M HCl solutions in a microwave field. The maximum time it takes to reduce ruthenium(IV) under microwave radiation (30 min) was an order of magnitude shorter than that for heating the solutions in a boiling water bath. Regardless of the type of alcohol, the most rapid reduction of ruthenium(IV) was observed at the boundaries of the studied range of hydrochloric acid concentrations. It was found that, under microwave radiation, thermal effects were the driving force of the process in a 0.6 M HCl solution at 98°C, whereas, in 8–10 M HCl solutions, the contributions of nonthermal and thermal effects were comparable. It was shown that the reducing ability of saturated monoatomic C₁–C₄ alcohols increases with the number of carbon atoms and is also due to specific features of microwave radiation.     

Synthesis and properties of a thermally stable methyltitanium(IV) compound: Cyclopentadienylmethyldichlorotitanium(IV)

η⁵C₅H₅Ti(CH₃)Cl₂ and η⁵-C₅H₅Ti(C₂H₅TiCl₂ have been synthesized. The reactivity of the methyl compound is much greater than that of the closely related sandwich compound, (η⁵-C₅H₅)₂Ti(CH₃)Cl, but the thermal stability is comparable.     

Antimony-doped tin(IV) oxide: Surface composition and electronic structure

Antimony-doped tin(IV) oxide Sn_1?xSb_xO₂ prepared by a high-temperature (1300 K) solid-state synthetic procedure has been studied over the composition range0 < x < 0.03by X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS) and high-resolution electron-energy-loss spectroscopy (HREELS). Pronounced enrichment by antimony close to the surface is evident from XPS with a heat of segregation approaching 30 kJ/mole. However, no increase in the surface free-carrier concentration is evident from the conduction-to-valence band intensity ratio in UPS or from the surface plasmon frequency in EELS. It is concluded that electrons associated with segregated Sb ions occupy a lone-pair-likesp hybrid surface state whose energy lies well below that of the conduction band.     

Calcium manganese(IV) oxides: biomimetic and efficient catalysts for water oxidation

CaMnO(3) and Ca(2)Mn(3)O(8) were synthesized and characterized by SEM, XRD, FTIR and BET. Both oxides showed oxygen evolution activity in the presence of oxone, cerium(IV) ammonium nitrate and H(2)O(2). Oxygen evolution from water during irradiation with visible light (λ > 400 nm) was also observed upon adding these manganese oxides to an aqueous solution containing tris(2,2'-bipyridyl) ruthenium(II), as photosensitizer, and chloro pentaammine cobalt(III) chloride, as electron acceptor, in an acetate buffer. The amounts of dissolved manganese and calcium from CaMnO(3) and Ca(2)Mn(3)O(8) in the oxygen evolving reactions were reported and compared with other (calcium) manganese oxides. Proposed mechanisms of oxygen evolution and proposed roles for the calcium ions are also considered.     

Ruthenium(II) phenylazopyridinearylazoimidazoles: Synthesis, spectral studies and redox properties

Tris-chelate complex [Ru(Pap)(RAaiR′)₂](ClO₄)₂ (I, II, III/a, b, c) (where RAaiR′ = 1-alkyl-(2-arylazo)imidazole, R = H, Me, Cl (a, b, c); R′ = Me, Et, CH₂Ph (I, II, III), and Pap = phenylazopyridine) was prepared by silver assisted synthetic route. IR spectra of the complexes support Ru-azo nitrogen π-bonding interaction. ¹H NMR spectra suggest that there are two types of streochemical orientation of RAaiR′ around ruthenium(II). Cyclic voltammetry of the complexes shows one metal oxidation Ru(II)/Ru(III) at 1.4–1.5 V and three successive ligand reduction couples at the negative side of the reference potential in the range from −0.5 to −0.56, −0.7 to −0.8, and from −1.25 to −1.40 V, respectively. The text was submitted by the author in English.     

Transition-metal oxides with triangular lattices: generation of new magnetic and electronic properties

The search for multifunctional materials as multiferroics to be applied in microelectronic or for new, chemically stable and nontoxic, thermoelectric materials to recover waste heat is showing a common interest in the oxides whose structures contain a triangular network of transition-metal cations. To illustrate this point, two ternary systems, Ba-Co-O and Ca-Co-O, have been chosen. It is shown that new phases with a complex triangular structure can be discovered, for instance, by introduction of Ga (3+) into the Ba-Co-O system to stabilize Ba 6Ga 2Co 11O 26 and Ba 2GaCo 8O 14, which both belong to a large family of compounds with formula [Ba(Co,Ga)O 3-delta] n [BaCo 8O 11]. In the latter, both sublattices contain triangular networks derived from the hexagonal perovskite and the spinel structure. Among the hexagonal perovskite, the Ca 3Co 2O 6 crystals give clear evidence where the coupling of charges and spins is at the origin of a magnetocapacitance effect. In particular, the ferrimagnetic to ferromagnetic transition, with a one-third plateau on the M( H) curve characteristic of triangular magnetism, is accompanied by a peak in the dielectric constant. A second class of cobaltites is the focus of much interest. Their 2D structure, containing CoO 2 planes isostructural to a CdI 2 slice that are stacked in an incommensurate way with rock salt type layers, is referred to misfit cobaltite. The 2D triangular network of edge-shared CoO 6 octahedra is believed to be responsible for large values of the Seebeck coefficient and low electrical resistivity. A clear relationship between the structuresincommensurability ratiosand the electronic properties is evidenced, showing that the charge carrier concentration can be tuned via the control of the ionic radius of the cations in the separating layers.     

Role of local and electronic structural changes with partially anion substitution lithium manganese spinel oxides on their electrochemical properties: X-ray absorption spectroscopy study

The electronic and local structures of partially anion-substituted lithium manganese spinel oxides as positive electrodes for lithium-ion batteries were investigated using X-ray absorption spectroscopy (XAS). LiMn(1.8)Li(0.1)Ni(0.1)O(4-η)F(η) (η = 0, 0.018, 0.036, 0.055, 0.073, 0.110, 0.180) were synthesized by the reaction between LiMn(1.8)Li(0.1)Ni(0.1)O(4) and NH(4)HF(2). The shift of the absorption edge energy in the XANES spectra represented the valence change of Mn ion with the substitution of the low valent cation as Li(+), Ni(2+), or F(-) anion. The local structural change at each compound with the amount of a Jahn-Teller Mn(3+) ion could be observed by EXAFS spectra. The discharge capacity of the tested electrode was in the order of LiMn(2)O(4) > LiMn(1.8)Li(0.1)Ni(0.1)O(4-η)F(η) (η = 0.036) > LiMn(1.8)Li(0.1)Ni(0.1)O(4) while the cycleability was in the order of LiMn(1.8)Li(0.1)Ni(0.1)O(4-η)F(η) (η = 0.036) ≈ LiMn(1.8)Li(0.1)Ni(0.1)O(4) > LiMn(2)O(4). It was clarified that LiMn(1.8)Li(0.1)Ni(0.1)O(4-η)F(η) has a good cycleability because of the anion doping effect and simultaneously shows acceptable rechargeable capacity because of the large amount of the Jahn-Teller Mn(3+) ions in the pristine material.     

Steric and electronic consequences of flexibility in a tetradentate redox-active ligand: Ti(IV) and Zr(IV) complexes

A redox-active, tetradentate ligand, N,N'-bis-(3-dimethylamino-propyl)-4,5-dimethoxy-benzene-1,2-diamide ([N(2)N(2)(cat)](2-)), was developed, and the six-coordinate metal complexes [N(2)N(2)(cat)]TiCl(2) (3) and [N(2)N(2)(cat)]ZrCl(2) (4) were synthesized. The tetradentate ligand was determined to be fluxional in 3 and 4, enabled by reversible dissociation of the neutral amine groups of the [N(2)N(2)(cat)](2-) ligand. Both amine arms of 3 could be replaced by N,N-dimethylaminopyridine with an overall free energy change of -4.64(3) kcal mol(-1) at 298 K. Cyclic voltammetry experiments were used to probe the redox capabilities of the [N(2)N(2)(cat)](2-) ligand: complex 3 exhibited two one-electron oxidations at -0.19 and -0.52 V versus [Cp(2)Fe](+/0) while 4 exhibited a single two-electron oxidation at -0.55 V. Substitution of the chlorides in 3 for an imide afforded the dimer {[N(2)N(2)(cat)]Ti(μ-p-NC(6)H(4)Me)}(2), in which the metal centers are five-coordinate because of dissociation of one amine arm of the [N(2)N(2)(cat)](2-) ligand. While the bis-azide complex [N(2)N(2)(cat)]Ti(N(3))(2) was stable toward elimination of N(2), the bis-phenylacetylide complex [N(2)N(2)(cat)]Ti(C≡CPh)(2) could be oxidized by PhICl(2), resulting in subsequent reductive elimination of 1,4-diphenylbutadiyne.     

Thermochemical reactivity of manganese(IV) oxides in reducing atmosphere

The thermochemical reduction of a series of structurally and morphologically different natural and synthetic manganese(IV) oxides has been investigated. Measurements have been performed by means of combined thermogravimetry/mass spectrometry, X-ray diffraction and analytical scanning electron microscopy. The mechanisms of the degradation of these materials have been characterized in order to establish standardized procedures for their reactivity as function of structure, morphology and experimental conditions. The corresponding results are the fundament with respect to a reproducible technical application.The authors would like to thank Dr. R. Lenck for compositional analyses of the investigated compounds. The financial support of the Chemetall GmbH, Frankfurt, Germany as well as the Fond der Chemischen Industrie is also acknowledged.     

Kinetics and mechanism of ligand substitution in bis(N-alkylsalicylaldiminato)oxovanadium(IV) complexes

Conventional and stopped-flow spectrophotometry was used to to study the kinetics of ligand substitution in a number of bis(N-alkylsalicylaldiminato)oxovanadium(IV) complexes (=VO(R-X-sal)(2)) by 1,1,1- trifluoropentane-2,4-dione (=Htfpd) in acetone, according to the following reaction: VO(R-X-sal)(2) + 2Htfpd --> VO(tfpd)(2) + 2R-X-salH. The acronym R-X-salH refers to N-alkylsalicylaldimines with substituents X = H, Cl, Br, CH(3), and NO(2) in the 5-position of the salicylaldehyde ring and N-alkyl groups R = n-propyl, isopropyl, phenyl, and neopentyl. Under excess conditions ([Htfpd](0) > [VO(R-X-sal)(2)](0)), substitution by Htfpd occurs in two observable steps, as characterized by pseudo-first-order rate constants k(obsd(1)) and k(obsd(2)). Both rate constants increase linearly with [Htfpd](0) according to k(obsd(1)) = k(s(1)) + k(1)[Htfpd](0) and k(obsd(2)) = k(s(2)) + k(2)[Htfpd](0), with k(s(1)) and k(s(2)) describing small contributions of solvent-initiated pathways. Depending on the nature of R and X, second-order rate constants k(1) and k(2) lie in the range 0.098-0.87 M(-1) s(-1) (k(1)) and 0.022-0.41 M(-1) s(-1) (k(2)) at 298 K. For ligand substitution in the system VO(n-propyl-sal)(2)/Htfpd, the activation parameters DeltaH++ = 35.8 +/- 2.8 kJ mol(-1) and DeltaS++ = -146 +/- 23 J K(-1) mol(-1) (k(1)) and DeltaH++ = 40.2 +/- 1.3 kJ mol(-1) and DeltaS++ = -142 +/- 11 J K(-1) mol(-1) (k(2)) were obtained. The Lewis acidity of the complexes VO(n-propyl-X-sal)(2) with X = H, Cl, Br, CH(3), and NO(2) was quantified spectrophotometrically by determination of equilibrium constant K(py), describing the formation of the adduct VO(n-propyl-X-sal)(2).pyridine. The adduct VO(tfpd)(2).n-propyl-salH, formed as product in the system VO(n-propyl-sal)(2)/Htfpd, was characterized by its dissociation constant, K(D) = (3.30 +/- 0.10) x 10(-3) M. The mechanism suggested for the two-step substitution process is based on initial formation of the adducts VO(R-X-sal)(2).Htfpd (step 1) and VO(R-X-sal)(tfpd).Htfpd (step 2).     

Infrared spectra and methyl group properties in dicyclopentadienyldimethyl-titanium(IV), -zirconium(IV) and -hafnium(IV)

Infrared spectra are reported for CH₃-, CD₃- and CHD₂-substituted Cp₂MMe₂ (Cp = η⁵-C₅H₅, M = Ti, Zr, Hf) in CCl₄ solution. The isolated CH stretching frequencies, ν(^isCH), measured in the CHD₂ species are lower than any previously observed in methylmetal compounds and the methyl CH bonds in Cp₂HfMe₂ are predicted to be the longest and weakest such bonds yet to have been characterised by this method. The methyl groups in Cp₂ZrMe₂ and Cp₂HfMe₂ have all three CH bonds equal, but in Cp₂TiMe₂ each methyl group contains two strong CH bonds and one weak one. This may be the result of steric overcrowding effects around the relatively small titanium atom. The symmetric deformation δ_s(CH₃) rises with increasing atomic number of the metal atom, the reverse of the trend observed for methyl derivatives of Main Group elements.     

Characterization of Cr(III)-Zr(IV) mixed and Sn(II) doped hydrous oxides

Surface and structural properties of chromium-zirconium mixed and Sn(II)doped hydrous oxide gels have been compared with chromium oxide hydrate gel by the use of thermal analysis, IR spectroscopy, X-ray diffraction, electron microscopy and magnetic measurements. The mixed and doped oxide gels were found to have a hexagonal close packed stacking of O, OH and H₂O ligands with chromium ions distributed in octahedral sites with little degree of order among them. The microstructure of the gels are characterized by the presence of large aggregates of chromium hydroxides, fine granular sheets due to HCrO₂ phase and Cr(OH)₃ microcrystallites. Magnetic susceptibility measurements indicate anti-ferromagnetic behaviour of these gels.