Redox behavior of molybdenum and tungsten in phosphate glasses

In this work, vitreous samples were prepared in the binary system (100 - x)NaPO3-xMO3 with M = Mo and W and x varying from 10 to 60. The transmittance properties in the UV, visible, and near-infrared were monitored as a function of MO3 concentration. In both cases, an increase in the amount of transition metal results in an intense and broad absorption band in the visible and near-infrared attributed to metal reduction under synthesis conditions. It was shown that this large absorption can be partially or totally removed using specific oxidizing agents or by improving synthesis parameters such as melting temperature or cooling rate of the melt. In addition, structural investigations by Raman and X-ray absorption spectroscopy suggest that reduction only occurs when the metal cation is in octahedral geometry and that the transmittance improvement is not related with any structural changes. These results were explained in terms of thermodynamic equilibrium of redox species in the melt and allowed to obtain for the first time transparent and chemically stable glasses containing high concentrations of MO3 with transition metals in octahedral geometry inside the glass network.     

X-ray photoelectron study of mixed valence metatungstate anions

X-ray photoelectron spectra of metatungstate H₂W₁₂O^6?₄₀ and of reduced derivatives with 5, 12, and 24 electrons have been recorded. W(4f) signals are consistent with the presence of tetravalent tungsten in the reduced species. In particular the 12e^? derivative does not contain W^V but W^IV and W^VI in equal amounts. Valence band spectra show the W(5d) levels near 2 eV in reduced forms, this energy being appreciably lower than in W bronzes and WO₂. This can be correlated with the relative inertness of reduced metatungstates towards oxidation.     

EXAFS studies of cobalt oxides and oxide glasses

The exafs of Co²⁺ has been studied in rare earth cobaltites and in sulphate and borate glasses. It has been found that the environment of Co²⁺ ions is very similar in these cases. It appears feasible to study local structures in glasses using probe ion exafs. Communication No. 151 from the Solid State and Structural Chemistry Unit.     

Highly mixed phases in ball-milled Cu/ZnO catalysts: an EXAFS and XANES study

New highly mixed phases have been identified in Cu/ZnO systems by EXAFS and XANES at both the Cu and Zn K-edge. The phases were generated by ball-milling Cu(2)O/ZnO mixtures under three different atmospheres of synthetic air (SA), SA + CO(2) and CO(2). The system milled in CO(2) shows disproportionation of Cu(2)O into Cu(0), Cu(1+) (cuprite Cu(2)O-type phase) and Cu(2+) (tenorite CuO-type phase), while most of the Zn(2+) is transformed into a nanocrystalline/amorphous ZnO-type zincite that forms a superficial mixture of oxide and carbonate phases. When synthetic air is added to the CO(2) atmosphere, ball milling results in the oxidation of nearly half the Cu(1+) into Cu(2+) with no Cu metal formed. The copper phase in this material is almost entirely amorphous. In SA, a significant amount of Cu(2+)- and Zn(2+)-based phases appears to react to form a nanocrystalline/amorphous Cu(1-x)Zn(x)O (x approximately 0.3) solid solution. This distorted rock saltlike solid solution, in which Zn and Cu feature different octahedral environments, was never reported before. It is thought to be formed by incorporation of Zn(2+) in the Cu fcc sublattice of the cuprite Cu(2)O matrix and the concomitant oxidation of Cu(1+) into Cu(2+). The formation of such a highly mixed Cu(1-x)Zn(x)O phase indicates strong Cu/Zn interaction in the Cu/ZnO system, which also suggests the presence of highly mixed phases in conventionally prepared activated catalysts.     

A unique coordination environment for an ion: EXAFS studies and bond valence model approach of the encapsulated cation in the Preyssler anion

X-Ray absorption spectroscopy was used to probe the coordination of different encrypted cations in the Preyssler anions [M(n+)P5W(30)O(110)]((15-n)-)(M(n+)= Sr2+, Am3+, Eu3+, Sm3+, Y3+, Th4+, U4+ in decreasing order of ionic radius, IR), hereafter abbreviated [M(n+)PA](15-n)-. The increase of the M-W distance and the decrease of the M-P distance with increasing M ionic radius reveal that the M cation is displaced along the C5 axis within the Preyssler cavity. The slight change (0.07 A) of the M-O distance that does not correspond to the IR difference of 0.27 A confirms that the cavity retains its rigidity upon cation substitution. Geometric modeling of the encapsulated cation in the channel was performed for comparison to the EXAFS results. The position of the cation in the cavity was calculated as well as the M-O10, -W5 and -P5 distances. This modeling confirms the cation displacement toward the center of the Preyssler anion as the cation size increases, which is understood in terms of the non-homogenous electrostatic potential present within the cavity. The bond valence model approach was applied to obtain experimental bond valences. Only the bond valence sum (BVS) of Am3+ is close to its actual charge. Sums smaller than the actual valences of the +3 and +4 ions (2.39-2.63 for +3 cations, Y, Sm, Eu; 3.17 and 3.38 for +4 cations, U and Th, respectively) were obtained, and a larger sum (2.89) was obtained for Sr2+. The deviations from the formal M sums of the encapsulated ions are attributed to the rigidity of the Preyssler framework. The tendency toward coordinative unsaturation for electroactive cations, such as Eu3+, is thought to be the driving force for facile reduction. Unlike other inorganic chelating ligands, the Preyssler anion provides a unique redox system to stabilize an electroactive cation in a low oxidation state.     

Local atomic structure of zinc selenide films: EXAFS data

This paper presents the results of our study of the structural state and local atomic structure of zinc selenide films obtained by thermal evaporation in supervacuum at condensation temperatures of ?150°C, 0, and 150°C. Structure-sensitive methods such as X-ray diffraction, atomic force microscopy, and EXAFS spectroscopy were used. The parameters of the local atomic environment (interatomic distances, coordination numbers) of zinc and selenium atoms were obtained by Fourier transformation.     

Local environment and dynamics of PO4 tetrahedra in Na-Al-PO3 glasses and melts

Glasses and melts in the system (NaPO3)(1-x)(Al(PO3)3)x were studied with the aim of obtaining information about the structure on the next larger scale beyond the PO4 group. Magic angle spinning NMR was applied to the pure NaPO3 glass and Raman scattering to systems with x = 0.00, 0.03, 0.06, 0.15, and 0.60 in the temperature range T = 300-1100 K. Comparison of the 31P chemical shift between glass and crystalline forms revealed that polymerization of the metaphosphate into tricyclophosphatelike (PO3)3(3-) rings is the dominant structure, ca. 80%, formed by the twofold vertex-joined PO4 groups in the glass. In the Raman study we focused on the prominent polarized band at ca. 1170 cm(-1) which is due to the symmetric breathing mode of the tetrahedral PO4 group. This band was decomposed into a few Gaussian lines. These component lines could be identified using the NMR results: two narrow components are due to PO4 groups in the tricyclophosphatelike rings, which have either a Na or an Al counterion and a third broad component is due to chain-polymerized (PO3(-))n. The variations of the component lines (peak positions, widths, and intensities) with respect to x and T are presented. We derive the shifts of the symmetric breathing mode frequency which are caused by Na or Al counterions, by ring closure, by x > 0, etc. The relative intensities of the narrow and broad components in the 1170-cm(-1) band of the Raman spectra are discussed. The amount of ring-to-chain transformation on addition of Al3+, and as functions of T and x, is derived. Indications for ordering on a next larger scale, derivable from Raman, NMR, and thermodynamics, are compared.     

Crystallization of iron phosphate glasses

Differential thermal analysis (DTA), thermogravimetric analysis (TG), X-ray diffraction and Mössbauer spectroscopy were employed in the investigation of crystalline products of FeO_x-P₂O₅ glasses generated by various heat treatments. In glasses with a high value of =Fe²⁺/(Fe²⁺ + Fe³⁺), absorption of oxygen occurs in a broad temperature range identified by TG. Depending on the value of , two exotherms appear in the DTA curves, the low-temperature one corresponding to crystallization of the Fe₃(PO₄)₂ type regions, and the high-temperature one being related to various phases with dominating FePO₄. Each exotherm has its own transformation region, identical in absolute value. The Mössbauer spectra of glasses which underwent thermal treatment at higher temperatures exhibit some indication of phases of the types Fe₃(PO₄)₂ · xH₂O and FePO₄ · xH₂O.

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Zusammenfassung Differentialthermoanalyse (DTA), thermogravimetrische Analyse (TG), Röntgendiffraktion und Mössbauerspektroskopie wurden bei der Untersuchung (von kristalliner Produkte) durch verschiedene thermische Behandlungen hergestellten FeO_x-P₂O₅ Gläsern eingesetzt. In den Glasarten mit einem hohen Wert von =Fe²⁺/(Fe²⁺ + Fe³⁺) erfolgt die Sauerstoffabsorption in einem durch TG [nachgewiesenen breiten Temperaturbereich. In Abhängigkeit von dem -Wert erscheinen zwei Exothermen in den DTA-Kurven, von denen der bei niedriger Temperatur die Kristallisation des Fe₃(PO₄)₂ entspricht, und jener bei höheren Temperaturen in verschiedenen Phasen dem FePO₄. Jede Exotherme hat ihr eigenes Umwandlungsgebiet, das in absolutem Wert identisch ist. Die Mössbauer-Spektren der Gläser welche einer Wärmebehandlung bei höheren Temperaturen unterzogen worden sind, weisen einige Indikationsphasen der Typen Fe₃(PO₄)₂ · xH₂O und FePO₄ · xH₂O auf.

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, , FeO_x-P₂O₅, . , =Fe²⁺/(Fe²⁺+Fe³⁺) . , - : Fe₃(PO₄)₂, – FePO₄. , . , , Fe₃(PO₄)₂ · ₂ FePO₄ · ₂.

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The authors thank Mrs. Simonyiová for assistance in the experimental part of these investigations, and Dr. Horváth and Dr. Vondrovic for performing the X-ray and TG analyses.     

Excited-state mixed valence in transition metal complexes

Mixed valence in the lowest-energy metal-to-ligand charge-transfer excited state of di-(4-acetylpyridine)tetraammineruthenium(II) complexes is defined and analyzed. The excited state has two interchangeably equivalent ligands with different oxidation states. The electronic absorption band energies, selection rules, and bandwidths are analyzed quantitatively in terms of the signs and orientations of the transition dipole moments, sign and magnitude of the coupling, and resonance Raman analysis of displaced normal modes.     

Thermal,optical and electrical properties of MnO2-doped mixed sodium potassium phosphate glasses

Journal of Thermal Analysis and Calorimetry - Glasses in the system (1???x)(0.5NaPO3–0.5KPO3)–xMnO2, with 0?≤?x?≤?50 mol%,...     

An EXAFS study of some gold and palladium cluster compounds

Gold L₃-edge EXAFS measurements at 80 K on Au₅₅(PPh₃)₁₂Cl₆ confirm that the Au-Au distances in this amorphous metal cluster compound are significantly shorter than in bulk gold. The nearest-neighbour Au-Au distances are all equal within experimental uncertainty. Outer-shell Au-Au distances have also been resolved. The results are consistent with the cuboctahedral structure originally proposed for this cluster, but not the polyicosahedral one recently suggested. Very similar results have been obtained from the sulphonated water-soluble derivative Au₅₅(PPh₂C₆H₄SO₃Na)₁₂Cl₆. In contrast, EXAFS of Au₁₁{PPh₂(p-ClC₆H₄)}₇I₃ has clearly resolved the two nearest-neighbour Au-Au distances associated with its icosahedral structure.Palladium K-edge EXAFS has been used to study the cluster Pd₅₆₁(phen)₃₆O₂₀₀. The Pd-Pd distance is nearly equal to that in bulk palladium. The results show a cubic close-packed cluster structure for this material, in contrast to the icosahedral structure reported for Pd₅₆₁(phen)₆₀(OAc)₁₈₀.     

Self-doping of molecular quantum-dot cellular automata: mixed valence zwitterions

Molecular quantum-dot cellular automata (QCA) is a promising paradigm for realizing molecular electronics. In molecular QCA, binary information is encoded in the distribution of intramolecular charge, and Coulomb interactions between neighboring molecules combine to create long-range correlations in charge distribution that can be exploited for signal transfer and computation. Appropriate mixed-valence species are promising candidates for single-molecule device operation. A complication arises because many mixed-valence compounds are ions and the associated counterions can potentially disrupt the correct flow of information through the circuit. We suggest a self-doping mechanism which incorporates the counterion covalently into the structure of a neutral molecular cell, thus producing a zwitterionic mixed-valence complex. The counterion is located at the geometrical center of the QCA molecule and bound to the working dots via covalent bonds, thus avoiding counterion effects that bias the system toward one binary information state or the other. We investigate the feasibility of using multiply charged anion (MCA) boron clusters, specifically closo-borate dianion, as building blocks. A first principle calculation shows that neutral, bistable, and switchable QCA molecules are possible. The self-doping mechanism is confirmed by molecular orbital analysis, which shows that MCA counterions can be stabilized by the electrostatic interaction between negatively charged counterions and positively charged working dots.     

An EXAFS study of the molecular structure of heterometallic chalcogenide clusters

Applicability of EXAFS spectroscopy for determination of the molecular structure of heterometallic chalcogenide clusters in the crystalline state and in solution was examined. The spatial structure of the metal core of the FeMoW(μ₃-Se)(CO)₇(η⁵-C₅H₅) cluster was determined using EXAFS data obtained at the K absorption edge for Fe, Mo, and Se and at the L_III-edge for W. The geometric parameters (the bond lengths and bond angles) obtained from EXAFS data are close to those determined by X-ray analysis. The Mo K-edge EXAFS study of the structurally similar FeMo₂Te(CO)₇(η⁵-C₅H₅) cluster both in the crystalline state and ino-xylene solution confirmed that the geometry of the metal core of the cluster is retained in solution. Published inIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1395–1398, August, 2000.     

Longitudinal lattice modes in linear mixed valence compounds

Mixed valence compounds of platinum or palladium have a crystal structure that consists of long, linear chains. The frequencies of the three collective (phonon) longitudinal k ≈ 0 optical modes bear little relevance to those of longitudinal modes of the independent molecules, being different in general and not in a one to one correspondence with them. Therefore, they have to be taken into account as such when the observed intensity peaks in resonance Raman or infrared spectra of these compounds are analyzed.     

Local structure of magnesium in silicate glasses: a 25Mg 3QMAS NMR study

We have reported the 25Mg triple-quantum magic-angle spinning (3QMAS) NMR spectra of silicate glasses. The two-dimensional spectra suggest that the magnesium ions in MgSiO3, CaMgSi2O6, Ca2MgSi2O7, Mg3Al2Si3O12, and Li2MgSi2O6 glasses are mainly in octahedral environments, although in Na2MgSi2O6, K2MgSi2O6, and K2MgSi5O12 glasses they form tetrahedral species. We discussed the coordination environments of Mg based on the field strength of competing Mg2+, Ca2+, Na+, K+, and Li+ cations, and convincingly demonstrated that there is a correlation between them. These results indicate that the two-dimensional NMR spectroscopy such as MQMAS technique is a very useful method to analyze the local environments of nonframework cations in noncrystalline solids.     

An EXAFS study of AgPO3 and AgBaP3O9

The local structure around silver atoms in the crystalline and glassy forms of AgBaP₃O₉ and AgPO₃ has been investigated using Extended X-ray Absorption Fine Structure Spectroscopy (EXAFS). The results are reported at the silver K-edge. The structure of AgBaP₃O₉ is representative of isolated silver whereas silver pairs have been evidenced in crystalline AgPO₃. The present investigation allows to detect the formation of Ag⁺-Ag⁺ pairs in both glasses. These data arc consistent with crystal chemistry of silver phosphates and the luminescent properties of these materials.