Composition-dependent microhardness and its relationship to bonding in sodium tungsten bronzes

The technique of microhardness measurements using diamond indenters is outlined and assessed for its potential use in quantifying bonding changes and studying reactions in nonstoichiometric crystals. Results are presented for both Vickers and Knoop hardness values on {001} and {011} crystal planes of cubic sodium tungsten bronzes, Na_xWO₃, with x in the range 0.5 to 0.75. The Knoop data show that in only one direction, 〈110〉 on {001}, is the hardness sensitive to changes in composition. Hardness in the 〈110〉 directions and the degree of anisotropy increase as the sodium content of the bronze increases. All the crystal faces examined showed marked anisotropic behavior, with 〈110〉 being about 50% harder than 〈100〉 on {001} faces, while on {011} planes hardness increases in the sequence $\text{〈100〉:〈21}\text{1}\text{〉:〈11}\text{1}\text{〉 ≈ 〈01}\text{1}\text{〉}$. Hardness results from isomorphous and isoelectronic ReO₃ are considered with the Na_xWO₃ data to show the dominant role played by Na⁺WO₃ matrix interactions in determining the properties of these materials. The results are discussed in terms of current bonding theories for bronzes.     

Investigation of the fermi edge in sodium tungsten bronzes by photoelectron spectroscopy

High-resolution photoelectron spectra of sodium tungsten bronzes Na _xWO₃ (0.26<x< 0.76) reveal a linear variation in the density of states at the Fermi energy as a function of sodium content. This behaviour does not appear to arise from filling of a rigid conduction band. Magnetic susceptibilities calculated from photoemission data using a simple independent-electron model agree with measured values.     

A series of lead tungsten bronzes

The phases in samples of gross composition Pb_xWO₃ (0.01 ? x ? 0.28) heated at temperatures between 973 and 1373°K have been investigated. At all temperatures a nonstoichiometric tetragonal tungsten bronze phase forms for compositions x > 0.18. At temperatures up to 1273°K a series of orthorhombic intergrowth bronzes also forms, but these appear to be unstable at higher temperatures and were not found in the preparations made at 1373°K. Aspects of the crystal chemistry of these latter materials are discussed, including structure, crystal habit, valence of the Pb atoms in these phases, and the relation of the phases found here to other related intergrowth bronze phases.     

A study of the sodium tungsten bronzes by high-resolution electron energy loss spectroscopy

Effective masses of conduction electrons in the sodium tungsten bronzes have been determined from the frequency of surface plasmon features in high-resolution electron energy loss spectra. Good agreement is found with effective masses from optical experiments. The results support the view that there is no major depletion in the carrier concentration at the surface of the bronzes.     

Phase transformations of ammonium tungsten bronzes

This article discusses the formation and structure of ammonium tungsten bronzes, (NH₄) _x WO_3−y . As analytical tools, TG/DTA-MS, XRD, SEM, Raman, XPS, and ¹H-MAS NMR were used. The well-known α-hexagonal ammonium tungsten bronze (α-HATB, ICDD 42-0452) was thermally reduced and around 550 °C a hexagonal ammonium tungsten bronze formed, whose structure was similar to α-HATB, but the hexagonal channels were almost completely empty; thus, this phase was called reduced hexagonal (h-) WO₃. In contrast with earlier considerations, it was found that the oxidation state of W atoms influenced at least as much the cell parameters of α-HATB and h-WO₃, as the packing of the hexagonal channels. Between 600 and 650 °C reduced h-WO₃ transformed into another ammonium tungsten bronze, whose structure was disputed in the literature. It was found that the structure of this phase—called β-HATB, (NH₄)_0.001WO_2.79—was hexagonal.     

The formation of a series of barium tungsten bronzes

The phases occurring in samples of gross composition Ba_xWO₃ (0.01 < x < 0.33) heated at temperatures between 1073 and 1373°K have been determined using X-ray diffraction and electron microscopy. At all temperatures a tetragonal tungsten bronze phase with a narrow homogeneity range of x = 0.20?0.21 was observed to form. In addition, at temperatures up to 1273°K, a series of orthorhombic intergrowth bronzes forms within a restricted composition range around x = 0.04. The latter phases are unstable at higher temperatures and were not found in preparations made at 1323°K. Similarly a new type of bronze phase forms at x = 0.14?0.16 at temperatures up to 1323°K, but not at 1373°K. The structure of this phase is unknown. Aspects of the crystal chemistry of the barium bronzes and the relationships to other bronze phases are discussed.     

Preparation and structure of mixed tungsten bronzes containing tin and europium

An isothermal section has been established for the system Eu_xWO₃Sn_yWO₃ at 1000°C from X-ray powder and single-crystal data and Mössbauer spectroscopy and chemical analyses. A single-phase region with the (12 × 3) tetragonal structure has been established. Single-crystal X-ray analysis shows that the Sn and Eu occupy pentagonal tunnel sites. Mössbauer spectroscopy shows that both Sn(II) and Eu(II) are present. The extent of the single-phase field is controlled by two factors: First, the ratio $\text{Eu}\text{Sn}$ must be <1, and second, the supernumerary electron concentration per WO₃ must be <0.6.     

Domain and surface structures of sodium tungsten bronzes,NaxWo3 (0.4 < x < 1)

Polarized-light microscopic observations have shown that the birefringent, twin-domain structure of metallic sodium tungsten bronze is exhibited by Na-deficient surface films and hence is not, as had been reported elsewhere, a bulk property. The film can be synthesized by anodic electrolysis in alkaline solution. It is chemically inert, translucent, and often laminates to a multiple layer. The domain structure of the film is hypersensitive to lateral stress and to thermal variation, exhibiting a marked change at the phase transition of the substrate through apparent epitaxial coherence. The domain-wall movement is often slow enough to be visible, and the thermally induced domain modulation is occasionally accompanied by audible high-pitched sound. The bulk structure of the substrate exhibits pseudoperiodic subboundaries that are probably caused by growth defects and the segregation of the sodium atoms. The near-surface of the substrate also shows the sodium segregation that tends to precipitate in periodic patterns. Optical and morphological properties of the substrate structures exhibited no detectable change due to thermal variation or external stress.     

The reaction of cubic sodium tungsten bronzes,NaxWO3, with metallic iron

The reaction of the cubic sodium bronzes, Na_xWO₃, with powdered iron metal has been studied by heating samples in vacuo and also at high pressure. Evidence for reaction is found at unexpectedly low temperatures. The reaction is an overall reduction which proceeds via an increase in the sodium content of the bronze phase up to some temperature-dependent limiting composition for which x < 1. The existence of this limit, its temperature dependence, and the identity of the other products of reduction have been explained in terms of the partial oxygen pressure of the system. The course of the reduction has been followed through the evolution of the bronze lattice parameter and a reaction mechanism is postulated. No evidence of significant incorporation of iron into a stable cubic sodium bronze phase has been found.     

Thermal neutron activation analysis for potassium,holmium and lanthanum in tungsten bronzes

Methods for the analysis of potassium, holmium and lanthanum in their tungsten bronzes, M _x WO₃, are described. Simultaneous thermal neutron activation of samples and comparison standards was followed by counting with Ge(Li) diodes. Analysis was performed by comparing appropriate full energy peaks in the spectra of the three elements with peaks in the¹⁸⁷W spectrum for sample and standard. Results with accuracy of 1 to 3% were obtained. Work was performed in the Ames Laboratory of the U.S. Atomic Energy Commission. Contribution No. 2700.     

Phase equilibria,thermal analysis,and reactivity of tin tungsten bronzes and related phases

Crystal chemistry and phase relations of the bronze forming region of the SnWO system have been investigated. Above 780°C the tin bronzes Sn_xWO₃ are shown to be thermally unstable and an equilibrium diagram is established at 700°C which shows that the composition limits of the tetragonal phase are 0.21 ? x ? 0.29. Below x = 0.21 a series of single and two phase regions containing orthorhombic bronzes exists for which the composition limits have been established. In the range 0.29 ? x ? 0.76 the system comprises the tetragonal bronze, Sn₂W₃O₈ and SnWO₄, while above 0.76 there is no bronze, only Sn₂W₃O₈, SnWO₄ and free Sn. The phase Sn₂W₃O₈ has been isolated and shown to have a hexagonal unit cell, a = 7.696 Å, c = 18.654 Å. The evidence of differential thermal analysis and X-ray studies suggests that this hexagonal phase arises from the decomposition of the tungsten bronze phase and is itself decomposed to cubic SnWO₄ above 700°C. Small thermal effects observed in the DTA scans of tin-containing tetragonal bronzes are interpreted in terms of an order-disorder phenomenon arising from asymmetric tunnel occupancy by Sn²⁺ ions caused by the presence of the lone pair of electrons. Hydrogen reduction of Sn_xWO₃ has been shown to result in complete removal of oxygen, producing Sn + α-W in the range 600–850°C. Some activation energy data are given for the reduction process.     

Migratory insertion of isocyanide into a ketenyl–tungsten bond as key step in cyclization reactions

Treatment of the side-on tungsten alkyne complex of ethinylethyl ether [Tp*W(CO)₂(η²-C,C′-HCCOCH₂CH₃)]⁺ {Tp* = hydridotris(3,4,5-trimethylpyrazolyl)borate} (2a) with n-Bu₄NI afforded the end-on ketenyl complex [Tp*W(CO)₂(κ¹-HCCO)] (4a). This formal 16 ve complex bearing the prototype of a ketenyl ligand is surprisingly stable and converts only under activation by UV light or heat to form a dinuclear complex [Tp*₂W₂(CO)₄(μ-CCH₂)] (6). The ketenyl ligand in complex 4a underwent a metal template controlled cyclization reaction upon addition of isocyanides. The oxametallacycles [Tp*W(CO)₂{κ²-C,O-C(NHXy)C(H)C(Nu)O}] {Nu = OMe (7), OEt (8), N(i-Pr)₂ (9), OH (10), O_1/2 (11)} were formed by coordination of Xy-NC (Xy = 2,6-dimethylphenyl) at 4a and subsequent migratory insertion (MI) into the W-ketenyl bond. The resulting intermediate is susceptible to addition reactions with protic nucleophiles. Compounds 2a-PF₆, 4a/b, and 7–11 were fully characterized including XRD analysis. The cyclization mechanism has been confirmed both experimentally and by DFT calculations. In cyclic voltammetry, complexes 7–9 are characterized by a reversible W(ii)/W(iii) redox process. The dinuclear complex 11 however shows two separated redox events. Based on cyclic voltammetry measurements with different conducting electrolytes and IR spectroelectrochemical (SEC) measurements the W(ii)/W(iii) mixed valent complex 11⁺ is assigned to class II in terms of the Robin-Day classification.

The prototype ketenyl ligand is bound end-on despite a formal 16 valence electron count at the metal. This situation opens a reaction pathway for a multicomponent cyclization centred on the migration of the ketenyl ligand.     

Lithium insertion chemistry of some iron vanadates

Lithium insertion into various iron vanadates has been investigated. Fe₂V₄O₁₃ and Fe₄(V₂O₇)₃ · 3H₂O have discharge capacities approaching 200 mAh g⁻¹ above 2.0 V vs. Li⁺/Li. Although the potential profiles change significantly between the first and subsequent discharges, capacity retention is unexpectedly good. Other phases, structurally related to FeVO₄, containing copper and/or sodium ions were also studied. One of these, β-Cu₃Fe₄(VO₄)₆, reversibly consumes almost 10 moles of electrons per formula unit (ca. 240 mAh g⁻¹) between 3.6 and 2.0 V vs. Li⁺/Li, in a non-classical insertion process. It is proposed that both copper and vanadium are electrochemically active, whereas iron(III) reacts to form LiFe^IIIO₂. The capacity of the Cu₃Fe₄(VO₄)₆/Li system is nearly independent of cycling rate, stabilizing after a few cycles at 120–140 mAh g⁻¹. Iron vanadates exhibit better capacities than their phosphate analogues, whereas the latter display more constant discharge potentials.     

The structures of intergrowth tungsten bronzes of Ba,Sn, Pb,and Sb

The structures of intergrowth tungsten bronzes (ITB) of compositions Ba_0.04WO₃, Sn_0.04WO₃, Pb_0.04WO₃, Sn_0.18WO₃, and Sb_0.25WO₃ have been deduced from high-resolution electron microscope images. Both the Pb_0.04, Sn_0.04, and Ba_0.04, ITB phases consist of single rows of hexagonal tunnels occupied by Pb, Sn, or Ba atoms intergrown in a WO₃-like matrix. The Sb_0.25, ITB phase is composed of similar rows of Sb-containing single hexagonal tunnels, the centers of which are separated by a WO₃-like matrix only two octahedra in thickness. The structure of the Sn_0.18, ITB phase consists of double rows of hexagonal tunnels containing Sn atoms joined by a single strip of WO₃-like octahedra. The structures are compared with the structures of other known ITB phases and the nonstoichiometric behavior of these phases is discussed.     

Catalase activity of coarse-grained and nanosized oxide tungsten bronzes obtained by electrolysis of molten salts

Nanocrystalline samples of K _x Li _y WO₃ with hexagonal structure obtained by electrodeposition of molten salts were several times more active in catalytic decomposition than the coarse-grained materials.     

Uncommon carbene insertion reactions

Transition-metal-catalysed carbene insertion reaction is a straightforward and efficient protocol for the construction of carbon–carbon or carbon–heteroatom bonds. Compared to the intensively studied and well-established “common” carbene insertion reactions, including carbene insertion into C–H, Si–H, N–H, O–H, and S–H bonds, several “uncommon” carbene insertion reactions, including carbene insertion into B–H, Sn–H, Ge–H, P–H, F–H, C–C, and M–M bonds, have been neglected for a long time. However, more and more studies on uncommon carbene insertion reactions have been disclosed recently, and clearly demonstrate the great synthetic potential of these reactions. The current perspective reviews the history and the newest advances of uncommon carbene insertion reactions, discusses their potential applications and challenges, and also presents an outlook of this promising field.

Transition-metal-catalysed carbene insertion reaction is a straightforward and efficient protocol for the construction of carbon–carbon or carbon–heteroatom bonds.