Electronic structure of rhenium oxides

The photoelectron spectra of Re 4? electrons observed in the course of reduction of ReO₃ and oxidation of metallic rhenium are reported. On the basis of SCF-Xα calculations for double-octahedral models of different rhenium oxides, the formation of multiple metal-metal bonds in clusters of edge-sharing octahedra on shear planes and the presence of 2+ and 5+ apparent oxidation states in ReO₂ and ReO₃ respectively, are inferred.     

Niobium based tetragonal tungsten bronzes as potential anodes for solid oxide fuel cells: synthesis and electrical characterisation

In this paper we report studies on a range of niobate based tungsten bronzes, with a view to analysing their potential as anode materials in SOFCs. Six systems were studied, (Sr_1−xBa_x)_0.6Ti_0.2Nb_0.8O₃, Sr_0.6−xLa_xTi_0.2+xNb_0.8−xO₃, (Sr_0.4−xBa_x)Na_0.2NbO₃, (Ba_1−xCa_x)_0.6Ti_0.2Nb_0.8O₃, Ba_0.5−xA_xNbO₃ (A=Ca, Sr), and Ba_0.3NbO_2.8, and the electrical conductivities were examined over a range of oxygen partial pressures (10⁻²⁰–1 bar). All the systems showed good conductivity in low oxygen partial pressures, with values as high as 8 S cm⁻¹ at 930°C (P(O₂)=10⁻²⁰ bar). As the oxygen partial pressure was raised the conductivity dropped showing in most cases an approximate [P(O₂)]^−1/4 dependence and good re-oxidation kinetics. Of all the samples studied the (Sr_1−xBa_x)_0.6Ti_0.2Nb_0.8O₃ and (Ba_1−xCa_x)_0.6Ti_0.2Nb_0.8O₃ systems appear most promising for potential use as anode materials in SOFCs.     

Electronic structure of AgNbO3 and NaNbO3 studied by X-ray photoelectron spectroscopy

The XPS examinations of the AgNbO₃ and NaNbO₃ single crystals and ceramics allowed estimate their average composition as Ag_1.1Nb_0.9O₃ and Na_1.2Nb_0.9O_2.9. The valence bands of the AgNbO₃ compound, formed mainly of the Nb 4d, Ag 4d and O 2p states, show an energy gap about 3 eV while for the NaNbO₃ compound consist of the O 2p states hybridized with the Nb 3d states and show an energy gap about 4 eV. The chemical shifts of these compounds suggest a mixed ionic and covalent character of the bonds. The broadening of the core level lines of AgNbO₃ suggests a stronger structural disorder in comparison with NaNbO₃ compound.     

Electronic structure of superconducting Cu oxides

On the basis of photoemission results on La_2-xSr_xCuO_4-y and YBa₂Cu₃O_y it is suggested that carriers doped in the otherwise insulating samples are oxygen p holes rather than Cu d holes. The p holes interact with the localized Cu d states through hybridization and may give rise to an anomaly near the Fermi surface as described by a periodic Anderson model with low carrier density.     

Electronic structure of semiconductor surfaces

Our present understanding of the electronic structure of semiconductor surfaces is reviewed. It is shown that photoemission and inverse photoemission are ideal techniques for probing occupied and unoccupied electronic states, respectively. All quantum numbers of an electron can be determined, i.e., energy, momentum, spin and angular symmetries. For simple systems, such as clean ordered surfaces with a small unit cell it is possible to understand the electronic structure from first-principles calculations. For complex systems, such as encountered during oxidation and dry etching one is restricted to measuring the properties determined by short-range order. Core level spectroscopy with synchrotron radiation is able to determine the oxidation state and the local bonding of surface and interface atoms.     

Interband coupling and enhanced Tc in the tungsten bronzes

Examining the possibility that the enhancement of T_c in the tungsten bronzes arises from interband coupling, we find that a single density of states provides reasonable quantitative agreement with the T_c enhancement, the heat capacity and the magnetic susceptibility.     

Electronic structure and interactions in well-defined coadsorbate layers

A synopsis of results and models for weakly and strongly interacting, well-defined coadsorbate systems is presented. Some experimental examples are given for various adsorbates on Ni(111) representing weak (different N₂ states, Ar/Xe+N₂) as well as strong (N₂+K) interaction. Models for coadsorbate interaction, in particular between molecules (CO, N₂) and alkali atoms, are discussed.     

Charge-density-wave instabilities and quantum transport in the monophosphate tungsten bronzes with m = 5 alternate structure

Resistivity, thermoelectric power and magnetotransport measurements have been performed on single crystals of the quasi two-dimensional monophosphate tungsten bronzes (PO₂)₄(WO₃)_2m for m =5 with alternate structure, between 0.4 K and 500 K, in magnetic fields of up to 36 T. These compounds show one charge density instability (CDW) at 160 K and a possible second one at 30 K. Large positive magnetoresistance in the CDW state is observed. The anisotropic Shubnikov-de Haas and de Haas-van Alphen oscillations detected at low temperatures are attributed to the existence of small electron and hole pockets left by the CDW gap openings. Angular dependent magnetoresistance oscillations (AMRO) have been found at temperatures below 30 K. The results are discussed in terms of a weakly corrugated cylindrical Fermi surface. They are shown to be consistent with a change of the Fermi surface below 30 K. Received 23 November 1999 and Received in final form 23 March 2000     

Assembly of tungsten oxide nanobundles and their electrochromic properties

Lenticular W₁₈O₄₉ nanobundles composed of ultra-thin nanowires with diameters of 5-10 nm have been synthesized through a simple solvothermal method with hexachloride as precursor and mixed cyclohexanol and ethanol as solvent. Electrochromic films were prepared by assembling the W₁₈O₄₉ nanobundle suspension onto tin-doped indium oxide (ITO) coated glass. Results showed that self-assembly of the W₁₈O₄₉ nanobundles was strongly influenced by the solvents employed to disperse the nanobundles. The W₁₈O₄₉ nanobundles coated films exhibited excellent electrochromic stability and reversibility. The W₁₈O₄₉ nanobundle films also showed much higher charge-insertion density compared with the WO₃ nanorod film, which may be due to the ultrathin feature of single nanowires constituting the nanobundles, unique oxygen vacancies of monoclinic W₁₈O₄₉, and the highly ordered assembly of the nanobundles.     

Electronic structure and band gap of the negative charge-transfer material Sr3Fe2O7

We studied the electronic structure of the Sr₃Fe₂O₇ compound using X-ray photoelectron spectroscopy (XPS). The charge-transfer satellites in the Fe 2p XPS spectra were analyzed using standard cluster model calculations. The analysis indicates that Sr₃Fe₂O₇ is in the negative charge-transfer regime, and that the ground state is dominated by the configuration (where denotes an O 2p hole in the oxygen band). These results are similar to those found in the related SrFeO₃ and Sr₂FeO₄ compounds. The band gap of the Sr₃Fe₂O₇ compound is split off by the relatively large value of the p-d transfer integral T_σ. The lowest lying excitations are and consequently the band gap is of the p-p type. The band gap in the Sr_n+1Fe_nO_3n+1 series can be understood taking into account the trend in the O 2p bandwidths.     

Electronic structure of PdAg(1 0 0) ordered surface alloys using synchrotron radiation

PdAg(1 0 0) ordered surface alloys have been prepared by e-beam evaporation technique in situ in UHV and were investigated using high energy photoemission and normal incidence X-ray standing wave (NIXSW) techniques with wiggler radiation. The line shapes of 3d core levels of Ag and Pd exhibited interesting changes with the increase of Pd concentration in the alloys. The Ag 3d core level exhibited a disorder induced broadening that gradually increased with Pd concentration. On the other hand Pd 3d core level exhibited an asymmetry on the high binding energy side of the peak which gradually increased with Pd concentration. Interestingly, Pd core level did not exhibit a broadening with increase of Pd content in the alloy.     

Electro-optic spectroscopy of electrochromic processes in tungsten trioxide

The nature of the electrochromic processes in solid-state electrochromic devices (ECD) ITO/a-WO₃/a-SiO₂/Au is investigated by analysing their electro-optical behavior during the current pulses and the linear sweeping of the bias voltage. It is shown that the character of the current-coltage curve of the ECD is determined by passivation, by hydrogen/oxygen evolution on the counter electrode (Au) and by redox reactions on the electrochronic electrode (amorphous WO₃ film: a-WO₃). The light transmittance versus voltage and the high-frequency differential impedance of the active component versus voltage curves are dependent mainly on electrochromic reactions.     

From beads-to-wires-to-fibers of tungsten oxide: electrochromic response

Suitable host lattice and morphology for easy intercalation and deintercalation process are crucial requirements for electrochromic device. In this investigation, the evolution of structural and morphological changes and their effect on electrochromic (EC) properties of spray-deposited WO₃ thin films are studied. Films of different morphologies were deposited from an ammonium tungstate precursor solution using a novel pulsed spray pyrolysis technique (PSPT) on tin-doped indium oxide (ITO) coated glass substrates by varying quantity of spraying solution. Interesting morphological transition from beads-to-wires-to-fibers as a function of quantity of sprayed solution has been demonstrated. The porosity, crystallinity and “open” structures in the films consisting of beads, wires, and fiber-like morphology enabled us to correlate these aspects to their EC performance. WO₃ films comprising wire-like morphology (20 cc spraying quantity) exhibited better EC properties both in terms of coloration efficiency (42.7 cm²/C) and electrochemical stability (10³ colored/bleached cycles) owing to their adequate open structure, porosity, and amorphicity, compared with the films having bead/fiber-like morphology.     

Synthesis and photoelectronic properties of polycristalline tantalum_substituted hexagonal potassium tungsten bronzes

Tantalum-substituted hexagonal potassium tungsten bronzes K_0.33Ta_yW_1?yO₃ (0.20 ? y ? 0.33) have been prepared and the lattice parameters determined. Values for the band gap were obtained from plots of ( rhν ) $\text{1}\text{2}$ against hν, where η is the quantum efficiency. Band gap variation with tantalum content is discussed.     

Aspects of the crystal growth of the intergrowth tungsten bronzes

The crystal growth of a few of the members of the family of the rubidium intergrowth tungsten bronzes has been investigated as part of a program to study the compositionally induced metal to insulator transition in this bronze system. Progress in the study of this nonstoichiometric material has been made possible by successful growth of large single crystal samples. Application of vapor-assisted seeded crystal growth has led to an increase of the product crystal size by two orders of magnitude, from 0.1 mm on an edge to about 2 cm on an edge. Scanning electron microscopy has been used to characterize cracks, voids and intergrowths in this material. Transmission electron microscopy has been used to image the lattice of this quasi-two dimensional material in order to observe the frequency and amplitude of stacking defects.     

Raman active soft modes and phase transitions in sodium tungsten bronzes

The temperature dependence of the Raman spectra of Na_xWO₃ for x > 0.5 is reported. A group theoretical analysis of the data confirms the space group assignments of Clarke⁸ and supports the existence of several phases derived from rigid tilts of the WO₆ octahedra. The phase diagram for high concentration Na_xWO₃ deduced from Raman measurements is in excellent agreement with that obtained from x-ray results particularly with regard to the previously undetected tetragonal to tetragonal transition.     

Study of the electronic structure of rhenium and tungsten oxides on the O K-edge

Oxygen K-edge x-ray absorption spectra were studied on the electrochromic amorphous thin film a-WO₃ in the comparison with crystalline oxides having variable electronic (d⁰, d¹, d²) and atomic structure: monoclinic m-WO₃ (insulator - d⁰), cubic Na_0.6WO₃ (metal - d¹), cubic ReO₃ (metal - d¹), layered-type hexagonal h-WO₃, WO₃H₂O and with intercalated H_xReO₃ (metal - d²), H_xWO₃ oxides having a metal-isulating transition. The changes in the XANES range 10–15 eV above the absorption edge are interpreted based on the known band-structure calculations. The high-energy features are related to the multiple-scattering processes (EXAFS) on the nearest atoms. The intensity of the feature at 550–560 eV is attributed for the first time to the value of the metal-oxygen-metal bond angle. Paper presented at the 5th Euroconference on Solid State Ionics, Benalmádena, Spain, Sept. 13–20, 1998.