Electron paramagnetic resonance studies of some ternary oxides of copper (II)

The electron paramagnetic resonance (EPR) of ternary oxides of Cu(II) has been studied between 4.2 and 300 K. The systems include those with 180 degrees Cu-O-Cu interactions (such as Ln2CuO4, Sr2CuO2Cl2, Sr2CuO3 and Ca2CuO3) or 90 degrees Cu-O-Cu interactions (such as Y2Cu2O5 or BaCuO2) as well as those in which the Cu2+ ions are isolated (such as Y2BaCuO5, La1.8Ba1.2Cu0.9O4.8 and Bi2CuO4). The change in the EPR susceptibility as a function of temperature is compared with that of the DC magnetic susceptibility. Compounds with extended 180 degrees Cu-O-Cu interactions which have a low susceptibility also do not give EPR signals below room temperature. For compounds such as Ca2CuO3 with one-dimensional 180 degrees Cu-O-Cu interactions a weak EPR signal is found the temperature dependence of which is very different from that of the DC susceptibility. For Y2BaCuO5 as well as for La1.8Ba1.2Cu0.9O4.8 the EPR susceptibility as well as its temperature variation are comparable with those of the static susceptibility near room temperature but very different at low temperatures. Bi2CuO4 also shows a similar behaviour. In contrast, for Y2Cu2O5, in which the copper ions have a very distorted nonsquare-planar configuration, the EPR and the static susceptibility show very similar temperature dependences. In general, compounds in which the copper ions have a square-planar geometry give no EPR signal in the ground state (0 K) while those with a distortion from square-planar geometry do give a signal. The results are analysed in the light of recent MS Xalpha calculations on CuO46- square-planar clusters with various Cu-O distances as well as distortions. It is suggested that in square-planar geometry the ground state has an unpaired electron in anionic orbitals which is EPR inactive. Competing interactions from other cations, an increase in Cu-O distance or distortions from square-planar geometry stabilise another state which has considerably more Cu 3d character. These states are EPR active. Both these states, however, are magnetic. For isolated CuO46- clusters the magnetic interactions seem to involve only the states which have mainly anionic character.

A study of cubic bismuth oxides of the type bi(26-X)m(X)O(40-delta) (m=ti, mn, fe, co, ni or pb) related to gamma-bi2O3

A structural investigation of cubic oxides (space group I23) of the formula Bi(26-x)M(x)O(40-delta) (M = Ti, Mn, Fe, Co, Ni and Pb) related to the Y-Bi2O3 phase has been carried out by the Rietveld profile analysis of high-resolution X-ray powder diffraction data in order to establish the cation distributions. Compositional dependence of the cation distribution has been examined in the case of Bi26-xCoxO40-delta (1 < x < 16). The study reveals that in Bi(26-X)M(X)O(40-delta) with M = Ti, Mn, Fe, Co or Pb, the M cations tend to occupy tetrahedral (2a) sites when x < 2 while the octahedral (24f) sites are shared by the excess Co or Ni cations with Bi atoms when x > 2. Also experimental magnetic moments of Mn, Co and Ni derivatives have been used to establish the valence state and distribution of these cations.

A new approach for understanding ion transport in glasses; example of complex alkali diborate glasses containing lead, bismuth and tellurium oxides

Mechanism of ion transport in glasses continues to be incompletely understood. Several of the theoretical models in vogue fail to rationalize conductivity behaviour when d.c. and a.c. measurements are considered together. While they seem to involve the presence of at least two components in d.c. activation energy, experiments fail to reveal that feature. Further, only minor importance is given to the influence of structure of the glass on the ionic conductivity behaviour. In this paper, we have examined several general aspects of ion transport taking the example of ionically conducting glasses in pseudo binary, yNa(2)B(4)O(7)center dot(1-y) M (a) O (b) (with y = 0 center dot 25-0 center dot 79 and M (a) O (b) = PbO, TeO2 and Bi2O3) system of glasses which have also been recently characterized. Ion transport in them has been studied in detail. We have proposed that non-bridging oxygen (NBO) participation is crucial to the understanding of the observed conductivity behaviour. NBO-BO switching is projected as the first important step in ion transport and alkali ion jump is a subsequent event with a characteristically lower barrier which is, therefore, not observed in any study. All important observations in d.c. and a.c. transport in glasses are found consistent with this model.

Effect of various alkaline-earth metal oxides on the broadband infrared luminescence from bismuth-doped silicate glasses

We report on the effect of various alkaline-earth metal oxides on the broadband infrared luminescence covering 1000-1600 nm wavelength region from bismuth-doped silicate glasses. The full width at half maximum (FWHM) of the infrared luminescence and the fluorescent lifetime is more than 200 nm and 400 mu s, respectively. The fluorescent intensity decreases with increasing basicity of host glasses. Besides the broadband infrared luminescence, luminescence centered at 640 nm was also observed, which should be ascribed to Bi2+ rather than to the familiar Bi3+. We suggest that the infrared luminescence should be assigned to the X-2 (2)Pi (3/2) -> X-1 (2)Pi(1/2) transition of BiO molecules dispersed in the host glasses. (c) 2006 Elsevier Ltd. All rights reserved.

Bismuth embrittlement of copper is an atomic size effect

Embrittlement by the segregation of impurity elements to grain boundaries is one of a small number of phenomena that can lead to metallurgical failure by fast fracture(1). Here we settle a question that has been debated for over a hundred years(2): how can minute traces of bismuth in copper cause this ductile metal to fail in a brittle manner? Three hypotheses for Bi embrittlement of Cu exist: two assign an electronic effect to either a strengthening(3) or weakening(4) of bonds, the third postulates a simple atomic size effect(5). Here we report first principles quantum mechanical calculations that allow us to reject the electronic hypotheses, while supporting a size effect. We show that upon segregation to the grain boundary, the large Bi atoms weaken the interatomic bonding by pushing apart the Cu atoms at the interface. The resolution of the mechanism underlying grain boundary weakening should be relevant for all cases of embrittlement by oversize impurities.

Identification of copper species present in Cu-ZSM-5 catalysts for NOx reduction

The structure of Cu-ZSM-5 catalysts that show activity for direct NO decomposition and selective catalytic reduction of NOx by hydrocarbons has been investigated by a multitude of modern surface analysis and spectroscopy techniques including X-ray photoelectron spectroscopy, thermogravimetric analysis, and in situ Fourier transform infrared spectroscopy. A series of four catalysts were prepared by exchange of Na-ZSM-5 with dilute copper acetate, and the copper loading was controlled by variation of the solution pH. Underexchanged catalysts contained isolated Cu2+OH-(H2O) species and as the copper loading was increased Cu2+ ions incorporated into the zeolite lattice appeared. The sites at which the latter two copper species were located were fundamentally different. The Cu2+OH-(H2O) moieties were bound to two lattice oxygen ions and associated with one aluminum framework species. In contrast, the Cu2+ ions were probably bound to four lattice oxygen ions and associated with two framework aluminum ions. Once the Cu-ZSM-5 samples attained high levels of exchange, the development of [Cu(μ-OH)2Cu]n2+OH-(H2O) species along with a small concentration of Cu(OH)2 was observed. On activation in helium to 500°C the Cu2+OH-(H2O) species transformed into Cu2+O- and Cu+ moieties, whereas the Cu2+ ions were apparently unaffected by this treatment (apart from the loss of ligated water molecules). Calcination of the precursors resulted in the formation of Cu2+O2- and a one-dimensional CuO species. Temperature-programmed desorption studies revealed that oxygen was removed from the latter two species at 407 and 575°C, respectively. © 1999 Academic Press.

Nanostructured copper oxide semiconductors : a perspective on materials, synthesis methods and applications

The oxides of copper (CuxO) are fascinating materials due to their remarkable optical, electrical, thermal and magnetic properties. Nanostructuring of CuxO can further enhance the performance of this important functional material and provide it with unique properties that do not exist in its bulk form. Three distinctly different phases of CuxO, mainly CuO, Cu2O and Cu4O3, can be prepared by numerous synthesis techniques including, vapour deposition and liquid phase chemical methods. In this article, we present a review of nanostructured CuxO focusing on their material properties, methods of synthesis and an overview of various applications that have been associated with nanostructured CuxO.

Sol-gel synthesis and characterization of cubic bismuth zinc niobium oxide nanopowders

Bismuth zinc niobium oxide (BZN) was successfully synthesized by a diol-based sol-gel reaction utilizing metal acetate and alkoxide precursors. Thermal analysis of a liquid suspension of precursors suggests that the majority of organic precursors decompose at temperatures up to 150°C, and organic free powders form above 350°C. The experimental results indicate that a homogeneous gel is obtained at about 200°C and then converts to a mixture of intermediate oxides at 350–400°C. Finally, single-phased BZN powders are obtained between 500 and 900°C. The degree of chemical homogeneity as determined by X-ray diffraction and EDS mapping is consistent throughout the samples. Elemental analysis indicates that the atomic ratio of metals closely matches a Bi1.5ZnNb1.5O7 composition. Crystallite sizes of the BZN powders calculated from the Scherrer equation are about 33–98 nm for the samples prepared at 500–700°C, respectively. The particle and crystallite sizes increase with increased sintering temperature. The estimated band gap of the BZN nanopowders from optical analysis is about 2.60–2.75 eV at 500-600°C. The observed phase formations and measured results in this study were compared with those of previous reports.

Electrochemical restructuring of copper surfaces using organic additives and its effect on the electrocatalytic reduction of nitrate ions

This work describes the fabrication of nanostructured copper electrodes using a simple potential cycling protocol that involves oxidation and reduction of the surface in an alkaline solution. It was found that the inclusion of additives, such as benzyl alcohol and phenylacetic acid, has a profound effect on the surface oxidation process and the subsequent reduction of these oxides. This results in not only a morphology change, but also affects the electrocatalytic performance of the electrode for the reduction of nitrate ions. In all cases, the electrocatalytic performance of the restructured electrodes was significantly enhanced compared with the unmodified electrode. The most promising material was formed when phenylacetic acid was used as the additive. In addition, the reduction of residual oxides on the surface after the modification procedure to expose freshly active reaction sites on the surface before nitrate reduction was found to be a significant factor in dictating the overall electrocatalytic activity. It is envisaged that this approach offers an interesting way to fabricate other nanostructured electrode surfaces.

Energy band engineering of complex metal oxides

This doctoral studies focused on the development of new materials for efficient use of solar energy for environmental applications. The research investigated the engineering of the band gap of semiconductor materials to design and optimise visible-light-sensitive photocatalysts. Experimental studies have been combined with computational simulation in order to develop predictive tools for a systematic understanding and design on the crystal and energy band structures of multi-component metal oxides.

Influence of lanthanum doping on the dielectric, ferroelectric and relaxor behaviour of barium bismuth titanate ceramics

Barium lanthanum bismuth titanate (Ba1−(3/2)xLaxBi4Ti4O15, x = 0–0.4) ceramics were fabricated using the powders synthesized via the solid-state reaction route. X-ray powder diffraction analysis confirmed the above compositions to be monophasic and belonged to the m = 4 member of the Aurivillius family of oxides. The effect of the partial presence of La3+ on Ba2+ sites on the microstructure, dielectric and relaxor behaviour of BaBi4Ti4O15 (BBT) ceramics was investigated. For the compositions pertaining to x ≤ 0.1, the dielectric constant at both room temperature and in the vicinity of the temperature of the dielectric maximum (Tm) of the parent phase (BBT) increased significantly with an increase in x while Tm remained almost constant. Tm shifted towards lower temperatures accompanied by a decrease in the magnitude of the dielectric maximum (εm) with an increase in the lanthanum content (0.1 < x ≤ 0.4). The dielectric relaxation was modelled using the Vogel–Fulcher relation and a decrease in the activation energy for frequency dispersion with increasing x was observed. The frequency dispersion of Tm was found to decrease with an increase in lanthanum doping, and for compositions corresponding to x ≥ 0.3, Tm was frequency independent. Well-developed P(polarization)–E(electric field) hysteresis loops were observed at 150 °C for all the samples and the remanent polarization (2Pr) was improved from 6.3 µC cm−2 for pure BBT to 13.4 µC cm−2 for Ba0.7La0.2Bi4Ti4O15 ceramics. Dc conductivities and associated activation energies were evaluated using impedance spectroscopy.

Hole Pairing In Antiferromagnetically Correlated Layered Oxides

We have considered a two-band Hubbard model having interlaced Cu-3d(x2−y2) and O-2p(x, y) orbitals representing the CuO2 square planes. Simple CuO2 -cluster calculation suggests that the additional holes created by doping stay mainly on oxygen. Motion of an oxygen hole interlacing with the antiferromagnetically correlated background of copper spins, creates a string of high energy spin configuration of finite length giving mass renormalization. Another hole of opposite spin can now anneal this string tension providing a triangular pairing potential for large pair momentum. The latter implies unusual Bose condensation of the wake-bound compact Bose-like pairs on a non-zero momentum shell. Effect of disorder favouring condensation at the mobility edge is pointed out.

Radical Enhanced Atomic Layer Deposition of Metals and Oxides

Atomic Layer Deposition (ALD) is a chemical, gas-phase thin film deposition method. It is known for its ability for accurate and precise thickness control, and uniform and conformal film growth. One area where ALD has not yet excelled is film deposition at low temperatures. Also deposition of metals, besides the noble metals, has proven to be quite challenging. To alleviate these limitations, more aggressive reactants are required. One such group of reactants are radicals, which may be formed by dissociating gases. Dissociation is most conveniently done with a plasma source. For example, dissociating molecular oxygen or hydrogen, oxygen or hydrogen radicals are generated. The use of radicals in ALD may surmount some of the above limitations: oxide film deposition at low temperatures may become feasible if oxygen radicals are used as they are highly reactive. Also, as hydrogen radicals are very effective reducing agents, they may be used to deposit metals. In this work, a plasma source was incorporated in an existing ALD reactor for radical generation, and the reactor was used to study five different Radical Enhanced ALD processes. The modifications to the existing reactor and the different possibilities during the modification process are discussed. The studied materials include two metals, copper and silver, and three oxides, aluminium oxide, titanium dioxide and tantalum oxide. The materials were characterized and their properties were compared to other variations of the same process, utilizing the same metal precursor, to understand what kind of effect the non-metal precursor has on the film properties and growth characteristics. Both metals were deposited successfully, and silver for the first time by ALD. The films had low resistivity and grew conformally in the ALD mode, demonstrating that the REALD of metals is true ALD. The oxide films had exceptionally high growth rates, and aluminium oxide grew at room temperature with low cycle times and resulted in good quality films. Both aluminium oxide and titanium dioxide were deposited on natural fibres without damaging the fibre. Tantalum oxide was also deposited successfully, with good electrical properties, but at slightly higher temperature than the other two oxides, due to the evaporation temperature required by the metal precursor. Overall, the ability of REALD to deposit metallic and oxide films with high quality at low temperatures was demonstrated.

Unusual effects of anisotropic bonding in Cu(II) and Ni(II) oxides with K2NiF4 structure

Geometric constraints present in A2BO4 compounds with the tetragonal-T structure of K2NiF4 impose a strong pressure on the B---OII---B bonds and a stretching of the A---OI---A bonds in the basal planes if the tolerance factor is t congruent with RAO/√2 RBO < 1, where RAO and RBO are the sums of the A---O and B---O ionic radii. The tetragonal-T phase of La2NiO4 becomes monoclinic for Pr2NiO4, orthorhombic for La2CuO4, and tetragonal-T′ for Pr2CuO4. The atomic displacements in these distorted phases are discussed and rationalized in terms of the chemistry of the various compounds. The strong pressure on the B---OII---B bonds produces itinerant σ*x2−y2 bands and a relative stabilization of localized dz2 orbitals. Magnetic susceptibility and transport data reveal an intersection of the Fermi energy with the d2z2 levels for half the copper ions in La2CuO4; this intersection is responsible for an intrinsic localized moment associated with a configuration fluctuation; below 200 K the localized moment smoothly vanishes with decreasing temperature as the d2z2 level becomes filled. In La2NiO4, the localized moments for half-filled dz2 orbitals induce strong correlations among the σ*x2−y2 electrons above Td reverse similar, equals 200 K; at lower temperatures the σ*x2−y2 electrons appear to contribute nothing to the magnetic susceptibility, which obeys a Curie-Weiss law giving a μeff corresponding to S = 1/2, but shows no magnetic order to lowest temperatures. These surprising results are verified by comparison with the mixed systems La2Ni1−xCuxO4 and La2−2xSr2xNi1−xTixO4. The onset of a charge-density wave below 200 K is proposed for both La2CuO4 and La2NiO4, but the atomic displacements would be short-range cooperative in mixed systems. The semiconductor-metallic transitions observed in several systems are found in many cases to obey the relation Ea reverse similar, equals kTmin, where varrho = varrho0exp(−Ea/kT) and Tmin is the temperature of minimum resistivity varrho. This relation is interpreted in terms of a diffusive charge-carrier mobility with Ea reverse similar, equals ΔHm reverse similar, equals kT at T = Tmin.