Particular transition metal oxides

Trace amounts of impurities, particularly transition-metal oxides, can have a profound effect on absorption due to electronic transitions alluded to earlier. Furthermore, the presence of Si—OH in the glass can cause significant absorption due to overtones of the O—H bond vibrations. It is important to control these impurities since they lie in the useful transmission window. For example, it has been estimated that one part per billion of these impurities could lead to 1 dB/km loss in silicate glass (see Prob. 16.25). 

The important properties of metal oxides that are of certain interest in different fields of applied science and technology are connected with magnetic, electrical, dielectric, optical, acid-base, and redox behavior. In particular, transition metal oxides with redox properties are of interest for applications in catalysis [1]. Metal oxides and in particular transition metal oxides can possess more than one stable oxidation state making them possible to catalyze reactions which necessitate electron exchanges between reactant(s) and surface active sites (e.g., oxidation, dehydrogenation, etc.). Conversely, other kinds of metal oxides are known to be almost complete irreducible. Oxides of copper, nickel, cobalt, molybdenum, are only few examples of reducible... 

Transition-metal oxides are particularly effective decomposition and burning-rate catalysts. The metal elements can demonstrate variable valence or oxidation states. 

When alkyl nitrates and in particular methyl and ethyl nitrates are added to acids (H2SO4, SnCl4, BF3) they give rise to a very violent reaction with the formation of large quantities of gas. The presence of impurities, nitrogen oxid, transition metal oxides increases the sensitivity of these mixtures to detonation. 

The ability of thioether macrocyclic complexes (and especially those of [9]aneS3) to support multi-redox behaviour at the coordinated metal centre is particularly notable. This allows a series of reversible stepwise one-electron oxidation and/or reduction processes, and stabilization of highly unusual transition metal oxidation states e.g. mononuclear [Pd([9]aneS3)2]2+/3+/4+,149 [Au([9]aneS3)2]+/2+/3+,150 [Ni([9]aneS3)2]2+/3+,151 and [Rh([9]aneS3)2]+/2+/3+.152 It appears to be the ability of the crown thioethers to readily adjust their... 

The interaction and sorption of metal ions with metal oxide and clay surfaces has occupied the attention of chemists, soil scientists, and geochemists for decades (1-4). Transition metal oxides receiving particular emphasis have included various oxides of manganese and iron (5). Interest in sorption phenomena is promoted by the desire to better understand incorporation of metals into minerals, especially marine deposits ( ), the removal of trace metal pollutants and radionuclides from rivers and streams, via sorption and/or precipitation phenomena (1,6), and the deposition of metals on solid substrates in the preparation of catalysts (7,8). 

Another method at the molecular level is the inhibition of oxidation catalysis by alkali and transition metal impurities. In particular, alkali metal oxides in traces serve as effective catalyst with almost ubiquitous presence in technological environments. The mechanism of operation is well described in the literature [64,72-77] despite its complex and multi-pathway behavior. 

In the following section, we restrict our discussion to templated mesoporous solids that are of potential interest as battery electrodes, including many transition-metal oxides and carbon. This slice of the literature still points the interested reader to many articles on the synthesis and physical characterization of relevant mesoporous materials. A much smaller number of electrochemical studies with templated mesoporous electrodes have been published, and these studies in particular will be noted. 

The ZSA phase diagram and its variants provide a satisfactory description of the overall electronic structure of stoichiometric and ordered transition-metal compounds. Within the above description, the electronic properties of transition-metal oxides are primarily determined by the values of A, and t. There have been several electron spectroscopic (photoemission) investigations in order to estimate the interaction strengths. Valence-band as well as core-level spectra have been analysed for a large number of transition-metal and rare-earth compounds. Calculations of the spectra have been performed at different levels of complexity, but generally within an Anderson impurity Hamiltonian. In the case of metallic systems, the situation is complicated by the presence of a continuum of low-energy electron-hole excitations across the Fermi level. These play an important role in the case of the rare earths and their intermetallics. This effect is particularly important for the valence-band spectra. 

Thermal desorption studies of preadsorbed oxygen on a number of transition metal oxides have been carried out by Iwamoto (170) and others who observed several desorption peaks for each oxide. In this type of work, reliable assignment of the desorption peaks to particular oxygen species is only possible when parallel spectroscopic studies are carried out. This is difficult for many of these oxides where only material of low specific surface area is available. 

The electrons in a solid interact both with one another and with the lattice vibrations. A theme of this book is the effect of the interaction between electrons in inducing magnetic moments and metal-insulator transitions. Interaction with phonons also has an important effect, particularly in some transitional-metal oxides. In this chapter both kinds of interaction are introduced. 

Since polarons are heavy, a gas of polarons in a conduction or valence band of a transitional-metal oxide can be non-degenerate at quite moderate temperatures even if the concentration is high. In this book we give examples of both degenerate and non-degenerate gases, particularly in oxides of mixed valence. We have to emphasize that the polaron concept is only valid if the number of carriers... 

This section does not attempt a survey of this large subject. Its aim is only to show briefly how metallic Ni, Co and Fe differ from the transitional-metal oxides that are of particular interest to us in connection with the metal-insulator transition. The former are in our view in a sense more complicated, because the number of electrons in the d-band is in all cases non-integral, while in metallic oxides (such as V203 under pressure and Cr02) this is not the case. 

Since pure mesoporous silica phases does not show any catalytic activity many successful attempts have been made to vary the inorganic composition towards transition metal oxides or metal chalcogenides [5-12], In particular the semiconducting properties of the latter offer a great range of possible applications in materials chemistry. 

The sulfate promoted transition metal oxides focussed considerable attention in recent years due to attractive catalytic properties. Most of the research carried out to date centered on sulfated zirconias,1 5 not surprisingly perhaps, as they exhibit the highest surface acidity (Ho <-16.04) among the members of this family of materials and appear to be able to initiate isomerization reactions in temperatures as low as 298 K. Far less interest attracted sulfated porous titanias, mainly owing to a lower surface acidity,6 although it may be a useful property in many catalytic situations. Thus closer inspection of the preparation procedures for sulfated titanias may be of interest, in particular as the reports on preparation and properties of these materials are scarce and we are not familiar with any work dealing with titania-sulfate aerogels. 

Studies of semiconductor films have shown many facets. The properties of epitaxial films have mainly been investigated on Ge and Si, and to a lesser degree on III—V compounds. Much work, lias been done on polycrystalline II-VI films, particularly with regard to the stoichiometry of the deposits, doping and post-deposition treatments, conductivity and carrier mobility, photo-conductance, fluorescence,electroluminescence, and metal-semiconductor junction properties. Among other semiconductors, selenium, tellunum. and a few transition metal oxides have found some interest. 

However, the most common examples of co-ordination compounds are those in which metals (and particularly transition metals) act as the acceptors. The metal may be in any oxidation state, and the donor-acceptor compounds may be neutral or charged (Fig. 1-2). 

Phenolic or non-phenolic oxidative coupling methods have been extensively used since the 1970 s to synthesize polyoxygenated bridged biaryl compounds, as will be illustrated later in the case of steganes. Remarkably enough, the use of transition metal oxidants is particularly suited to perform intramolecular biaryl coupling whereas most other methods are better suited to intermodular coupling. 

A concise review of data relative to the photofixation of dinitrogen on transition metal oxides is presented. Analysis of data is focussed upon the nature and potential efectivenes of a process which can occur in nature on the surface of particular inorganic materials when exposed to direct sunlight. The significance of this process is discussed. Finally, some new and selected results are presented which outline particular mechanistic aspects of the photoassisted nitrogen process. 

Since the discovery of high temperature superconductivity in cuprates, there has been intense interest in transition metal oxides with strongly layered, (quasi) two-dimensional (2D) crystal structures and electronic properties. For several years now alkali-metal intercalated layered cobaltates, particularly Na CoCL (NxCO)withx 0.50 — 0.75, have been pursued for their thermoelectric properties [1] IAX C0O2 is of course of great interest and importance due to its battery applications. The recent discovery[2] and confirmation[3-5] of superconductivity in this system, for x 0.3 when intercalated with H20, has heightened interest in the NxCO system. 

Among the phase transitions where electronic factors play a major role, the most well-known are the metal-insulator transitions exhibited by transition-metal oxides, sulfides, and so on. This subject has been discussed at length.2,23,24 A recent observation26 of some interest is that the metal-nonmetal transition occurs at a critical electron concentration as given by the particular form of the Mott criterion, = 0.26 ... 

In this article, we discuss spin, charge and orbital orders in transition metal oxides, in particular, in the rare-earth manganites which have become famous since 1993 owing to the phenomenon of colossal magnetoresistance (CMR) exhibited by them (Rao Raveau 1998). We also touch upon the issue of electronic inhomogeneities we discuss the signatures of the presence of such inhomogeneities and theoretical approaches to understand the same. 

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