Ionic-covalent mixed oxides

After a survey of the basic theory and some experimental aspects of photoelectron spectroscopy which are relevant to actinide solids, two systems are illustrated elemental actinide metals, in which the Mott transition between plutonium and americium is evidenced in a photographic way by photoemission, and strongly ionic oxides, in which the 5f localized behaviour is clearly seen, and indications of f-p or d-p covalent mixing are investigated. 

The difficulty of the problem is enhanced because it is now known that the bonds between atoms in compounds and in crystals can be partly ionic and partly covalent in character, and not simply one or the other. This applies in particular to many mixed oxides so that aluminates, titanates, zincates, etc., can be regarded as built up entirely from metal and oxygen ions, though to some extent negative aluminate, titanate, etc., ions seem to be present. 

The three types of solids, metals, covalent semiconductors or insulators, and ionic compounds (including oxides) have characteristic surface reactions. In organic catalysis only metals and ionics are considered (Table 6.5), while in CVD all three types of solid surfaces are of interest. Metals absorb hydrogen and nitrogen dissociatively while ionic substrates have redox reactions or acid/base reactions with molecules. Oxidation of gases is often catalyzed by the surface of metal oxides. So is deposition of oxides by oxidation and hydrolysis of metal-containing precursors. When mixed oxides (e.g., perovskites) are deposited care must be taken to ensure a sufficient availability of the separate components. 

Based on the concept of mixed-framework lattices, we have reported a novel class of hybrid solids that were discovered via salt-inclusion synthesis [4—7]. These new compounds exhibit composite frameworks of covalent and ionic lattices made of transition-metal oxides and alkali and alkaline-earth metal halides, respectively [4]. It has been demonstrated that the covalent frameworks can be tailored by changing the size and concentration of the incorporated salt. The interaction at the interface of these two chemically dissimilar lattices varies depending upon the relative strength of covalent vs. ionic interaction of the corresponding components. In some cases, the weak interaction facilitates an easy... 

In actinide binary compounds an equation of state can also be developed on the same lines. The difference in electronegativity of the actinide and the non-actinide element plays an important role, determining the degree of mixing between the actinide orbitals (5 f and 6 d) and the orbitals of the ligand. A mixture of metallic, ionic and covalent bond is then encountered. In the chapter, two classes of actinide compounds are reviewed NaCl-structure pnictides or chalcogenides, and oxides. 

A variety of defect formation mechanisms (lattice disorder) are known. Classical cases include the - Schottky and -> Frenkel mechanisms. For the Schottky defects, an anion vacancy and a cation vacancy are formed in an ionic crystal due to replacing two atoms at the surface. The Frenkel defect involves one atom displaced from its lattice site into an interstitial position, which is normally empty. The Schottky and Frenkel defects are both stoichiometric, i.e., can be formed without a change in the crystal composition. The structural disorder, characteristic of -> superionics (fast -> ion conductors), relates to crystals where the stoichiometric number of mobile ions is significantly lower than the number of positions available for these ions. Examples of structurally disordered solids are -> f-alumina, -> NASICON, and d-phase of - bismuth oxide. The antistructural disorder, typical for - intermetallic and essentially covalent phases, appears due to mixing of atoms between their regular sites. In many cases important for practice, the defects are formed to compensate charge of dopant ions due to the crystal electroneutrality rule (doping-induced disorder) (see also -> electroneutrality condition). 

Unraveling the relationship between the atomic surface structure and other physical and chemical properties is probably one of the most important achievements of surface science. Because of the mixed ionic and covalent bonding in metal oxide systems, the surface structure has an even stronger influence on local surface chemistry as compared to metals or elemental semiconductors [1]. A vast amount of work has been performed on Ti02 over the years, and this is certainly the best-understood surface of all the metal oxide systems. 

On the basis mainly of results obtained in the oxidation of isobutene to methacrolein, the oxidative dehydrogenation of butene to butadiene and the oxygen-aided dehydration of formamide to nitriles, it was possible to show that oxides present in catalysts are located on a scale reflecting donor-acceptor properties (fig. 5). Some oxides are essentially acceptors (e.g., M0O3, some tellurates) they can potentidly cany active and selective sites, provided they receive spillover oxygen. Others are essentidly donors a-Sb204, in this respect, is typical it produces spillover oxygen but carries no sites active for oxidation. Other oxides have mixed properties. The acceptors are relatively covalent, the donors are more ionic [63,77]. 

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