Anion-deficient structures

Pt(II) compound reactivation, 37 201 Pt(IV) compound reduction, 37 201 rate-determining step, 37 199-201 tetrachloride, 4 187-188 tetracyanide anions, as one-dimensional electrical conductors, 26 235-268 anion-deficient structures anhydrous compounds, 26 252-254 dimerization, 26 249-251 hydrated derivatives, 26 245-252 physics, 26 260-263 with potassium bromide, 26 248-249 with rubidium chloride, 26 249-250 cation-deficient compounds, 26 244, 254-256... 

In anion-deficient structures, polyhedra may share comers, edges, or faces. 

Table 2,2 summarises the main phases observed in the region CeO -CeOj 714. It will be seen that there is some conflict in the literature concerning both the composition and the structure of intermediate phases, only some of which have been fully characterised by structural refinement. Unfortunately, several anion-deficient structures remain unexplained. There are two major problems involved. First, it is difficult to obtain a single crystal. Second, the utility of X-ray powder diffraction... 

The Kang-Eyring system interpreting fluorite related anion deficient structures has recently been extended to a wide range of fluorite related structures with vacancies in both anion and cation sites [28, 29]. 

Perovskite-like anion deficient structure of the RBa2Cu30e+6 phases (R123, R - rare earth cation) (a b... 

The parent structure of the anion-deficient fluorite structure phases is the cubic fluorite structure (Fig. 4.7). As in the case of the anion-excess fluorite-related phases, diffraction patterns from typical samples reveals that the defect structure is complex, and the true defect structure is still far from resolved for even the most studied materials. For example, in one of the best known of these, yttria-stabilized zirconia, early studies were interpreted as suggesting that the anions around vacancies were displaced along < 111 > to form local clusters, rather as in the Willis 2 2 2 cluster described in the previous section, Recently, the structure has been described in terms of anion modulation (Section 4.10). In addition, simulations indicate that oxygen vacancies prefer to be located as second nearest neighbors to Y3+ dopant ions, to form triangular clusters (Fig. 4.11). Note that these suggestions are not... 

The final anion-deficient fluorite structure type material to mention is 8-Bi203. The formula of this phase makes it surprising that a fluorite structure form exists, but such a structure occurs at high temperatures. The resulting phase is an excellent O2- ion conductor with many potential applications. Unfortunately, the high-temperature form is not maintained when the compound is cooled to room temperature. However, fluorite structure anion-deficient phases of the same type can be prepared by reaction with many other oxides, and these are stable at room temperature. The majority of these materials have a modulated anion substructure (Section... 

The structure of pyrochlore is considered to be an anion deficient derivative of the fluorite structure type. Ca atoms are in eight-fold coordination, while Nb atoms are in six-fold coordination. Steady-state luminescence spectra of pyrochlore revealed emission of REE, such as trivalent Dy and Nd (Gorobets and Rogojine 2001). The natural pyrochlore in our study consisted of four... 

Anion-deficient fluorite oxides are also present, for example, U02- c, Ce02-x The presence of anion vacancies in reduced fluorites has been confirmed by diffraction studies. In reduced ceria for example, some well-ordered phases has been reported (Sharma et al 1999). The defective compounds show very high anion mobilities and are useful as conductors and as catalytic materials as will be described later. However, the structures of many anion-deficient fluorite oxides remain unknown because of the shear complexity of the disordered phases. There are, therefore, many opportunities for EM studies to obtain a better understanding of the defect structures and properties of these complex materials which are used in catalysis. 

Ordering of vacancies also plays a key role in selective oxidation catalysis over perovskite-based catalysts such as CaMnOs oxides. CaMnOs has a CaTiOs (AMO3) perovskite structure which is made up of cations coordinated to 12 0 anions. They, in turn, are connected to corner-sharing MoOe octahedra. CaMnOs was used as a model catalyst on a laboratory scale by Thomas et al (1982) in propene oxidation to benzene and 2-methyl propene to paraxylene. In such reactions the compounds are found to undergo reduction to form anion-deficient metastable phases of the type CaMnOs-x where 0 < x < 0.5, forming several distinct phases. 

The principle of cs is best understood with reference to the ReOj structure. The ReOj structure is built up of ReOg octahedra sharing all its corners. A projection of the ReOj structure along one of its crystallographic axes is shown in Fig. 5.17(a). When anion deficiency is introduced in the structure, the anion vacancies are eliminated by shearing one part of the structure relative to the other along a certain crystallographic direction. 

Anion-deficient perovskites and vacancy-ordered structures. Anion-vacancy... 

One of the best-characterized perovskite oxides with ordering of anion vacancies is the brownmillerite stmctme exhibited by Ca2Fe205 and Ca2FeA105 (Grenier et al, 1981). The compositions could be considered as anion-deficient perovskites with one-sixth of anion sites being vacant. The orthorhombic unit cell of the brownmillerite structure (a = 5.425, b = 5.598 and c = 14.768 A for Ca2Fe205) arises because of vacancy-ordering and is related to the cubic perovskite as a - c-... 

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