Anion-excess structures

There are large numbers of anion excess fluorite-related structures known, a small number of which are listed in Table 4.4. The defect chemistry of these phases is enormously complex, deserving of far more space than can be allocated here. The defect structures can be roughly divided into three categories random interstitials, which in... 

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... 

Some of the earliest examples of modulated structures to be unraveled were the fluorite-related vernier structures. These structures occur in a number of anion-excess fluorite-related phases and use a modulation to accommodate composition variation. They can be illustrated by the orthorhombic phases formed when the oxyfluoride YOF reacts with small amounts of YF3 to give composition YOxF3 with x in the range 0.78-0.87, but similar phases occur in the Zr(N, O, F) system with x taking values of 2.12-2.25 and other systems in which the Zr is replaced by a variety of lanthanides. 

Anion-excess fluorite structure nonstoichiometric phases prepared by heating CaF2 and LaF3 contain ... 

Catlow and Lidiard calculated, by computer assisted cluster calculations in an ionic model, that the 2 2 2 and 4 3 2 clusters are particularly stable. Similar clusters are reported to exist in other ionic fluorite-structure solids, e.g. Cap2 -I- YF3 , indicating that they are a feature of anion-excess fluorite compounds. 

Defects in perovskite oxides can be due to cation vacancies (A or B site), amon vacancies or anion excess. Cation-deficient oxides such as A,WOj give rise to oxide bronze structures, W03 itself representing the limiting case of the A-sile deficient oxide A-site vacancies are seldom ordered in these metallic systems. B-site vacancies are favoured in hexagonal perovskites and ordering of these vacancies gives rise to superstructures in some of the oxides. 

Anion excess is found in some perovskites, though not commonly. LaMn03 + v is an example of an anion-excess perovskite with cation vacancies, while LaTi03 5 is a case of anion-excess perovskite with a layered structure. Anion excess also results in the formation of new structures, an example being the A B 03 + 2 system of oxides. 

After an introductory discussion of such misfit structures, various terms that have previously been applied are reviewed, and degrees of incommensurability are used as the basis for a systematic nomenclature. The known structures of specific examples are then discussed graphite intercalates minerals with brudte-like layers as one component (koenenite, valleriite, tochilinites) silicates heavy metal sulphides (cylindrite, incaite, franckeite, cannizzarite, lengenbachite, lanthanum-chromium sulphide) anion-excess, fluorite-related yttrium oxy-fluorides and related compounds. 

The MX2+X phases contain interstitial anions. As with the anion-deficient phases, these interstitials are not random point defects, but ordered or clustered. The earliest cluster geometry to be postulated was the [2 2 2] cluster in UO2+J, the prototype anion-excess flnorite phase. The cluster is composed of 2 interstitial oxygen atoms displaced along (110), two interstitial oxygen atoms displaced along (111) in UO2+J (Figure 5). Other cluster geometries have also been proposed in this oxide, and the defect structure of this well studied phase is still not completely resolved. 

Fluorite type oxides are particularly prone to nonstoichio-metric effects. This most commonly occurs in the form of cation nonstoichiometry induced by partial reduction of the cation or by replacement of a portion of the oxide by flnoride. Anion excess phases can occur as a result of cation oxidation or by replacement with higher valence impurities. The dominant defect in this structure involves the migration of oxygen to the large cuboidal interstice resulting in the formation of a vacancy at a normal lattice site. A vacancy of this type is called a Frenkel defect. 

Anion vacancy in perovskites is more common than cation vacancy. Unlike the well-known case of W03, anion-deficient nonstoichiometry is not accommodated by the crystallographic shear mechanism, but by assimilation of vacancies into the structure, resulting in supercells of the basic network. The review by Rao et al. (24) contains numerous examples of this kind of behavior. Anion excess has been described in a more limited number of systems. Structural details of this type of compounds can be found in Rao et al. (24) and Smyth (25). 

Comment on the structural and compositional implications of (a) the Fe-deliciency of iron(II) oxide, and (b) the anion-excess nature of uranium(IV) oxide. 

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