Anion-Deficient Fluorites

The phase diagrams and ionic conductivities of anion-deficient ZrO2 compounds have been widely investigated, to probe the influence of both dopant size and concentration (e.g.. Ref. [95]). Whilst Sc + doped systems possess the highest values of ionic conductivity, factors such as cost and long-term stability have favored the use of (Zri j,Yj,)O2 x/2 as the current best material, in which the m t and t c transitions are observed at x 0.05 and xw0.16, respectively [96]. On increasing dopant (anion vacancy) concentration, the ionic conductivity initially increases with X, but reaches a maximum close to the lower limit of stability of the c phase and then decreases rapidly [97]. Many theoretical studies have attempted to provide an explanation for this effect, and it is generally accepted that vacancies are trapped 

Numerous other anion-deficient fluorite-structured oxides have been investigated, 

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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... 

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. 

Apart from aimealing times, the structures described above are also sensitive to the preparation temperature. This is well illustrated by phases in the Bi203-Te02 system. At high temperatures, a nonstoichiometric anion-deficient fluorite structure seems to be stable between the composition limits Bi2Te05 and Bi2Tc207, (MO1.6667-MO1.7500). This broad... 

This approach makes it possible to describe several structural features of anion-deficient fluorite-related binary oxides of Ce, Pr and Tb but it is fundamentally descriptive rather than predictive. This limit was pointed out by Khang and Eyring who recently built a model that is also capable of predicting the structural features of unknown phases in the series... 

J. Kilner and B.C.H. Steele, Mass transport in anion-deficient fluorite oxides, in O. Sorenson (Ed.), Nonstoichiometric Oxides. Academic Press, New York, 1981, pp. 233-269. 

There has been a strong effort to rationalise and elucidate a structural principle which will account for all the anion-deficient, fluorite-related, mixed-valent binary oxides of cerium, praseodymium and terbium. This is a key step not only for the solid-state chemistry of these materials but also for a large class of fluorite-related materials involved in applications such as fast oxygen conductors and as catalysts. The two main theoretical approaches to the problem were developed by Martin" and by Kang and Eyring, and will be illustrated in the following sections. 

Structure and Structural Defects in Anion-Deficient Fluorite-Related Materials High-Resolution Electron Microscopy, L. Eyring, High Temp. ScL, 20,183-201 (1985). 

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