Anion-Excess Fluorite Structures

Bevan, D.J.M., J. Strahle and O. Greis, 1979, Structural Principles in Fluorite-related Anion-excess Super-lattice Phases The Crystal Structure of Tveitite, an Ordered Ytrrofluorite Mineral, and the Synthetic Compound Ca2Ybp7, in Proc. of the Fifty European Crystallographic Meeting, Copen hagen, Denmark. 

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. 

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. 

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. 

The MXz+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 U02+, the prototype anion-excess fluorite 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. 

A structural essay on anion-excess, fluorite-related structures... 

From these relations, from the fundamental principle of anion-excess fluorite-related structures (see section 2.2), and from the chemical compositions, it is possible to establish the chemical formulae of the different superstructures as shown in table 11 for AF2-RF3 systems. In the case of RF2-RF3, A must be replaced by R". All observed phases belong to one or two homologous series, M F2n+5 and M F2 +6. The mineral tveitite (rhy) and the phase rhjS can be... 

Fig. 10. Constituent structural elements of rhombohedral fluorite-related superstructures with anion excess (AFg cube, icosahedron, and A R F37,.24 clusters, seen from the outside and from the interior).

Phases of the composition ARF4 are reported for A = Li, Na, Ag, K, Rb, and Cs. From a structural view-point, they can all be considered as metal difluorides with a disordered or partially/fully ordered cation sublattice. Disordered phases can be expected mainly if A and have similar radii and in fact, cubic fluorite-related (Na, R)F2 phases are observed at high temperatures as well as a few (K, R)F2 phases. In most cases, they are part of solid solutions (A, R)F2-j, which are structurally related to yttrofluorites as far as the anion-excess phases... 

J. P. Laval, B. Frit, Defect structure of anion-excess fluorite-related Cai xYxF2+x solid solutions, J. Solid State Chem., 49, 237-246 (1983). 

---