Fluorite Type Structure

There can be considered that the present netwoik may result from an overlap of a compact cubic netwoik of the reference ions (the base is compulsory represented by the Ca ions) with a simple cubic netwoik of the counter-ions (F ) through the gliding of the last one along the A axis so that the comer of its elementary cell to be placed to a quarter of the length of the large diagonal of the elementary cell of the base. This means that in the cubic cell with centered faces of the mandatory reference (Ca ) lattice, all the tetrahedral interstices are occupied ion anions F . Thus the elementary cell contains  

FIGURE 4.32 Structure of type CaFj ( Ca F ) (a) elementary cell of fluorite type (b) atomic positions projected on the base of the elementary cell (c) union of the coordination polyhedra of Ca after Heyes (1999). 

Very important is the fact that a similar arrangement is obtained if the positions of the ions are changed so that the reference ion becomes the natural one an anion. 

In this case the stmcture of anti-fluorite is obtained, as characteristic to the oxides of the alkaline elements. For example face centered cubic the compounds with fluorite and anti-fluorite stmcture are in Table 4.8 exposed. 

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M-M multiple bonding has long been known in metal oxide structures. The first Mo=Mo bond was seen in one crystalline form of Mo02 which has a distorted rutile structure wherein the Mo(4+) ions occupy adjacent octahedral holes throughout the lattice ( 4). The octahedra are distorted because of the short Mo-Mo distances 2.51 X. La. RejO has a fluorite type structure in which 02 is substituted for F and four of the five Ca2 sites are occupied by La3 ions. The remaining Ca2 site is occupied by an (Re=Re)8 unit with an Re-Re distance 2.259(1) A... 

Another important recent result is the preparation of the oxide sulfate Na60(S04)2 from the reaction of Na20 and Na2S04.72 The structure of this sulfate can be seen as a variant of the fluorite-type structure The [ONa6] octahedra are in a ccp arrangement and the sulfate tetrahedra fill all of the tetrahedral voids (Figure 9). 

Sulphides. The partially ionic alkali metal sulphides Me2S have the anti-fluorite-type structure (each Me surrounded by a tetrahedron of S, and each S atom surrounded by a cube of Me). The NaCl-structure type (6/6 coordination) is adopted by several mono-sulphides (alkaline earth, rare earth metals), whereas for instance the cubic ZnS-type structure (coordination 4/4) is observed in BeS, ZnS, CdS, HgS, etc. The hexagonal NiAs-type structure, the characteristics of which are described in 7.4.2.4.2, is observed in several mono-sulphides (and mono-selenides and tellurides) of the first-row transition metals the related Cdl2 (NiAs defect-derivative) type is formed by various di-chalcogenides. Pyrite (cP 12-FeS2 type see in 7.4.3.13 its description, and a comparison with the NaCl type) and marcasite oP6-FeS2 are structural types frequently observed in several sulphides containing the S2 unit. 

Figure 1.11. Schematic diagrams of (a) and (b) Fei j O point defect clusters (after Koch and Cohen 1969) (c) clustering in fluorite-type structures and (d) the Cap2 structure.

Fig. 2.34 Structure model of Y7O5E) (1) based on a Fluorite-type structure, projected on (001) (half of the unit cell is drawn).

In the Y O iF + 2 system, compounds n = 4,5,6,7,8 have been confirmed, the structures of which can be understood in terms of the vernier structure, similar to structure model (II) (Fig. 2.36). Closely related compounds have also been observed in the Nb-Zr-O and Zr-N-F systems. Figure 2.37 shows the structure of ZriogNggFijg drawn in the same way as YjOgFg in Fig. 2.35. This structure is also an example of a vernier structure based on a Fluorite-type structure. A vernier structure with layer type [a] (as in Fig. 2.32) has been critically and systematically reviewed by Makovicky and Hyde. °... 

TI2O3 has a cubic crystal structure, K7h, la 3, and a = 10.5344 A. It is a fluorite-type structure with vacancies in oxygen layers (Figure 6.19). 

Ce02 with fluorite-type structure is a component in N02 dissociation catalysts [57] and biomass gasification catalysts [58],... 

Yttria-stabilized zirconia f[Zrlj YJ02, /2) is known in the literature as YSZ and has a fluorite-type structure [67] (see Figure 2.16). This material has a high oxygen ion conductivity and is, therefore, applied as a high-temperature electrolyte material, for example, in high-temperature fuel cells [68,73],... 

It had the nonstoichiometric composition YbH2i55 and a fluorite-type structure, and, therefore, appears to be analogous to the first class of rare earth hydrides. 

Recently, higher hydrides of niobium and of vanadium have been prepared by special techniques (5, 32). They both have the cubic fluorite-type structure of the rare earth hydrides. These compounds are discussed in more detail by Gibb (11),... 

Substitution Variants. The fluorite-type structure is maintained in principle when alkaline earth elements are replaced partially by rare-earth elements. Charge compensation is achieved by occupation of additional interstitial anionic sites.The coordination of the metal atoms may increase from 8 to 9 or even 10 by this. Another way of charge balance is the partial replacement of fluorine by oxygen to form oxyfluorides. Since the possible interstitial positions provide pathways for anion disorder and movement, this class of materials shows fluoride ionic conductivity. 

Praseodymium dioxide crystallizes in the fluorite-type structure (space group Fm3m) with four praseodymium atoms and eight oxygen atoms per unit cell. This structure may be visualized easily as an infinite array of coordination cubes (each consisting of a Pr atom at the center with eight O atoms at the corners) stacked so that all cube edges are shared. 

Amo.5 Pao.5 )02 of fluorite-type structure with a statistical distribution of the metal ions (18). (Amo.s Pao.s )O2, like the analogous compounds of the rare earth elements, forms a double oxide with BaO, Ba(Amo.5, Pao.5)03 with an ordered perovskite-type of structure (19). 

Cerium (Ce) is a second element of lanthanides in periodic table. Cerium oxide (Ce02) has a cubic fluorite-type structure with a lattice constant (a) of 0.5411 nm. Ce02 thin films are highly attractive for electronic and electrochemical device applications as insulating buffer layers, ion-conducting layers, or ion-storage layers. Recently, a lot of interest has been generated in nano-structured cerium oxide for various electro-catalytic applications due to its... 

Hydrides of the 3d, 4d, and 5d metals. We now comment briefly on the remaining hydrides of Table 8.1. All the (h.c.p.) elements Ti, Zr, and Hf form dihydrides with fluorite-type structures which are cubic above their transition points and have lower (tetragonal) symmetry at ordinary temperatures. Both phases have ranges of composition, which depend on the temperature, and the following figures (for room temperature) show that for Ti the cubic phase includes the composition TiH ... 

The earlier solid oxide electrolytes were solid solutions of divalent or triva-lent metal oxides (Y2O3, Yb203 on CaO) in oxides of tetravalent metals having a fluorite type structure A4O8 like Zr02, Th02 or Ce02 [97]. The introduction of... 

For BkO, 63pm/140pm = 0.450, is also within the 0.414-0.732 range but this time we have to look under ABi structure-type column thus, we can predict TiO, structure type. For this oxide, however, this is only a predicted structure—BkO2 actually has a fluorite-type structure. Thus, this example shows a limitation of this method. Even if we were to use ionic radii for correct coordination numbers (8 for Bk and 4 for O in CaFa structure type), the y value would still fall within the Ti( structure range. 

Praseodymium is one of the possible "new" additives, which today attracts more and more attention. In fact, it was demonstrated that the oxygen exchange occurs at lower temperature on cerium-praseodymium mixed oxides than on ceria [7]. Furthermore, high temperature pretreatment does not affect the oxygen exchange capacity (OSC) of the mixed oxides. Moreover, high surface area PrOy-Zr02 materials with a fluorite-type structure were also prepared by the sol-gel way [7, 8]. 

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