Fluorite-Structured Compounds

The Ag + and Cu + superionic phases discussed above are generally characterized by 

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Frenkel defects on the cation sublattice of a sodium chloride structure compound. Frenkel defects on the anion sublattice of a fluorite structure compound. 

It is important to note that in the halite-structure materials the displacement sequence is along a close-packed direction enabling momentum transfer to occur such that the interstitial becomes separated from the vacancy by several atomic positions. In the fluorite-structure compounds and in crystalline SiC however, such a displacement sequence is not possible since the STE is not aligned along a close-packed direction. As a result stable, well-separated interstitials and vacancies are very unlikely in these materials at all but the very lowest temperatures. Transient defect formation only is observed (9,16). 

There are four calcium and eight fluorine atoms in the unit cell. The number of molecules of CaF2 in the unit cell is four, so that, for all fluorite structure compounds, Z = 4. In this structure, each calcium atom is surrounded by eight fluorine atoms at the comers of a cube. Each fluorine atom is surrounded by four calcium atoms at the vertices of a tetrahedron (see also Chapter 7). A perspective view of the stmcture is shown in Figure 1.12a, and a projection of the stmcture down the c-axis in Figure 1.12b. 

Frenkel defects on the anion sublattice of fluorite structure compounds. 

The compounds of the MMe205F type, where Me = Nb or Ta M = Rb, Cs, Tl, crystallize in cubic symmetry and correspond to a pyrochlore-type structure [235-237]. This structure can be obtained from a fluorite structure by replacing half of the calcium-containing cubic polyhedrons with oxyfluoride octahedrons. 

Other binary compounds include MAs3 (M = Rh, Ir), which has the skutterudite (CoAs3) structure [33] containing As4 rectangular units and octahedrally coordinated M. The corresponding antimonides are similar. M2P (M = Rh, Ir) has the anti-fluorite structure while MP3 has the CoAs3 structure. In another compound of this stoichiometry, IrSi3, 9-coordination exists for iridium. 

The Fluorite and Rutile Structures.—Compounds of the type MXS also have their choice of two ionic structures, but the factors influencing them... 

Many complex ions, such as NH4+, N(CH3)4+, PtCle", Cr(H20)3+++, etc., are roughly spherical in shape, so that they may be treated as a first approximation as spherical. Crystal radii can then be derived for them from measured inter-atomic distances although, in general, on account of the lack of complete spherical symmetry radii obtained for a given ion from crystals with different structures may show some variation. Moreover, our treatment of the relative stabilities of different structures may also be applied to complex ion crystals thus the compounds K2SnCle, Ni(NH3)3Cl2 and [N(CH3)4]2PtCl3, for example, have the fluorite structure, with the monatomic ions replaced by complex ions and, as shown in Table XVII, their radius ratios fulfil the fluorite requirement. Doubtless in many cases, however, the crystal structure is determined by the shapes of the complex ions. 

The theoretical result is derived that ionic compounds MXS will crystallize with the fluorite structure if the radius ratio Rm/Rx is greater than 0.65, and with the rutile (or anatase) structure if it is less. This result is experimentally substantiated. 

This compound has the cubic fluorite structure with one octahedral interstice per Ce atom. Therefore, a =1, and S = 2 for CeH2. We can therefore write ... 

R. Klenata, B. Daouda, M. Sahnoun, H. Baltache, M. Rerat, A. H. Reshak, B. Bouhafs, H. Abid, and M. Driz, Structural, Electronic and Optical Properties of Fluorite-type Compounds, Eur. Physical Jour. B, 47,63 (2005). 

The fluorite structure is a common one for compounds that have 1 2 stoichiometry. A great many compounds have formulas that have twice as many cations as anions. Examples include compounds such as Li20 and Na2S. These compounds have crystal structures that are like the fluorite structure but with the roles of the cations and anions reversed. This structure is known as the antifluorite structure, in which there are eight cations surrounding each anion and four anions surrounding each cation. The antifluorite structure is the most common one for compounds that have formulas containing twice as many cations as anions. 

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 fluorite (CaF2) type structure is a structure often encountered in ionic solids. It follows the same general principles as described above for the ionic AnX compounds the packaging ensures that the chief contacts are between atoms of opposite sign and that each atom is surrounded by the maximum number of atoms of opposite sign. The cations in the fluorite structure are surrounded by eight equidistant anions at the corners of a cube. Inversely, each anion has around it four cations at the corners of a tetrahedron. As a rule, this structure is only formed if the ratio radius cation/radius anion is greater or equal to 0.73. 

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. 

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