Anti-fluorite type

When the positions of cations and anions are interchanged, the same structure types result for the CsCl, NaCl and zinc blende type. In the case of the fluorite type the interchange also involves an interchange of the coordination numbers, i.e. the anions obtain coordination number 8 and the cations 4. This structure type sometimes is called anti-fluorite type it is known for the alkali metal oxides (Li20,..., Rb20). 

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. 

Very important is the structural type of Cap2, where the metal atom is surrounded by 8 atoms of fluorine occupying the vertices of a cube. Examples of this type are presented in Table 5.8. The last column of this table lists the oxides and chalcogenides of alkali metals, which have stmctures of the anti-fluorite type, i.e. with the metal... 

Structure of (anti) Fluorite Type Cell s Parameter a[A]... 

In the sulphides, selenides, tellurides and arsenides, all types of bond, ionic, covalent and metallic occur. The compounds of the alkali metals with sulphur, selenium and tellurium form an ionic lattice with an anti-fluorite structure and the sulphides of the alkaline earth metals form ionic lattices with a sodium chloride structure. If in MgS, GaS, SrS and BaS, the bond is assumed to be entirely ionic, the lattice energies may be calculated from equation 13.18 and from these values the affinity of sulphur for two electrons obtained by the Born-Haber cycle. The values obtained vary from —- 71 to — 80 kcals and if van der Waal s forces are considered, from 83 to -- 102 kcals. 

The heat capacity is based on the drop calorimetry of May (6) (400-1500 K), The pre-melt S-shaped enthalpy curve is reinterpreted as incorporating a lambda transition in view of the enthalpy measurements on K2S by Dworkin and Bredig (7 ) and the occurrence of lambda transitions in other materials having the fluorite or anti-flourite type of structure ( ). The adopted heat capacity shows the maximum of the lambda transition at 50.65 cal... 

Two hydrides of Cr have been made (electrolytically), CrH with an anti-NiAs type of structure (Cr—6 H, 1-91 A, Cr—Cr, 2 71 A) and CrH2 with apparently the fluorite (f.c.c.) structure. 

Carbides of type (a) yield CH4 on hydrolysis. Examples are Be2C, with the anti-fluorite structure, and AI4C3. The structure of the latter is rather more complex and its details do not concern us here. It is sufficient to note that each carbon atom is surrounded by A1 atoms at distances from 1-90 to 2-22 A, the shortest C-C distance being 3-16A. As in Be2C therefore there aie discrete C atoms, accounting for the hydrolysis to CH4. The alkaline-earth carbides, type (b), crystallize at room temperature with the CaC2 structure (Fig. 22.6). (There is some... 

A majority of the important oxide ceramics fall into a few particular structure types. One omission from this review is the structure of silicates, which can be found in many ceramics [1, 26] or mineralogy [19, 20] texts. Silicate structures are composed of silicon-oxygen tetrahedral that form a variety of chain and network type structures depending on whether the tetrahedra share comers, edges, or faces. For most nonsilicate ceramics, the crystal structures are variations of either the face-centered cubic (FCC) lattice or a hexagonal close-packed (HCP) lattice with different cation and anion occupancies of the available sites [25]. Common structure names, examples of compounds with those structures, site occupancies, and coordination numbers are summarized in Tables 9 and 10 for FCC and HCP-based structures [13,25], The FCC-based structures are rock salt, fluorite, anti-fluorite, perovskite, and spinel. The HCP-based structures are wurtzite, rutile, and corundum. 

A mechanism for this solvolytic decomposition reaction has been postulated (Liu and Eick 1991). The reaction is topochemical with the structure of the product controlled by symmetry and size considerations. Since the R Cl2 +1 vernier structures are closely related to the fluorite structure (Haschke 1979), synthesis of fluorite-type modifications upon leaching is not surprising. However, when the M14X33 phase is leached either the fluorite or the anti-Fc2P-type structure is obtained. If M represents Sm or Eu, and X = Cl, the fluorite modification is obtained. If M = Ba and La or Nd and X = Br, the anti-FejP-type modification is obtained. The Mj4X33 and the fluorite structures both contain layers of cations and anions perpendicular to the three-fold axes. The anti-FejP structure contains both cations and anions in the layers. Thus conversion of the M 34013 3 structure to the fluorite modification requires only in-layer movement of the unsolvated M ions in the planes perpendicular to the three-fold axis. On the other hand, conversion to the anti-Fc2P-type structure requires both in-plane and out-of-plane movement of the M and Br ions. The latter motion results with larger cations which cannot fit easily in the planes. Reaction behavior is... 

TABLE 4.8 Fluorite (CaF ) and Anti-Fluorite (Na O) Types of Crystalline Substances after (Chiriac-Putz-Chiriac, 2005)... 

One of the most important salts is cryolite whose structure (Fig. 9-2) is important since it is adopted by many other salts containing small cations and large octahedral anions and, in its anti-form, by many salts of the same type as [Co(NH3)6]I3. It is closely related to the structures adopted by many compounds of the types M2[AB6]2- and [XY6]2 + ZJ. The last two structures are essentially the fluorite (or antifluorite ) structures (see Fig. -2=3,-page 51), except that the anions (or cations) are octahedra whose axes are oriented parallel to the cube edges. The unit cell contains four formula units. 

Anti-Frenkel disorder similar to Frenkel disorder except that the interstitials are anions and vacancies are therefore in the anion sublattice. In Zr02 the reaction is 0 kS + 0[ and the anti-Frenkel equilibrium constant is K p = [ko ][On- This type of thermal defect is found in lattices that have a fluorite structure (CaF2, Zr02), which means that there are many large interstitial sites where the anions can be accommodated, but not the cations because their charge is larger, and they are less well screened from each other. 

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