Anti-fluorite

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. 

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. 

Fig. 4 Lattice of CajP2 and MgjPj (anti fluorite lattice with vacant sites)...

Lithium-containing ternary nitrides most commonly form anti-fluorite related structures (anti-CaF2).9 Nitrogen occupies the 8-coordinate calcium... 

Table 10.1.5. Some compounds with fluorite and anti-fluorite structures and a values...

Only the alkalis and alkaline earths form sulfides that appear to be mainly ionic. They are the only sulfides that dissolve in water and they crystallize in simple ionic lattices, for example, an anti-fluorite lattice for the alkali sulfides and a rock salt lattice for the alkaline earth sulfides. Essentially only SH" ions are present in aqueous solution, owing to the low second dissociation constant of H2S. Although S2" is present in concentrated alkali solutions, it cannot be detected below 8 M NaOH owing to the reaction... 

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. 

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... 

Lithium oxide crystallizes with the anti-fluorite structure, with lithium ions filling all the tetrahedral holes in a close-packed array of oxide ions. 

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. 

Anti-fluorite m2o 4 8 Na20, Li20, K.O Fluxes, prone to hydration... 

Even though most of the simple MO oxides have the halide structures where the metal ions are octahedrally coordinated by the oxygen ions there are a few MO oxides where the metal ions are tetrahedrally coordinated (26). The alkali metal oxides, Li20, Na20, K2O and Rb20 possess the anti-fluorite structure with oxygen ions considered as close-packed and cations occupying all of the tetrahedral sites. 

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