Anti-fluorite structure

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. 

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

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. 

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. 

Many of the oxides, sulphides, selenides and tellurides of the alkali metals (e.g. Li20, Li2S, etc.) have the so-called anti-fluorite structure, i.e. a fluorite structure in which the positions of the anions and cations are interchanged. Most of these can be regarded as essentially ionic compounds. The co-ordination is 4 8. 

Postulated structures of U2NH (a) the simple cubic anti-fluorite structure with orientationally disordered (NH) " groups (H atoms not shown) (b) the doubled anti-fluorite low-temperature structure with an ordered distribution of hydrogen in the imide units. 

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