Antifluorite

Betyllium, because of its small size, almost invariably has a coordination number of 4. This is important in analytical chemistry since it ensures that edta, which coordinates strongly to Mg, Ca (and Al), does not chelate Be appreciably. BeO has the wurtzite (ZnS, p. 1209) structure whilst the other Be chalcogenides adopt the zinc blende modification. BeF2 has the cristobalite (SiOi, p. 342) structure and has only a vety low electrical conductivity when fused. Be2C and Be2B have extended lattices of the antifluorite type with 4-coordinate Be and 8-coordinate C or B. Be2Si04 has the phenacite structure (p. 347) in which both Be and Si... 

The predominantly ionic alkali metal sulfides M2S (Li, Na, K, Rb, Cs) adopt the antifluorite structure (p. 118) in which each S atom is surrounded by a cube of 8 M and each M by a tetrahedron of S. The alkaline earth sulfides MS (Mg, Ca, Sr, Ba) adopt the NaCl-type 6 6 structure (p. 242) as do many other monosulfides of rather less basic metals (M = Pb, Mn, La, Ce, Pr, Nd, Sm, Eu, Tb, Ho, Th, U, Pu). However, many metals in the later transition element groups show substantial trends to increasing covalency leading either to lower coordination numbers or to layer-lattice structures. Thus MS (Be, Zn, Cd, Hg) adopt the 4 4 zinc blende structure (p. 1210) and ZnS, CdS and MnS also crystallize in the 4 4 wurtzite modification (p. 1210). In both of these structures both M and S are tetrahedrally coordinated, whereas PtS, which also has 4 4... 

Lattice energies and thermochemistry of hexahalometallate(IV) complexes, A2MX6, which possess the antifluorite structure. H. D. B. Jenkins and K. F. Pratt, Adv. Inorg. Chem. Radiochem., 1979, 22, 1-111 (81). 

Lattice Energies and Thermochemistry of HexahalometallatedV) Complexes, AjMXg, which Possess the Antifluorite Structure... 

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 length of the edge of the unit cell for Na20 (which has the antifluorite structure) is 555 pm. For Na20, determine the following ... 

The edge of the unit cell in a crystal is sometimes called the lattice or cell constant. The structure known as antifluorite is the structure of K20 and its cell constant is 644 pm. Determine the value for each of the following ... 

The sulfides of the group IIA metals generally have the sodium chloride structure, but those of the group IA metals have the antifluorite structure because the ratio of anions to cations is 2. Solutions of the sulfides are basic as a result of the hydrolysis reaction... 

LijN has a low decomposition potential, 0.445 V, which limits its usefulness as a practical solid electrolyte in batteries. Various Li3N-derivative phases have been synthesised, such as cubic Li5Nl2 with an antifluorite-derivative structure and a related phase in the system, LijN-Lil-LiOH. Some of these have high conductivity and the latter materials in particular are comparable to those of H-doped LijN. They also have a much higher decomposition voltage, 1.5V, but do suffer from increased sensitivity to chemical attack. 

One might also point out that while the compound K2S itself has the antifluorite structure (Cl) under normal conditions, Cl often transforms to C23 under pressure, which may be expected to be the (not very) high-pressure form of K2S. Thus (a situation we shall return to) P-K2SO4 may be regarded as an anion-stuffed, high-pressure K2S polymorph. 

Cubic close-packed 4 8 M2O All tet Antifluorite U2O, Na20, K2O, Rb20, sulfides... 

The symmetry between cations and anions in the bond valence model can best be seen in the compounds of the alkali metals and alkaline earths where the cation valences are similar to those of the anions. Binary compounds such as NaCl (18189), CsCl (22173), and ZnO (67454) are invariant under the interchange of the cations and anions since both kinds of ions occupy equivalent sites. For compounds such as CaF2 (29008) which crystallizes with the fluorite structure, changing the signs of ions gives the antifluorite structure adopted by the alkali metal oxides such as Na20 (60435). Although the antifluorite... 

If we make the shaded spheres in Fig. 4.14 the cations, and the unshaded ones the anions, we have the antifluorite structure, which is typified by lithium oxide (an fee array of O2- with Li+ in every T-hole). 

The sphalerite, wurtzite, antifluorite (Li2S), cadmium iodide, cadmium chloride, and many other arrangements also involve closest packing of the large ions. 

Many difluondes and dioxides arc found with the fluonie structure Examples are the fluorides of Ca, Sr, Ba, Cd, Hg, and Pb, and the dioxides of Zr, lit, and some lanthanides and actinides, ff the numbers and positions or the cations and anions are reversed, one obtains the antifluorite structure which is adopted by the oxides and the sulfides of Li, Na, K, and Rb. 

The zinc blende structure of ZnS (3 2PT) has S2- filling ccp P layers and Zn2+ filling one T layer. For Li20 (3 3PTT, the antifluorite structure), the O2- ions fill ccp P layers with Li+ ions filling both T layers. The NaCl structure has the 3 2PO structure, with CP ions filling ccp P layers and Na+ ions filling O layers. Cadmium chloride (CdCh) has a... 

Table 6.3. Compounds with the Li20 (antifluorite) (3 3PTT) structure.

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