Antifluorite lattice

Cation Chloroacetates M+C1CH2C02 Layer lattice type NQRv MHz at 77 Decomp. Temperature (K) ) Cation Hexachlorostannates (M+)2SnCl62-Cubic antifluorite lattice NQRv MHz at 298 KW Decomp. Temperature (K) ... 

In KgPtCl there exists resonance between the covalent and ionic structures. The KgPtCl crystal consists of K+ ions and octahedral PtCl -ions arranged in a similar manner to the Li+ and ions in an antifluorite lattice. Each potassium ion is surrounded by twelve chlorine atoms and the closeness of the packing will evidently be due to the attraction between the positive potassium ions and the partially negative atoms of chlorine. 

Ionic Sulfides Sulfide Ions. 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 antifluorite lattice for the alkali sulfides and a rock salt lattice for the alkaline-earth sulfides. It is not absolutely certain that they contain S2- and not SH ions. 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 solutions14 it cannot be detected below about 8M NaOH owing to the reaction... 

In excess tetrahedral phases of the Mg2Ge type, the number of cations exceeds the number of anions, and cations occupy both systems of tetrahedral voids in the fee anion sublattice (the antifluorite lattice). 

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 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 ... 

All of the Group IA and IIA metals form sulfides, some of which are used rather extensively. The sulfides of Group IIA metals consist of M2+ and S2 ions arranged in the sodium chloride type lattice (see Chapter 3). The compounds of the Group IA metals consist of M+ and S2, but as a result of there being twice as many cations as anions, the structure is of the antifluorite type (see Chapter 3). The sulfide ion is a base so there is extensive hydrolysis in solutions of the sulfides, and the solutions are basic ... 

X-Ray powder diffraction data of a large number of NF4+ salts are described by various workers. The data (Table I) indicate these salts have a tetragonal lattice. Single crystals of 98.9% pure (NF4)2NiF6 showed that the compound has a body-centered tetragonal cell, space group /4/m (37). The salt is made up of octahedral NiF62- ions and tetrahedral NF4+ cations and has the antifluorite structure. The interatomic N-F distance in the NF4+ tetrahedron is 130-140 pm and the F---F distance is 220 pm. 

Lattice type antifluorite NaCl comndum beta-silica oligomer oligomer molecular ... 

If the cation in the crystal lattice exhibits a cubic environment (coordination number of 8), the fluorite structure is commonly observed (Figure 2.16). Lattices of this variety consist of an fee arrangement of cations, with all 8 tetrahedral interstitial sites e.g, (l/4,l/4,l/4), etc.) occupied by the anionic species. Of course, this will only be prevalent when the size of the anion is much smaller than the cation, such as Cap2. For structures with relatively smaller cations, the anions will form the fee lattice, with cations situated within the interstitials. Since the relative positions of cations and anions are reversed in the latter case, the anti prefix is used, designating the structure as antifluorite. 

FIGURE 7-9 Fluorite and Antifluorite Crystal Structures, (a) Fluorite shown as Ca in a cubic close-packed lattice, each surrounded by eight F in the tetrahedral holes. 

Figure 7-9(b), has the fluoride ions in a simple cubic array, with calcium ions in alternate body centers. The ionic radii are nearly perfect fits for this geometry. There is also an antifluorite structure in which the cation-anion stoichiometry is reversed. This stmc-ture is found in all the oxides and sulfides of Li, Na, K, and Rb, and in Li2Te and Be2C. In the antifluorite stmcture, every tetrahedral hole in the anion lattice is occupied by a cation, in contrast to the ZnS structures, in which half the tetrahedral holes of the sulfide ion lattice are occupied by zinc ions. 

Figure 2 The relationship between the lattice parameter L(in Angstroms), and pressure P with the same simple exponential relation for comparative purposes P=A exp(- BL) given for ice X and the antifluorite structure in (a) and (b) respectively, where in (a) A = 5.80x10, B =4.04 and correlation = 0.9999 and (b) A = 8.95x10, B = 3.41, correlation = 0.998,...

Fluorite (CaF2) structure (Fig. 4-U)- Comparison of the fluorite structure (Fig. 4.14) with Fig. 4.12 shows that fluorite can be described as an fee array of Ca + with F ions in all the tetrahedral holes (forming a simple cubic sublattice of fluorides). In this case, the Ca " " and F sites are not interchangeable. This is to be expected, since we have twice as many F as Ca " " as noted earlier, there are indeed twice as many T-holes as lattice atoms. The coordination numbers of Ca + and F are eight and four, respectively. Other solids with this structure include the nuclear fuel U02-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 0 - with Li+ in every T-hole). 

E24.28 In Na2C o, all of the tetrahedral holes are filled with sodium cations within the close-packed array of fulleride anions—in other words, it has an antifluorite structure (see Section 3.9(a)). In NajC >, all of the tetrahedral holes and all of the octahedral holes are filled with sodium cations within the fee lattice of fulleride anions. 

A Potassium has an ionic radius of 1.5 A compared with lithium at 0.9 A. This difference is reflected in their coordination numbers, where potassium is normally six- or eight-coordinate and lithium is often found to be four-coordinate. For example, lithium oxide forms the antifluorite structure with lithium in the tetrahedral holes ill a close-packed lattice of oxygen ions. 

---