Crystal antifluorite

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... 

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 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 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... 

Structures for some common crystal types in which the ratio of cation to anion is 1 2. (a) The fluorite structure. The fluorite structure is a common structural type for 1 2 compounds. If the compound has a 2 1 formula, the role of the cation and anion are reversed, and this gives the antifluorite structure that is shown by compounds such as Na2S. (b) The Ti02 or rutile structure. 

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. 

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. 

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. 

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. 

Armstrong RL (1989) Displacive order-disorder crossover in perovskite and antifluorite crystals undergoing rotational phase transitions. Prog NMR Spect 21 151-173... 

The fact that compounds such as Mg2Si to MgjPb have such high resistances and crystallize with the antifluorite structure does not mean that they are ionic crystals. Wave-mechanical calculations show that in these crystals the number of energy states of an electron is equal to the ratio of valence electrons atoms (8/3) so that, as in other insulators, the electrons cannot become free (that is, reach the conduction band) and so conduct electricity. That the high resistance is characteristic only of the crystalline material and is not due to ionic bonds between the atoms is confirmed by the fact that the conductivity of molten MgjSn, for example, is about the same as that of molten tin. 

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... 

Figure 3.8. Examples of composite crystal structures, a) Antifluorite structure, provided the octahedra are not occupied, (h) Perovskite structure (CaTiO ). At the center of each cuboctahedron is a Ca ion. Each Ca cuboctahedron is surrounded by eight titania octahedra. Also see Fig. 3.9.

Yellow-red, finely crystalline powder, slowly decomposing in moist air. Crystallizes in structure type Cl (antifluorite type). 

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, (b) Fluorite shown as F in a simple cubic array, with Ca + in alternate body centers. Solid lines enclose the cubes containing Ca + ions. If the positive and negative ion positions are reversed, as in LijO, the structure is known as antifluorite. 

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