Antifluorite structure

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 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... 

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. 

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). 

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... 

Some compounds, such as Na20 and K2S, have the inverse fluorite (or antifluorite) structure, which is a ccp structure of anions with all the tetrahedral interstices occupied by cations. 

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. 

Examples of methides are Be2C and AI4C3 the former having an antifluorite structure. Aluminum carbide, AI4C3, hydrolyzes according to... 

Many ionic compounds of AX2 stoichiometry possess the CaF2 (fluorite), or Na20 (antifluorite) structures shown in Figure 3.15. Fluorite is similar to CsCl, but with every other eight coordinate cation removed. Each fluoride anion is tetrahedrally coordinated by calcium ions. This structure is adopted by several fluorides and oxides. In the antifluorite structure, the coordination numbers are the inverse. Most oxides and other chalcogenides of the alkali metals (e.g. Na2Se, K2Se) possess the antifluorite structure, but so do some more covalent compounds, such as the silicides of Mg, Ge, Sn, and Pb. 

Theoretical and experimental optical susceptibilities for fluorite and antifluorite structures. Theoretical values are from Eq. (14-7), with E. = fi -. ... 

Group 2 elements typically form ionic carbides of formnla MC2. When BeO is heated with C at 1900-2000 °C, a brick red colored carbide of formula Bo2C results. This ionic compound adopts the antifluorite structure, that is, like the Cap2 structure except that the positions of the cations and Anions are interchanged. It is unusual because it reacts with water, forming methane, and is thus called a methanide (equation 9). 

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