Antifluorite structure type

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... 

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. 

Every second octahedral site of the elpasolite-type structure is unoccupied in compounds A2M F6 of the K2SiF6 type, known also as the K2PtCl6-type (s.g. Fm3m, Figure 18(b)). This structure can also be seen as an antifluorite structure if the whole complex [MFg] anion is compared with Ca. It can be observed for nearly all alkali hexafluorometallates(IV) of the first transition metal series as well as some noble metals like /3-Cs2PtF6. However, for many such compounds other polymorphs are known, too, and the cubic form is the HT-phase only. These alternative structures are mentioned in the next section. A survey on all of the main structme types mentioned up to now derived from the ReOs/VFs type structnre is given in Table 3. 

If the cation and anion sites in Figure 5.18a are exchanged, the coordination number of the anion becomes twice that of the cation, and it follows that the compound formula is M2X. This arrangement corresponds to the antifluorite structure, and is adopted by the group 1 metal oxides and sulfides of type M2O and M2S CS2O is an exception. 

One of the most important salts is cryolite whose structure (Fig. 9-2) is important since it is adopted by many other salts containing small cations and large octahedral anions and, in its anti-form, by many salts of the same type as [Co(NH3)6]I3. It is closely related to the structures adopted by many compounds of the types M2[AB6]2- and [XY6]2 + ZJ. The last two structures are essentially the fluorite (or antifluorite ) structures (see Fig. -2=3,-page 51), except that the anions (or cations) are octahedra whose axes are oriented parallel to the cube edges. The unit cell contains four formula units. 

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

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... 

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 ... 

A certain number of types of compound with n < 4 is known these conopounds therefore have more cations than anions. They are characterized by ordered antifluorite-type structures, which we shall call excess structures to distinguish them from the aforementioned sphaler-Ite-type structures with defects. At the same time, they can be regarded as defect structures compared with an antifluorite-t3q)e structure. They can be most conveniently described as close packings of anion spheres in which the cations occupy the tetrahedral voids of the first and second kinds (i.e., the differently oriented tetrahedra) to an equal extent. 

Valence compounds, like elements, satisfy the Hume-Rothery rule, as can be seen by calculating the average coordination number for these compounds. At the same time, they retain the tetrahedral distribution of the atoms, i.e., the tetrahedral bonds. The structure of defect diamond-like phases has been studied in some detail but corresponding data for excess phases are not available. The problem of the change in the structure of excess phases with varying valence electron concentration is more complicated since it is neither immediately apparent nor known how a sphalerite-type structure is transformed into a defect antifluorite-type structure. 

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