Zinc sulfide coordination number

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 zinc blende and wurtzite structures. Zinc sulfide crystallizes in two distinct lattices hexagonal wurtzite (Fig. 4.2a) and cubic zinc blende (Fig. 4.2b). We shall not elaborate upon them now (see page 121), but simply note that in both the coordination number is 4 fbr both cations and anions. The space groups are Ptync and F43m. Can you tell which is which ... 

The structure of cubic zinc sulfide (zinc blende, sphalerite) may be described as a ccp of S atoms, in which half of the tetrahedral sites are filled with Zn atoms the arrangement of the filled sites is such that the coordination numbers of S and Zn are both four, as shown in Fig. 10.1.7. The crystal belongs to space group 7 2 — / 43m. Note that the roles of the Zn and S atoms can be interchanged by a simple translation of the origin. 

Both zinc sulfide and zinc selenide are converted from the zinc blende type (coordination number = 4) into the sodium chloride structure (coordination number = 6) at 117 kbar and 100 kbar respectively. In zinc sulfide the Zn—S distance is increased from 233.9 pm to 250 pm while the Zn—Zn distance is decreased from 382 to 353 pm. In zinc selenide the Zn—Se distance is increased from 246 pm to 254 pm while the Zn—Zn distance is considerably decreased, namely from 401 to 359 pm. Since the Zn—Zn distances are more strongly reduced, than the Zn—Se-bonds are lengthened, the volume is also decreased 33). 

Zinc sulfide exists in two forms, as cubic zinc blende and as hexagonal wurtzite. Of the two modifications, the latter is stable at higher temperatures. The ZnS films obtained by simultaneous vapour condensation of sulphur and zinc onto unheated substrates are crystalline even from 5 nm mass thickness. However, the diffraction pattern shows that these small, isolated, three-dimensional ZnS microcrystals contain a large number of stacking faults in all three spatial coordinates. In thicker films (from about 20 nm), the diffraction pattern shows better-ordered crystallites of the zinc blende type. ZnS films deposited at normal incidence have a clearly distinct <111> growth texture becoming noticeable from about 100 nm, as can be seen in Fig. 2 [17b],... 

ZnS has two common crystalline forms, both with coordination number 4. Zinc blende is the most common zinc ore and has essentially the same geometry as diamond, with alternating layers of zinc and sulfide (Figure 7.8(a)). It can also be described as having zinc ions and sulfide ions, each in face-centered lattices, so that each ion is in a tetrahedral hole of the other lattice. The stoichiometry requires half of these tetrahedral holes to be occupied, with alternating occupied and vacant sites. 

Finally, you should also note that Table 7.4 indicates the maamum coordination number for a given radius ratio. For example, in ZnS, the radius ratio comes out to be 0.52, indicating that the maximum coordination number of the zinc cations is 6. In accordance with the above discussion, the coordination number cannot be 8 because the small zinc cation would rattle around in the bigger cubic hole. The coordination number could, however, be 4 as this would result in the doubly negative sulfide (S2-) ions arranged in a tetrahedron around the zinc cation being forced apart. In fact, the zinc cations turn out to occupy tetrahedral holes and have a coordination number of 4. The crystal structures for ZnS will be discussed in more detail in the next section. 

A greater disproportion between the sizes of the cations and anions in a compound makes a coordination number of even 6 physically impossible. For example, in ZnS (Zn radius = 74 pm radius = 184 pm) the crystal structure, shown in Figure 11.53 , has a coordination number of only 4. We can visualize this structure, called the zinc blende structure, as sulfide anions occupying the lattice sites of a face-centered cubic structure with the smaller zinc cations occupying four of the eight tetrahedral holes located directly beneath each comer atom. A tetrahedral hole is the... 

A third AX structure is one in which the coordination number is 4—that is, all ions are tetrahedrally coordinated. This is called the zinc blende, or sphalerite, structure, after the mineralogical term for zinc sulfide (ZnS). A unit cell is presented in Figure 12.4 all corner and face positions of the cubic cell are occupied by S atoms, whereas the Zn atoms fill interior tetrahedral positions. An equivalent structure results if Zn and S atom positions are reversed. Thus, each Zn atom is bonded to four S atoms, and vice versa. Most often the atomic bonding is highly covalent in compounds exhibiting this crystal structure (Table 12.1), which include ZnS, ZnTe, and SiC. 

Structural data are available (Table 30) for a range of binary, ternary and quaternary sulfides of manganese, almost invariably Mn", and these set the scene for the structures to be expected in the compounds with the more discrete polyhedra.319 Indeed, the structural pattern is established in the simple binary compound MnS. Whereas, the stable modification of this (a-MnS) is green and has the cubic rock salt structure with [MnS6] octahedra, the well-known flesh-coloured precipitates of the qualitative analysis system are metastable / - and y-modifications, which have [MnS4] tetrahedra with respectively the zinc blende or diamond (cubic) and wurtzite (hexagonal) structures. And so, in the rest of the known solids, there are almost equal numbers of four-coordinate tetrahedra and six-coordinate octahedra with no other polyhedra having been detected. 

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