Zinc sulfide, hexagonal form, wurtzite

Zinc sulfide.ZnS,crystallizes in two different forms, or polymorphsra hexagonal form (wurtzite) and a face-centered cubic form (zinc blende, or sphalerite), which is shown here. Sulfide ions, S ,are at the corners and at the centers of the faces of the unit cell Zn ions are in alternate subcubes of the unit cell.The unit cell can also be described as having Zn " ions at each corner and at the center of each face, with ions in... 

Zinc sulfide is white to gray-white or pale yellow powder. It exists in two crystalline forms, an alpha (wurtzite) and a beta (sphalerite). The wurtzite form has hexagonal crystal structure refractive index 2.356 density 3.98 g/cm3 melts at 1,700°C practically insoluble in water, about 6.9 mg/L insoluble in alkalis soluble in mineral acids. The sphalerite form arranges in cubic crystalline state refractive index 2.368 density 4.102 g/cm changes to alpha form at 1,020°C practically insoluble in water, 6.5 mg/L soluble in mineral... 

Wurtzite structure. Zinc sulfide can also crystallize in a hexagonal form called wurtzite that is formed slightly less exothermically than the cubic zinc blende (sphalerite) modification (Afff = —192.6 and —206.0 kJ mol-1, respectively) and hence is a high temperature polymorph of ZnS. The relationship between the two structures is best described in terms of close packing (Section 4.3) in zinc blende, the anions (or cations) form a cubic close-packed array, whereas in wurtzite they form hexagonal close-packed arrays. This relationship is illustrated in Fig. 4.13 note, however, that this does not represent the actual unit cell of either form. 

The wurtzite structure is the high-temperature form of zinc sulfide, where only half of the tetrahedral holes are filled in an ordered manner, such that either 3/8 and 7/8 or 5/8 and 1/8 are filled (Figure 1.19). It is noteworthy that there is no hexagonally close-packed fluorite equivalent, as Nature never simultaneously fills holes which are only one quarter of the unit cell apart. 

Zinc Sulfide. Zinc Blende. SZn mol wt 97.45. S 32,91%. Zn 67.09%. ZnS. Occurs in nature as the minerals wurtzlte (hexagonal, d 4,087) and sphalerite (cubic, d 4.102). Precipitated zinc sulfide ol commerce usually contains 15-20% water of hydration. The dried precipitate may have been heated to 725° in the absence of air to obtain substantial conversion to wurtzite. the form peferred by the pigment industry. 

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],... 

Krunks et al. [119] studied zinc thiocarbamide chloride as a single-source precursor for obtaining thin films of zinc sulfide by spray pyrolysis. By heating this compound to 1200 C, they demonstrated that cubic ZnS (sphalerite) forms below 300 °C and stays in this form until 760 C, when it transforms to hexagonal ZnS (wurtzite). 

The zinc blende lattice Is named after its parent compound, ZnS. Zinc sulfide also exists in a different structure known as the wurtzite lattice. Molecules that can exist in more than one type of crystalline form exhibit polymorphism. The wurtzite lattice is comprised of one type of ion forming a hexagonal closest-packed unit cell, with the other type of ion occupying half of the tetrahedral holes. The following molecules can assume the wurtzite lattice ZnO, ZnS, ZnSe, ZnTe, BeO, Agl, CdS, MnS, SiC, AIN, and NH4F. Both types of lattices consist of corner-shared tetrahedrons, but the tetrahedrons in wurtzite are canted in alternating layers. 

Like zinc sulfide, cadmium sulfide has two forms, cubic (zinc blende, see Figure 3-2) or hexagonal (wurtzite, see Figure 3-3). 

Heaton (1928) reports that when synthetic zinc sulfide was prepared by precipitation it gave a white substance of great opacity . Supposedly introduced as a pigment in 1852, it was not widely used because of a grey cast caused by traces of iron sulfide group (. v.) compounds. A purer product was marketed from 1927 under the trade name Sachtolith this also contained 3% zinc oxide q.v.). According to the Merck Index (1996), synthetic zinc sulfide often contains 15-20% water of hydration it is therefore heated in the absence of air to 725°C to convert it to wurtzite, the hexagonal form of ZnS preferred by the paint industry. 

The wurtzite form of ZnS, Figure 7-8(b, c), is much rarer than zinc blende, and is formed at higher temperatures than zinc blende. It also has zinc and sulfide each in a tetrahedral hole of the other lattice, but each type of ion forms a hexagonal close-packed lattice. As in zinc blende, half of the tetrahedral holes in each lattice are occupied. 

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