Sphaleritic structure

When the radius ratio of an ionic compound is less than about 0.4, corresponding to cations that are significantly smaller than the anion, the small tetrahedral holes may be occupied. An example is the zinc-blende structure (which is also called the sphalerite structure), named after a form of the mineral ZnS (Fig. 5.43). This structure is based on an expanded cubic close-packed lattice of the big S2 anions, with the small Zn2+ cations occupying half the tetrahedral holes. Each Zn2+ ion is surrounded by four S2 ions, and each S2" ion is surrounded by four Zn2+ ions so the zinc-blende structure has (4,4)-coordination. 

FIGURE 5.43 Hie zinc-blende (sphalerite) structure, rhe tour zinc ions (pink) form a tetrahedron within a face-centered cubic unit cell composed of sulfide ions (vellow).The zinc ions occupy half the tetrahedral holes between the sulfide ions, and the parts or the unit cell occupied by zinc ions are shaded blue. The detail shows how each zinc ion is surrounded by four sulfide ions each sulfide ion is similarly surrounded by four zinc ions. 

Ziegler-Natta catalyst A stereospecific catalyst for polymerization reactions, consisting of titanium tetrachloride and triethylaluminum. zinc-blende structure A crystal structure in which the cations occupy half the tetrahedral holes in a nearly close packed cubic lattice of anions also known as sphalerite structure. 

The Transition to the Sphalerite Structure.—The oxide, sulfide and selenide of beryllium have neither the sodium chloride nor the cesium chloride structure, but instead the sphalerite or the wurzite structure. The Coulomb energy for the sphalerite arrangement is... 

The zincblende (ZB), or sphalerite, structure is named after the mineral (Zn,Fe) S, and is related to the diamond structure in consisting entirely of tetrahedrally-bonded atoms. The sole difference is that, unlike diamond, the atoms each bond to four unlike atoms, with the result that the structure lacks an inversion center. This lack of an inversion center, also characteristic of the wurtzite structure (see below), means that the material may be piezoelectric, which can lead to spurious ringing in the free-induction decay (FID) when the electric fields from the rf coil excite mechanical resonances in the sample. (Such false signals can be identified by their strong temperature dependence due to thermal expansion effects, and by their lack of dependence on magnetic field strength). 

Evidence of a more ordered chalcopyrite structure can be shown in Fig. 6.26, where the (101) and (211) diffractions at 17.9° and 37.3°, respectively, can be clearly shown in the annealed film. Another characteristic of the chalcopyrite structure not shown in the sphalerite structure is the peak splitting in... 

Sodium chloride structure crystals have all octahedral sites filled, and so cation diffusion will be dependent upon vacancies on octahedral sites. In the zinc blende (sphalerite) structure, adopted by ZnS, for example, half of the tetrahedral sites are empty, as are all of the octahedral sites, so that self-diffusion can take place without the intervention of a population of defects. 

The cubic zinc blende or sphalerite structure of ZnS is similar to that of diamond but with alternating sheets of Zn and S stacked parallel to the axes, replacing C. 

According to Pearson (1972), when a point representing a specific phase has a larger value of the strain parameter than that of a particular contact line, then the contacts corresponding to that line are to be considered compressed, on the basis of the Dx and DY assumed for the components. If, on the other hand, the experimental points lie below a line then those contacts have not been established. Fig. 4.24(a)-(c) represent the data and the trends for a few structure types. For compounds having the cF8-ZnS sphalerite structure it can be seen that the X-Y (Zn-S)... 

In terms of a combination of invariant lattice complexes the sphalerite structure may therefore be described as ZnS F + F". 

Sphalerite and wurtzite structures general remarks. Compounds isostructural with the cubic cF8-ZnS sphalerite include AgSe, A1P, AlAs, AlSb, BAs, GaAs, InAs, BeS, BeSe, BeTe, BePo, CdS, CdSe, CdTe, CdPo, HgS, HgSe, HgTe, etc. The sphalerite structure can be described as a derivative structure of the diamond-type structure. Alternatively, we may describe the same structure as a derivative of the cubic close-packed structure (cF4-Cu type) in which a set of tetrahedral holes has been filled-in. This alternative description would be especially convenient when the atomic diameter ratio of the two species is close to 0.225 see the comments reported in 3.7.3.1. In a similar way the closely related hP4-ZnO... 

Fig. 2.7. Comparison of the primary and tertiary bonding around Zn (small circle) in (a) the sphalerite structure and (b) the wurtzite structure. In (b) the distances (in pm) are those found in ZnO (67454).

One example is the tertiary bond found in the wurtzite structure of ZnO (67454). All members of the Zn chalcogenide series crystallize with structures based on the close packing of the chalcogenide ions, with Zn occupying half the tetrahedral cavities. The higher members, ZnSe and ZnTe (31840), crystallize with the cubic sphalerite structure while ZnO crystallizes with the hexagonal wurtzite structure. ZnS (60378, 67453) is known in both forms. 

In the sphalerite structure the anions form a cubic close packed array. The structure has a single adjustable parameter, the cubic cell edge. The 0 ions are too small for them to be in contact in this structure (see Fig. 6.4) so ZnO adopts the lower symmetry hexagonal wurtzite structure which has three adjustable parameters, the a and c unit cell lengths and the z coordinate of the 0 ion, allowing the environment around the Zn " ion to deviate from perfect tetrahedral symmetry. In the sphalerite structure the ZnX4 tetrahedron shares each of its faces with a vacant octahedral cavity (one is shown in Fig. 2.6(a)), while in the wurtzite structure one of these faces is shared with an empty tetrahedral cavity which places an anion directly over the shared face as seen in Fig. 2.6(b). The primary coordination number of Zn " in sphalerite is 4 and there are no tertiary bonds, but in wurtzite, which has the same primary coordination number, there is an additional tertiary bond with a flux of 0.02 vu through the face shared with the vacant tetrahedron. 

In the crystal of ZnS (sphalerite structure, not dealt with in the text) the closest approach between positive and negative ions is 258 pm. The enthalpy of atomization of zinc metal is 130 kJ moT1 and that of elementary sulfur is 223 kJ mol . The two electron attachment enthalpies of sulfur are -206 and +526 kJ mol, respectively. The first and second successive ionization enthalpies of zinc are... 

B3 is the sphalerite structure (cubic) and B4 the wurtzite structure (hexagonal). The experimental values are from Strukturbericht and Structure Reports, except for BN, which is from R. H. Wentorf, Jr., J. Ckem. Phys. 26, 956 (1957). 

Among the oxides and sulphides, only CdO adopts the octahedral rock salt structure found with group 2 element, although the solid is normally very deficient in oxygen and the electrons not used in bonding give rise to metallic properties. ZnO and ZnS are prototypes of the tetrahedrally coordinated wurtzite and zinc blende (or sphalerite) structures in fact, ZnS can adopt either structure, as can CdS and CdSe. HgO and HgS have chain structures with linear two-coordination of Hg. 

AIN, GaN and InN crystallise in the wurtzite structure which is characterised by lattice parameters a and c, as well as by u-value (u = b/c, where b is a bond-length in the c-direction). For the ideal wurtzite structure, c/a = 1.633 and u = 0.375. In contrast to the cubic sphalerite structure, the wurtzite structure offers two possibilities to deviate from the ideal arrangement, by changing the c/a ratio and by changing the u value. Such deviations are often observed in wurtzite-type structures [1] but there exists a strong correlation between the c/a ratio and the u parameter if c/a decreases, then u increases in such a way that the four tetrahedral distances remain nearly constant and the tetrahedral angles are distorted [2]. The bond lengths would be equal if ... 

The c/a ratio also correlates with the differences of the electronegativities the compounds with the greatest differences show the largest departure from the ideal c/a ratio [3], The distortions were explained by long-range polar interactions. Only wurtzite structures with c/a ratios lower than the ideal value of 1.633 are stable (otherwise the sphalerite structure is a stable one). The structure parameters for the Ill-nitrides are given in TABLE 1. 

The sphalerite structure exhibits a unit cell with a ccp anion lattice with one type of tetrahedral hole occupied with cations. What is the compound stoichiometry ... 

Ax) and the average, h, of the principal quantum number, n, of the valence electrons on the A, B atoms. Two of these Mooser Pearson diagrams are shown in Figures 8 and 9. Notice that for the AB octets such indices sort the structures of the three different coordination number structures from each other well, and also separate the wurtzite structure from the sphalerite structure. The wurtzite structure is found for the more ionic examples. 

When the hard-sphere cation-anion radius ratio exceeds 0.732, as it does for the cesium halides, a different crystal structure called the cesium chloride structure, is more stable. It may be viewed as two interpenetrating simple cubic lattices, one of anions and the other of cations, as shown in Figure 21.17. When the cation-anion radius ratio is less than 0.414, the zinc blende, or sphalerite, structure (named after the structure of ZnS) results. This crystal consists of an fee lattice of... 

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