Zinc Blende-Type Structure

No matter what kind of interaction exists between A and B in, for example, the zinc-blende structure type with a coordination number of four (see Figure 1.1(c)), the atoms (or ions) can be expected to minimize their interatomic A-B distance because of energy optimization so shorter distances should indicate stronger interaction. For a purely electrostatic one (see Section 1.2), this is particularly easy to calculate the distance between A and B can then be approximated by the sum of the ionic radii of the cation and anion, Vc and Va. 

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... 

MnS and MnSe are the only transition-element compounds which have a zinc blende modification. The ZnS structure is the cubic version of the ZnO structure, i. e. the cations occupy half the tetrahedral holes in a cubic close-packed anion sublattice. As in the rocksalt structure the anion and the cation sublattices are identical to one another, i.e. the NaCl and ZnS structures are their own antitype. Like in the case of ZnS itself one should expect several polytypes to occur for MnS and MnSe. MnTe can be stabilized in the zinc blende structure by adding B3-type tellurides. Cubic mixed crystals Zni Mn Te were synthesized up to x = 0.86 171), Cdi-zMnzTe up to x — 0.75 172) and Hgi- Mn Te up to x = 0.8 172). 

Structures for some common crystal types in which the ratio of cation to anion is 1 1. (a) The NaCI or rock salt structure, (b) The CsCI structure, (c) The zinc blende structure for ZnS. (d) The wurtzite structure for ZnS. 

Explain why ZnO has the zinc blende structure but CdO has a structure of the sodium chloride type. 

The III-V compounds have the zinc blende structure, which is the same as the diamond structure except that two types of atoms are present each group III atom has four group V atoms as its nearest neighbors and vice versa. Although the crystalline structure of the surfaces of the compounds is as that of the group IV elements (Fig. 1), the electronic configuration of the surface atoms of the m-V compounds is different from that of the group IV elements. 

CdS may exhibit different structures and its color varies from yellow to orange-brown. The stable a-form (greenockite) has a wurtzite type structure. The /3-form, however, shows a cubic zinc blende structure. At 20kbar, CdS adopts a rocksalt form. By doping with other metals... 

The diversity in structure and bonding possible for phosphides is effectively demonstrated by the monophosphides. Monophosphides MP of the group 1 and 2 elements (El, E2) are polyphosphides with i(P ) chains and P2" dumbbells, respectively. Ell and E12 monophosphides are not known. The E3 and E13 monophosphides are the so-called normal compounds with 3x = (M) (see Section 2). With El3, they form the zinc blende structure with tetrahedral heteroatomic bonds. Ternary derivatives such as MgGeP2 and CuSi2P3 have a random distribution of the M atoms, whereas CdGeP2, crystallizes in the ordered chalcopyrite type with a TO[GeP4/2] tetrahedral net (see Section 6.4). The E3 monophosphides form the NaCl structure. CeP is remarkable because of its physical properties (metal-semiconductor transition heavy-fermion behavior). The E14 monophosphides show the break usually observed when passing the Zintl border. Binary lead phosphides are not known SiP and GeP... 

A comparison of the structures of compounds of Be, Mg, Zn, and Cd reveals an interesting point we exclude Hg from this series because there is practically no resemblance between the structural chemistries of Zn and Hg apart from the fact that one form of HgS has the zinc-blende structure. In the following group of compounds italic type indicates tetrahedral coordination of the metal atom in the crystal in other cases the metal has 6 octahedral neighbours. 

Zinc oxide may crystallize in either the wurtzite or, more rarely, the zinc blende structure, the former being more stable at lower temperatures. In both structures the zinc and oxygen ions are tetrahedrally co-ordinated to each other. The bonding is intermediate between the completely ionic and the completely covalent, both ions being more polarizable than in a perfect ionic crystal and carrying an effective charge of only 0.5e. As is well known, non-stoicheio-metric ZnO, with excess of Zn, is an n-type semiconductor with a band gap of 3.2 eV. 

Some intermetallic compounds having the diamond or zinc blende structures, such as GaSb and InSb show an altered surface structure (19) as indicated by the presence of fractional order diffraction beams after cleaning by the argon-ion bombardment and annealing technique. For crystals of this type there is asymmetry in opposite directions perpendicular to (111) planes. This asymmetry results in atoms of type A on one surface and those of type B on the opposite surface (20). The possibility of detecting effects of the asymmetry of clean surfaces by electron diffraction has been considered. However, in the cases of GaSb and InSb, no difference in the diffraction patterns from these two surfaces has been detected. 

The situation is analogous for the transformation of the hexagonal B-12 type structure of BN at >60 kbar and 1300 °C into the cubic B-3 (zinc blende) structure (borazon). The interlayer distances are drastically decreased from 334 pm to 157 pm, while the intra-layer B—N distances are less dramatically increased from 145 pm to 157 pm. The net-effect may account for the increase in density from 2.30 to 3.45 g/cm 19). 

Both in the wurtzite and zinc blende structures (coordination number = 4) of silver iodide, stable at atmospheric pressure, the Ag—I-distances are 278 pm and 280 pm respectively. At 4 kbar the NaCl-type structure (coordination number = 6) becomes stable, in which the Ag—I-distances are remarkably greater, namely 303 pm. Increase in pressure to 100 kbar contracts all of these distances to 283 pm with a decrease in molar volume from 33.8 to 27.6 cm /mol. Thus the transformation with increase in coordination number from the low-pressure modification into the high pressure modification involves an increase in Ag—I-distances by 8.4%, but further increase in pressure does not produce another geometrical rearrangement and hence all of the equidistant bonds within the crystal lattice are shortened 34). 

When only a fraction of the holes of a given type are occupied there are several possibilities. The most symmetrical way of filling half the tetrahedral holes gives the zinc blende structure with ccp, and the very similar 4 4 wurtzite (ZnO) structure with hep. Both the rutile and Cdl2 structures can... 

Slowly hydrolyzed by cold water, rapidly by hot water. M.p. above 1200°C. Crystallizes in the zinc blende structure lattice type. 

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