Wurtzite structure compounds

Nevertheless, some conclusions may be drawn from the set of results presented here. First, with the notable exception of InN, the group III nitrides form a family of hard and incompressible materials. Their elastic moduli and bulk modulus are of the same order of magnitude as those of diamond. In diamond, the elastic constants are [49] Cu = 1076 GPa, Cn = 125 GPa and Cm = 577 GPa, and therefore, B = (Cn + 2Ci2)/3 = 442 GPa. In order to make the comparison with the wurtzite structured compounds, we will use the average compressional modulus as Cp = (Cu + C33)/2 and the average shear modulus as Cs = (Cu + Ci3)/2. The result of this comparison is shown in TABLE 8. 

The (110) surface of the zincblende structure compound semiconductors and the ( lOllO) surface of the wurtzite structure compound semiconductors... 

Other wurtzite structure compounds MnS MnSe SiC MnTe... 

By substituting alternately the carbon atoms in cubic diamond by zinc and sulfur atoms, one obtains the structure of zinc blende (sphalerite). By the corresponding substitution in hexagonal diamond, the wurtzite structure results. As long as atoms of one element are allowed to be bonded only to atoms of the other element, binary compounds can only have a 1 1 composition. For the four bonds per atom an average of four electrons per atom are needed this condition is fulfilled if the total number of valence electrons is four times the number of atoms. Possible element combinations and examples are given in Table 12.1. 

Since these structures are formed by filling the open spaces in the diamond and wurtzite structures, they have high atomic densities. This implies high valence electron densities and therefore considerable stability which is manifested by high melting points and elastic stiffnesses. They behave more like metal-metalloid compounds than like pure metals. That is, like covalent compounds embedded in metals. 

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... 

The wurtzite structure is composed of an hep array of sulfide ions with alternate tetrahedral holes occupied by zinc ions. Each zinc ion is tetrahedrally coordinated by four sulfide ions and vice versa. Compounds adopting the structure include BeO, ZnO, and NH4F. 

The wurtzite structure is closely related to the zinc blende structure, having the same 4 4 tetrahedral coordination arising from a hexagonal close packing of anions in which half the tetrahedral sites are occupied by cations. Examples of AB compounds crystallizing in this structure are CuCl, CuBr, Cul, Agl, BeO, ZnO, ZnS, MnS, MnSe,... 

Fig. 3.4 The Phiilips-Van Vechten structure map ( c, ) for the sp-valent octet AB compounds. The four-fold coordinated zinc blende and wurtzite structure types are separated from the six-fold coordinated NaCI structure type by the straight line corresponding to the degree of ionicity a = 0.785. (After Phillips and Van Vechten (1969).)...

Many divalent metal oxides and sulfides such as BeO, ZnO, BeS, MnS, ZnS. CdS, and HgS adopt the zinc blende or wurtzite structures, or occasionally both. Other compounds with these structures include Agl, NH F, and SiC. 

Agl, in its low temperature form, crystallizes in the (hep) wurtzite structure. The silver ions occupy tetrahedrally coordinated sites. The fast ion conducting Agl (7 >420K) is bcc. One can stabilize structural disorder at low temperature not only by incorporating cations such as Rb+, NH, etc., but also by adding S2 to reconstruct the anion sublattice and obtain Ag3SI. This compound exists in three different structures. At high temperature (>519K), it is bcc and both the cations and... 

For structures of type D (fiA bridging), three alternatives are possible, all rather rare. Type D1 was first detected in the compound [Co4S2(CO)10].73 More recent examples are [Mo4(NO)4S,3]4 (ll)3 and [Ni9(/i4-S)3(/i3-S)6(PEt3)6]2 (12).74 Type D2 has been found in [SZn4(S2 AsMe2)6]75 and type D3 in [Fe4S(SR)2(CO)12].76 Here the coordination corresponds to that of the zincblende or wurtzite structure. 

Many compounds of the type AB have structures in which each A is tetrahedrally bonded to four B, and vice versa. The zinc blende and wurtzite structures are of this type, and differ in details which need not concern us. Examples include ZnS, Cul, BeO, BN and A1P. The diamond lattice is of the zinc blende type. 

Compounds with the wurtzite structure are shown in Table 6.2. 6.2.2. The 2-2PT Crystal Structure of I/, Ice... 

The majority of unipolar ionic conductors identified to date are polymorphic compounds with several phase transitions, where the phases have different ionic conductivities owing to modifications in the substructure of the mobile ions [28], One of the first studied cationic conductors was a-Agl [21]. Silver iodide exhibits different polymorphic structures. Agl has a low-temperature phase, that is, [3-Agl, which crystallizes in the hexagonal wurtzite structure type, and a high-temperature cubic phase, a-Agl, which shows a cubic CsCl structure type [20,22] (see Section 2.4.5). 

Zinc oxide (ZnO) is an oxidic compound naturally occurring as the rare mineral zincite, which crystallizes in the hexagonal wurtzite structure P63inc [16]. The mineral zincite was discovered in 1810 by Bruce in Franklin (New Jersey,... 

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. 

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