Zinc, crystal structure energy

The total energy of the system is one of the most important results obtained from any of the calculational techniques. To study the behavior of an impurity (in a particular charge state) in a semiconductor one needs to know the total energy of many different configurations, in which the impurity is located at different sites in the host crystal. Specific sites in the diamond or zinc-blende structure have been extensively studied because of their relatively high symmetry. Figure 1 shows their location in a three-dimensional view. In Fig. 2, some sites are indicated in a (110) plane... 

In metallic and many semiconducting crystals, the valence electrons are delocalized throughout the solid, so that antisite defects are not accompanied by prohibitive energy costs and are rather common. For example, an important defect in the semiconducting material GaAs, which has the zinc blend structure (Supplementary Material SI), is the antisite defect formed when an As atom occupies a Ga site. 

The carbonic anhydrase (Cam) in M. thermophila cells is elevated several fold when the energy source is shifted to acetate, suggesting a role for this enzyme in the acetate-fermentation pathway. It is proposed that Cam functions outside the cell membrane to convert CO2 to a charged species (reaction A4) thereby facilitating removal of product from the cytoplasm. Cam is the prototype of a new class (y) of carbonic anhydrases, independently evolved from the other two classes (a and P). The crystal structure of Cam reveals a novel left-handed parallel P-helix fold (Kisker et al. 1996). Apart from the histidines ligating zinc, the activesite residues of Cam have no recognizable analogs in the active sites of the a- and P-classes. Kinetic analyses establish that the enzyme has a zinc-hydroxide mechanism similar to that of Cab (Alber et al. 1999). 

The larger ions, however, will not make contact for the same ratio of ionic radii in the zinc-blende structure, with the result that the crystal energy is a minimum for this structure, which is, therefore, the stable one. 

Silicon crystallizes in the diamond structure,16 which consists of two interpenetrating face-centered cubic lattices displaced from each other by one quarter of the body diagonal. In zinc blende semiconductors such as GaAs, the Ga and As atoms lie on separate sublattices, and thus the inversion symmetry of Si is lost in III-V binary compounds. This difference in their crystal structures underlies the disparate electronic properties of Si and GaAs. The energy band structure in... 

However, Sacconi, in his early work [90] on some nickel and zinc chelates of V-methylsalicylideneaniline, showed that compounds with different crystal structures may still form mixed crystals since conformational changes in the molecular geometry of the minor component are induced by the dominating major component. The energy required for such conformational changes is presumably small by the formation of the mixed crystal and is, in any case, very small. 

Many studies on the direct reaction of methyl chloride with silicon-copper contact mass and other metal promoters added to the silicon-copper contact mass have focused on the reaction mechanisms.7,8 The reaction rate and the selectivity for dimethyldichlorosilane in this direct synthesis are influenced by metal additives, known as promoters, in low concentration. Aluminum, antimony, arsenic, bismuth, mercury, phosphorus, phosphine compounds34 and their metal complexes,35,36 Zinc,37 39 tin38-40 etc. are known to have beneficial effects as promoters for dimethyldichlorosilane formation.7,8 Promoters are not themselves good catalysts for the direct reaction at temperatures < 350 °C,6,8 but require the presence of copper to be effective. When zinc metal or zinc compounds (0.03-0.75 wt%) were added to silicon-copper contact mass, the reaction rate was potentiated and the selectivity of dimethyldichlorosilane was enhanced further.34 These materials are described as structural promoters because they alter the surface enrichment of silicon, increase the electron density of the surface of the catalyst modify the crystal structure of the copper-silicon solid phase, and affect the absorption of methyl chloride on the catalyst surface and the activation energy for the formation of dimethyldichlorosilane.38,39 Cadmium is also a structural promoter for this reaction, but cadmium presents serious toxicity problems in industrial use on a large scale.41,42 Other metals such as arsenic, mercury, etc. are also restricted because of such toxicity problems. In the direct reaction of methyl chloride, tin in... 

Tossell, J. A., and D. J. Vaughan (1981). Relationships between valence orbital binding energies and crystal structures in compounds of copper, silver, gold, zinc, cadmium and mercury. Inorg. Chem. 20, 3333-40. 

Indium antimonide (InSb) forms crystals with the zinc blende structure (similar to diamond). These crystals are semiconductors. Describe the bonding in a crystal of InSb, and draw an orbital energy-level diagram for the compound. Atomic orbitals for Sb are lower in energy than those for In. 

In Chapter 5 we will see that packing is the most important consideration in determining the structure adopted by predominantly ionically bonded crystals. The difference in between some crystal structures is very small. In such cases, for example, the zinc blende and wurtzite structures (named after the two crystalline forms of ZnS), the difference in the resulting electrostatic energy is small. For zinc blende and wurtzite it is -0.2%. When the energy difference between structure types of the same stoichiometry is small, we often encounter polymorphism the compound can form with more than one structure. We will examine this useful complication in Chapter 7. 

Figure 2 Relative total energy per CdS pair (dashed curve) and the energy gap between the occupied and unoccupied orbitals (solid curve) for finite Cd S clusters as a function of n for clusters derived from either the wurtzite (upper panel) or the zinc blende (lower panel) crystal structure (From ref. 19).

Under ambient conditions, CdSe crystallizes in the zinc blende structure. Ab initio calculations show that the energy of the zinc blende structure is lower than that of the wurtzite structure by 1.4 meV/atom. Under increasing pressure, the wurtzite structure transforms to the NaCl structure, the, 6-tin structure, and as yet unidentified structures. If not stated otherwise, the data in this section concern CdSe in the zinc blende structure. 

---