Heteropolar crystals

Figure 5.12 Glide activation energies from high temperature data vs. energy band gaps. Note that the data for the homopolar crystals (C, SiC, Si, and Ge) lie quite close to the correlation line, while the data for The heteropolar crystals show some scatter. The reason why GaP Is an exception is not known. Also, note that the slope of the correlation line is two. 

E4 - energy obtained by binding polar groups of the surfactant with ions of the heteropolar crystal surface ... 

The cause of these cracks may perhaps be that the natural packing, at any rate of a heteropolar crystal (one composed of ions such as Na and Cl) in bulk is some 5 per cent, greater than the natural packing, under the known electrostatic forces, of the atoms in a surface layer.3 The attempt on the part of the atoms in a newly formed surface to adjust themselves from the more widely separated packing of the interior to the closer packing of a surface would probably produce considerable tensions and cracks in the surface layer. 

In many cases, heteropolar crystals conduct electric current through the motion of ions, and they can be electrolyzed by means of a sufficiently high voltage. Even when, in certain ranges of component activities, electronic partial conductivity predominates in an ionic crystal, its absolute value is always small in comparison with that of normal semiconductors or metals which will be discussed later. One final characteristic property should be mentioned Ionic crystals absorb strongly in the infrared by virtue of vibrations of the totality of the cations and anions in their sublattices. 

Evjen HM. On the Stability of Certain Heteropolar Crystals. Phys Rev. 1932 39 675. 

The surface energies of simple crystals have been examined from the point of view of the lattice energy of heteropolar compounds by Born Atomtheorie des festen Zustands, 2nd ed. 1923), and by Jones and Taylor (Proc. Boy. Soc. A, cii. 496, 1925). The following are typical of the data obtained ... 

When either arsenolite or senarmontite is sublimed on to mica, it is deposited in oetahedra, respectively isotropic and birefringent, oriented so that similar dimensions of the crystal meshes coincide,8 for example -. As4Oe 13-54, Sb406 13-64, mica 13-66 A. Such orientation appears only to occur with minerals of ionic structure 9 and when both substances concerned have heteropolar linkings,10 so that the phenomenon is said to provide evidence of this type of linking. 

Sherman (114) and other workers have compared crystal energies obtained by the Born-Haber cycle from experimental thermochemical data with theoretical values calculated assuming strict ionic character. The differences obtained have been used to indicate deviations from strict heteropolarity. Sherman 114) in his review gives results for 50 crystals, the computations being made with the Born-Lande equation. 

The Schomaker-Stevenson relationship states that heteropolar bonds are always stronger and shorter than hypothetical, purely covalent bonds between the same atoms. In an ionic crystal, would you expect some covalency to shorten or lengthen the bond Explain. (Shannon, R. D. Vincent, H. Sirtict. Bonding (Berlin) 1974. 19, I.)... 

Furthermore, the actual Coulomb forces are also important. If the molecules have polar substituents or a permanent dipole moment, or if they are electrically charged in a heteropolar fashion, i.e. the crystals are salts, then the intermolecular interactions are naturally also determined by the static monopole, dipole, and Coulomb forces with their long range. This of course also influences the crystal structures... 

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