Crystallographic polarity

The atoms of the ill surface layer (let us say B atoms) are triply bonded to the lattice (Fig. 1). The atoms of the layer immediately below (in this case A atoms) are singly bonded to the lattice and triply bonded to die surface layer, ft the B layer is removed, the A layer immediately below is highly unstable. Thus the 111 surfaces with this particular orientation, (111), must terminate with B atoms. For the same reasons, the ill surfaces with the opposite orientation, (111), must terminate with A atoms. Consequently " steps" on Ul surfaces must consist of at least two atomic layers (Fig. 3). This must be also true in the case of the group IV elements, but in the case of die III-V compounds two types of ill surfaces result viz., those terminating with A atoms and those terminating with B atoms. The two types have distincdy different properties as will be pointed out below. One speaks of crystallographic polarity along the < 111 > directions. 

Bhalla, A. S., Tongson, L. L. and Newnham, R. E. Relationship of crystallographic polarity to piezoelectric, pyroelectric and chemical etching effects in LijGeOs and LiGa02 single crystals. J. Appl. Cryst. 16, 138-140 (1983). 

As shown in Fig. 10, the zinc blende structure has crystallographic polarity along (111) axes by nature. That is, there are (lll)B and (lll)N surfaces in cBN crystals, which results in the characteristic crystal habit and surface morphology. The crystallographic polarity of cBN should also affect etching behavior, crystal growth, dislocations, strain and abrading properties, chemical reactivity, impurity concentrations, etc. as observed in other sphalerite compounds (97). 

Absolute crystallographic polarity of cBN crystals was determined by Rutherford back-scattering spectroscopy (RBS) using helium ion beam (98). 

X-ray crystallographic studies (59) have defined the conformations and hydrogen bonding of the tetracyclines under nonpolar and polar conditions. These are shown ia Figure 3. It is beheved that the equiUbrium between the 2witterionic and nonioni2ed forms is of importance for the broad-spectmm antibacterial activity, membrane permeation, and pharmacokinetic properties. 

Maximum information is obtained by making Raman measurements on oriented, transparent single crystals. The essentials of the experiment are sketched in Figure 3. The crystal is aligned with the crystallographic axes parallel to a laboratory coordinate system defined by the directions of the laser beam and the scattered beam. A useful shorthand for describing the orientational relations (the Porto notation) is illustrated in Figure 3 as z(xz) y. The first symbol is the direction of the laser beam the second symbol is the polarization direction of the laser beam the third symbol is the polarization direction of the scattered beam and the fourth symbol is the direction of the scattered beam, all with respect to the laboratory coordinate system. 

Carbocations are intermediates in several kinds of reactions. The more stable ones have been prepared in solution and in some cases even as solid salts, and X-ray crystallographic structures have been obtained in some cases. An isolable dioxa-stabilized pentadienylium ion was isolated and its structure was determined by h, C NMR, mass spectrometry (MS), and IR. A P-fluoro substituted 4-methoxy-phenethyl cation has been observed directly by laser flash photolysis. In solution, the carbocation may be free (this is more likely in polar solvents, in which it is solvated) or it may exist as an ion pair, which means that it is closely associated with a negative ion, called a counterion or gegenion. Ion pairs are more likely in nonpolar solvents. 

Figure5.9. Hypothetical particle of cubic MgO, exhibiting three crystallographic surfaces. The (100) and (110) surfaces are nonpolar, implying that Mg and O ions are present in equal numbers, but the polar (111) surface can be terminated either by Mg cations or O anions. In practice, MgO crystals predominantly show (100) terminated surfaces. 

Recently, the In situ Raman scattering from Fe-TsPc adsorbed onto the low Index crystallographic faces of Ag was examined and the results obtained are shown In Fig. 5 (15). On the basis of the similarities of these spectra with those obtained for the macrocycle In solution phase, as well as the polarization behavior characteristics, It has been concluded that the most likely configuration Is that with the macrocycle edge-on with respect to the surface. This Is In agreement with conclusions reached from the UV-vlslble reflectance spectra. The preferred configuration, however, may depend on the particular macrocycle, as well as on the nature of the adsorption site. 

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