Crystals crystal face, description

Figure 3. Computer draining of crystal from face description...

Active crystal face of vanadyl pyrophosphate for selective n-butane oxidation catalyst preparation, 157-158 catalyst weight vs. butane oxidation, 162,163/ catalytic activity, 162,1 (At catalytic reaction procedure, 158 experimental description, 157 flow rate of butane vs. butane oxidation, 162,163/ fractured SiOj-CVO PjO scanning electron micrographs, 160,161/ fractured scanning electron... 

Obviously, much of the development of crystallography predates the discovery of diffraction of X-rays by crystals. Early studies of crystal structures were concerned with external features of crystals and the angles between faces. Descriptions and notations used were based on these external features of crystals. Crystallographers using X-ray diffraction are concerned with the unit cells and use the notation based on the symmetry of the 230 space groups established earlier. 

For a mathematical description of crystal faces, take any three non-parallel faces (chosen to be mutually orthogonal, if possible) and take their intersections as reference axes, which are labeled OA, OB, and OC with the origin at O, as shown in Fig. 9.1.2(a). Let another face (the standard face or parametral face A B C ) meet these axes at A, B, and C, making intercepts OA = a, OB = b and OC = c, respectively. The ratios a b c are called the axial ratios. 

Zincite is usually colored red or orange by manganese impurities. Photographs of zincite are shown in Fig. 1.2. Zinc oxide crystals exhibit several typical surface orientations. The most important surfaces are the (0001) and (0001) (basal plane), (1010) and (1120) (prism planes) and (1121) (pyramidal plane) crystal faces. In principle, the (0001) planes are terminated by Zn atoms only, while the (0001) surfaces are terminated by oxygen atoms only. However, this simple picture does not hold in reality (see description of the surface structure in Sect. 4.2.1 of this book). Nevertheless, the etching behavior is noticeably different for these two surfaces [17] (see also Chap. 8). 

A perfect crystal face should be completely free of any surface defects. In view of its further application for crystal growth studies, however, a face not intersected by screw dislocations can be considered conditionally as perfect. All other defects have either little or no effect on the growth behavior of the face. To meet this situation, the term quasi-ideal or quasi-perfect" has been introduced for the description of faces free of screw dislocations [5.14]. A quasi-perfect face is characterized by extended atomically smooth terraces separated by monatomic steps and absence of emergence points of screw dislocations. A smooth quasi-perfect face without steps can be described as an intact quasi-perfect face". 

The most recent experimental work has involved studies of organic adsorption at the single crystal faces of polarizable solid metal electrodes [57]. These experiments provide details of the role of the metal in organic adsorption. By examining these data within the context of the new molecular descriptions of interfacial adsorption the theory of this important process will be greatly advanced. 

The description of the system in terms of this set of variables implies that we have to regard the solid phase as a solid solution of the sorbate in the zeolite, the concentration of the solid solution being given by Wg and. The mass of the solid sorbent is an essential variable, whereas the area of the phase boundary between solid and gas is not important. The situation is thus different from the case of adsorption on the surface of a solid, where we can define a surface concentration as the amount of matter from the gas per unit area of phase boundary, which is in excess of the concentration in the gas. In the case of sorption by zeolites, the sorbed amount is generally found to be proportional to the mass of the solid sorbent, independent of crystallite size or extent of phase boundary sorption in the bulk volume is generally large compared with adsorption on the crystal faces, and therefore the latter will be neglected in the present considerations. 

Crystal habit refers to the external appearance of the crystal. A quantitative description of a crystal means knowing the crystal faces present, their relative areas, the lengths of the axes in the three directions, the angles between the faces, and the shape factor of the crystal. Shape factors are a convenient mathematical way of... 

Morphology. The morphology of a crystal is the geometric description of its outward appearance in terms of the crystal faces (i.e., size, shape, and angular relationships of the facets—see Figure 11). 

Figure 2.9 Description of a crystal face by its Miller index hid. The large face intersects the three axes atx = 1 a, y = 2 b, and z = 4 c the small face intersects atx = (1/4) o,...

In the preceding sections we have considered a well-defined surface structure typical for crystal faces prepared under ultra-high-vacuum (UHV) conditions. In practice, one more frequently deals with surfaces which are rough. These require a different description. 

That benzene hexachloride isomer mixture is then the raw material for lindane production. The production of lindane per se is not a chemical synthesis operation but a physical separation process. It is possible to influence the gamma isomer content of benzene hexachloride to an extent during the synthesis process. Basically, however, one is faced with the problem of separating a 99%-plus purity gamma isomer from a crude product containing perhaps 12 to 15% of the gamma isomer. The separation and concentration process is done by a carefully controlled solvent extraction and crystallization process. One such process is described by R.D. Donaldson et al. Another description of hexachlorocyclohexane isomer separation is given by R.H. Kimball. 

The term crystal structure in essence covers all of the descriptive information, such as the crystal system, the space lattice, the symmetry class, the space group and the lattice parameters pertaining to the crystal under reference. Most metals are found to have relatively simple crystal structures body centered cubic (bcc), face centered cubic (fee) and hexagonal close packed (eph) structures. The majority of the metals exhibit one of these three crystal structures at room temperature. However, some metals do exhibit more complex crystal structures. 

In Ref. 42 a similar approach was chosen as in Ref. 39 using stereoisomers of the type Fmoc-L-Asp-L-Asp-D-Xaa-D-Xaa (Xaa = Gly, Ala, Phe, His, Ser, Tyr). Interestingly, in part the findings are different. The ACE/MS hyphenation caused a number of practical problems affecting the reliability of the system. Surprisingly, the authors faced problems with positive ESI and were forced to use negative ionization. Because of the use of the nonvolatile Tris buffer, crystallization problems occurred frequently. Only high-EOF conditions prevented this knockout scenario. However, the description of problems and related solutions is very instructive. 

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