Differences in crystal structure and

A white pigment for rubbers and plastics characterised by high tinctorial power, fastness to light, and chemical stability. Titanium dioxide pigments are made in two crystal forms, mtile and anatase, which differ in crystal structure and crystal size. 

In Fig. XI-3, the pressure scale is changed in the other direction, so that we show up to 12,000 atm. Here the gaseous phase, which exists for pressures only up to a few hundred atmospheres, cannot be shown on account of the scale. On the other hand, a great deal of detail has appeared in the region of the solid. It appears that, in addition to the familiar form of ice, there arc at least five other forms (the fifth exists at higher pressures than those shown in the figure). These forms, called polymorphic forms, presumably differ in crystal structure and in all their physical properties, as density, specific heat, etc. The regions whore these phases exist separately are divided by equilibrium lines, on... 

The use of the thermodynamic data for a discussion of stability of crystals is demonstrated in Fig. 4.52. The differences in crystal structure and melting temperatures of successive n-alkanes have been linked to the symmetries of odd and even CHj-sequences. Note, that planar zig-zag chains of odd-numbered sequences of carbon atoms in a molecular backbone point with their final bonds into the same direction, while even ones point into opposite directions. Odd paraffins have an orthorhombic, rectangular layer-stmcture, while the even ones up to C24H50 are triclinic with oblique layers. From structural analyses, orthorhombic crystals can accommodate even and odd chains without difference in packing density. The triclinic... 

At high pressures, a substance may have additional triple points for two solid phases and the liquid, or for three solid phases. This is illustrated by the pressure-temperature phase diagram of H2O in Fig. 8.4 on the next page, which extends to pressures up to 30 kbar. (On this scale, the liquid-gas coexistence curve lies too close to the horizontal axis to be visible.) The diagram shows seven different solid phases of H2O differing in crystal structure and... 

In this chapter the structure and thermodynamics of the various interfaces which can form between different phases in sohd systems are described. Essentially, all tbe features described in Chapter 4 for single-phase systems apply, with added complexity being introduced by differences in crystal structure and/or composition of the adjoining phases. Particular emphasis is placed on the effects of surfaces and interfaces on chemical reactions involving thin films. 

KYANITE. (Cyanite.) 3Al203-3Si02. This mineral has the same chemical composition as andalusite and sUlimanite, but differs in crystal structure and physical properties. Kyanite ore has a specific gravity of 3.5-3.7 and a variable hardness, 4—5 parallel to the long direction of its blades and 6-7 across them. 

Fibers can also be spun from isotropic solutions of cellulose in NMMO, as a variation of the Lyocell process. Theoretically, fibers from isotropic solutions would exhibit different structures and inferior mechanical properties vis-a-vis those spun from mesophases. But high tensile properties, strength and modulus can be obtained by conditioning at elevated temperatures. Chae et al. [82] spim cellulose fibers from isotropic cellulose spinning dope in NMMO monohydrate. They foimd little difference in crystal structure and tensile strength between their fibers and conventional Lyocell libers. However, the eellulose fibers from isotropic solution had a lower tensile modulus, which may be helpful for cellulose fibers used in textiles. 

The two most familiar allotropes of sulfur, rhombic and monoclinic, have the same molecular formula, S8. However, they differ in crystal structure. Using the phase diagram shown in Figure C. you can deduce how to convert either of these allotropes to the other. Notice that rhombic sulfur is the stable allotrope at temperatures below about 95°C. If it is heated to that temperature at... 

Phosphate and cadmium uptake in HH. In the experiments described here the phosphate concentration has been varied between 5.5 and 6.5 M with sulfate concentrations between 0.02 and 0.3 M. In this whole range the incorporation of phosphate can be described by a D-value of 1.5-2 10, which is 10 times lower than for DH. As pointed out in section 3.2, this can be caused by differences in crystal structure of HH and DH or in solution composition, or by the higher temperature of HH crystallization. 

MAO A and B differ in primary structure and in substrate specificity [5,7]. The two isozymes, located on the mitochondrial outer membranes, have 70% homology in peptide sequence and share common mechanistic details. It is now recognized that these are different proteins encoded by different genes, but probably derived from a common ancestral gene. Crystal structures for both MAO A and B complexes with inhibitors have recently been reported [8]. Serotonin is selectively oxidized by MAO A, whereas benzylamine and 2-phenylethylamine are selective substrates for MAO B. Dopamine, norepinephrine, epinephrine, trypt-amine, and tyramine are oxidized by both MAO A and B in most species [9]. In addition, MAO A is more sensitive to inhibition by clorgyline (1), whereas MAO B is inhibited by low concentrations of L-deprenyl ((f )-( )-deprenyl) (2) [5,6cj. Development of inhibitors that are selective for each isozyme has been an extremely active area of medicinal chemistry [8]. 

Most metals of practical importance are actually mixtures of two or more metals. Recall from Section 1.1.3 that these intimate mixtures of metals are called alloys, and when the bond between the metals is partially ionic, they are termed intermetallics. For the purposes of this chapter, and especially this section, we will not need to distinguish between an intermetallic and an alloy, except to note that when a compound is indicated on a phase diagram (e.g., CuAb), it indicates an intermetallic compound. We are concerned only with the thermodynamics that describe the intimate mixing of two species under equilibrium conditions. The factors affecting how two metal atoms mix has already been described in Section 1.1.3. Recall that the solubility of one element in another depends on the relative atomic radii, the electronegativity difference between the two elements, the similarity in crystal structures, and the valencies of the two elements. Thermodynamics does not yet allow us to translate these properties of atoms directly into free energies, but these factors are what contribute to the free energy of... 

The differences in the HMX data of Refs 14 19 is probably due in part to differences in crystal structure. Over the temp range of Ref 14, HMX is primarily in the 0 form, while in Ref 19 it is mostly in the 8 form. Of course there is also a difference in measurement methods — the Langmuir method in Ref 14 and the effusion method in Ref 19... 

Ideally, hydroxyapatite has the formula mentioned above. The synthetic material usually contains fewer than 10 Ca-ions and more than 2 OH-ions per crystal unit. Important differences in crystal structure, composition and specific surface exist between synthetic and biologic apatite. These differences result from the processing method of the raw materials and the synthetic method used. 

The English physicist William Barlow began as a London business man later he became interested in crystal structures and devoted his life to that study. In 1894, he published his findings of the 230 space groups. It is amazing that from consideration of symmetry three scientists in different countries arrived at the 230 space groups of crystals at about this time. Barlow then worked with ideas of close packing. He pictured the atoms in a crystal as spheres, which, under the influence... 

Five chemical compositions constitute these pigment classes. Medium chrome yellow is essentially PbCr04 and is the reddest hue of yellow. Solid solutions of PbCr04 with PbS04 in varying proportions yield primrose and lemon chrome yellows. Their hue difference is also attributable to the their polymorphic nature— primrose yellow is orthorhombic in crystal structure and lemon yellow is monoclinic. The less stable orthorhombic crystal is treated during primrose yellow manufacture to minimize hue shifts and conversion. 

In the normal alkanes (Table 31) one finds the alternation of the increase of melting point also well known in the fatty acids. This is a consequence of the difference in crystal structure of even and odd molecules resulting from a difference in symmetry (centre or plane through the middle). 

Theses are calculated from the 3D molecular structure, which is defined with coordinates of all atoms in the molecule. The step from 2D to 3D description of molecules is the crucial one [40-42]. The 3D structure is an ambiguously defined quantity, which depends on molecular environment, i.e., it is different in crystal structure, in solutions, or in vacuo. If it is theoretically determined it depends on the computational method. However, the 3D structures form a basis for a broad range of descriptors. Examples are mass distribution descriptors such as moment of inertia or gravitation index, and shape indices such as shadow indices, surface area indices, and van der Waals indices [29,40,41]. 

Seismological evidence has led to the realization that the Earth is a layered body, with the basic subdivisions into crust, mantle, and core being further refined to give an upper and lower mantle separated by a transition zone, and a liquid outer core and solid inner core. The different layers show fairly sharp contrasts in density and elastic properties, leading to speculation that changes in crystal structure (and possibly in electronic structure) as well as in overall composition are responsible for the layer-... 

In the description of crystals and crystal structures the two terms/om and habit have very specific and very different meanings. Form refers to the internal crystal structure and etymologically is the descendant of the Greek morph. Hence, polymorph refers to a number of different crystal modifications or different crystal structures, and the naming of different structures as Form F or a Form follows directly from this definition and usage. As we have seen above, the difference in crystal structure is very much, although not exclusively, a function of thermodynamics. Certainly, only the structures which are thermodynamically accessible can ever exist, but there often is a question of thermodynamic vs kinetic control over which particular structure may be obtained under any particular set of crystal growth conditions. 

Since polymorphs represent different crystal structures essentially every physical or chemical property may vary among the polymorphic structures of a material. A consequence of this is the fact that virmally any technique that measures the properties of a solid material may in principle be used to detect polymorphism and to characterize the similarities and difference among polymorphic structures. Some techniques are more sensitive to the differences in crystal structure or molecular environment, as opposed to molecular structure, and in many cases these are to be preferred in detecting and characterizing polymorphs. 

X-ray crystallographic methods, which reflect differences in crystal structure, in most cases can be definitive in the identification and characterization of polymorphs, and whenever possible should be included in the analytical methods utilized to define a polymorphic system. 

In terms of the structural features that are probed with various analytical methods, solid state nuclear magnetic resonance (SSNMR) may be looked upon as representing a middle ground between IR spectroscopy and X-ray powder diffraction methods. The former provides a measure of essentially molecular parameters, mainly the strengths of bonds as represented by characteristic frequencies, while the latter reflect the periodic nature of the structure of the solid. For polymorphs differences in molecular environment and/or molecular conformation may be reflected in changes in the IR spectrum. The differences in crystal structure that define a polymorphic system are clearly reflected in changes in the X-ray powder diffraction. Details on changes in molecular conformation or in molecular environment can only be determined from full crystal structure analyses as discussed in Section 4.4. 

AgF, AgCl and AgBr crystallize with a sodium chloride lattice and CuCl, CuBr, and Cul with the lattice given in Figure 62 in which copper has a coordination number of four. This difference in crystal structure of the two salts is not determined by the difference in the dimensions of the Ag"+ and Cu+ ions, since according to Pauling the latter has the value 0-96 A, which is not outside the limits for a sodium chloride lattice. The decrease of the coordination number from six to four and the formation of a tetrahedral configuration are more probably determined by the covalent character of the bonds between copper and chlorine. However, it is to be emphasized that the bonds are not entirely covalent any more than they are entirely ionic, but are of an intermediate... 

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