Crystal defective/disordered

Internal boundaries in a crystal, when disordered, form extended defects. However, if the boundaries become ordered, they simply extend the unit cell of the structure and hence are no longer regarded either as boundaries or defects (Fig. 3.20c). In addition, some boundaries can change the composition of a solid locally and, if present in large numbers, can change the macroscopic composition noticeably. When these are ordered, new series of compounds form. Boundaries that do cause significant composition changes are described in Chapter 4. 

Heat capacity of disordered systems Crystal defects... 

The notion of point defects in an otherwise perfect crystal dates from the classical papers by Frenkel88 and by Schottky and Wagner.75 86 The perfect lattice is thermodynamically unstable with respect to a lattice in which a certain number of atoms are removed from normal lattice sites to the surface (vacancy disorder) or in which a certain number of atoms are transferred from the surface to interstitial positions inside the crystal (interstitial disorder). These forms of disorder can occur in many elemental solids and compounds. The formation of equal numbers of vacant lattice sites in both M and X sublattices of a compound M0Xft is called Schottky disorder. In compounds in which M and X occupy different sublattices in the perfect crystal there is also the possibility of antistructure disorder in which small numbers of M and X atoms are interchanged. These three sorts of disorder can be combined to give three hybrid types of disorder in crystalline compounds. The most important of these is Frenkel disorder, in which equal numbers of vacancies and interstitials of the same kind of atom are formed in a compound. The possibility of Schottky-antistructure disorder (in which a vacancy is formed by... 

Probably the most important reaction mechanism is the liquid-mediated process (Hi). This is because most drugs, even those not particularly susceptible to hydrolysis, become less stable as the surrounding moisture levels increase. It has been speculated that degradation proceeds via a thin film of moisture on the surface of the drug substance [23], However, studies have indicated that the moisture is concentrated in local regions of molecular disorder, rather than in thin films [24], These regions that are crystal defects or amorphous areas, equate to the reaction nuclei of mechanisms (i) and (ii). 

There is still another type of internal solid state reaction which we will discuss and it is electrochemical in nature. It occurs when an electrical current flows through a mixed conductor in which the point defect disorder changes in such a way that the transference of electronic charge carriers predominates in one part of the crystal, while the transference of ionic charge carriers predominates in another part of it. Obviously, in the transition zone (junction) a (electrochemical) solid state reaction must occur. It leads to an internal decomposition of the matrix crystal if the driving force (electric field) is sufficiently high. The immobile ionic component is internally precipitated, whereas the mobile ionic component is carried away in the form of electrically charged point defects from the internal reaction zone to one of the electrodes. 

The totality of defects in a unit volume of the crystal is termed the disorder of the crystal. This disorder is assumed to be small and composed of defects having either a biographical or a thermal origin. The biographical disorder, denoted by X, is irreversible and preserved... 

Surface area is also directly proportional to the dissolution rate of a solute. Particle size reduction is another common and often efficient means by which to achieve higher levels of drug in solution at earlier time points.As particle size decreases, the surface area per unit volume of solute increases and consequently more drug is exposed to the solvent. Also, as particle size decreases the surface molecules are of higher free energy which increases dissolution. And finally, the processing of solid material can often lead to crystal defects within a particle or surface area where crystallinity is lost (amorphous), both of which can increase the apparent solubility. Mosharraf et al. have demonstrated the effect of crystal structure disorder on solubility and dissolution rate. ... 

Millard et al. 1992 Maekawa et al. 1997), because of the distinct chemical shift ranges for " Al and Al in oxides. In spinel, " Al gives a narrow peak near +70 ppm, whereas the Al gives a broader peak near +10 ppm (Fig. 27). The tetrahedral site in spinel has cubic point symmetry which requires Cq = 0, but crystal defects and the [Mg,Al] disorder results in a small distribution of EFG s at the Al site, so that the peak at 70 ppm contains only the central transition. The earlier studies suffered from low MAS rates, which yield spinning sidebands that overlap the centerbands, and unequal excitation of the two Al NMR signals. 

Ice Ih, the proton-disordered hexagonal phase of water, has been by far the most studied phase of ice, however, a complete understanding of many of its properties still remains elusive. The role played by crystal defects is among the important issues requiring better comprehension. It is in fact quite remarkable that not even the most basic disruptions of crystalline order, the molecular vacancy and self-interstitial, are well understood. ... 

All transitional aluminas crystallize in disordered (and distorted) defect spinel lattices. The unit cell contains a cubic close-packed array of 32 0 ions. Electroneutrality demands that of the 24 sites in the cation sublattice only 21f are occupied, leaving 2 vacant positions. In essence, the various transitional aluminas differ in the uniformity of the anionic stacking and in the distribution of aluminum atoms over the octahedral and tetrahedral sites. In contrast to the AIO(OH) and Al(OH)3 precursors and to a-Al203, the transitional aluminas show a significant fraction of tetrahedral aluminum sites, which are readily identified and quantitated by Al MAS-NMR 58,59], The fraction of four-coordinate aluminum sites decreases in the order t] 7 8 0, in accordance... 

It is well known (66) that the a-relaxation process of crystalline polymers consists of at least two processes, referred to as ai and U2 in the order of lower temperature, respectively. The ai-process (67-77) is pronounced in melt crystallized samples and is associated with the relaxation of grain boundaries, such as dislocation of lamellae with a frictional resistance related to disordered interface layers. The magnitude of the ai-process increases with the increase in the crystal defects. The o 2-process (71,73,78-83) is pronounced in single crystal mats and is ascribed to incoherent oscillations of the chains about their equilibrium positions in the crystal lattice in which intermolecular potential suffers smearing out. The magnitude of the Q 2-process increases with the increase in the lamellar thickness and/or the degree of crystallization (39). 

For the limiting case (Cd g) > we find that (VAg) — (CdAg). That is, the vacancy concentration is completely fixed by the addition of CdBr2. This is called the region of exclusively extrinsic disorder, as opposed to the region of intrinsic disorder. In the extrinsic region, those physical properties of the crystal which depend upon the point defect disorder are functions only of the concentration of dopant. However, in deriving eq. (4-26), it has been tacitly assumed that point defects do not form complexes. This assumption, as shown later, must eventually be modified. 

The polymer may be pictured as a matrix of crystalline material with units of various forms such as lamellae and fibrils, containing various kinds of crystal defects and interfaced by regions of greater disorder. Both ordered and disordered regions will contribute to the viscoelastic behavior. 

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