Crystal defect mechanism

These observations were the basis for the proposal that polymers, like ionic crystals, exhibit shock-induced polarization due to mechanically induced defects which are forced into polar configurations with the large acceleration forces within the loading portion of the shock pulse. Such a process was termed a mechanically induced, bond-scission model [79G01] and is somewhat supported by independent observations of the propensity of polymers to be damaged by more conventional mechanical deformation processes. As in the ionic crystals, the mechanically induced, bond-scission model is an example of a catastrophic shock compression model. 

At very low temperatures, Holstein predicted that the small polaron would move in delocalized levels, the so-called small polaron band. In that case, mobility is expected to increase when temperature decreases. The transition between the hopping and band regimes would occur at a critical temperature T, 0.40. We note, however, that the polaron bandwidth is predicted to be very narrow ( IO Viojo, or lO 4 eV for a typical phonon frequency of 1000 cm-1). It is therefore expected that this band transport mechanism would be easily disturbed by crystal defects. 

So important are lattice imperfections in the reactions of solids that it is considered appropriate to list here the fundamental types which have been recognized (Table 1). More complex structures are capable of resolution into various combinations of these simpler types. More extensive accounts of crystal defects are to be found elsewhere [1,26,27]. The point which is of greatest significance in the present context is that each and every one of these types of defect (Table 1) has been proposed as an important participant in the mechanism of a reaction of one or more solids. In addition, reactions may involve structures identified as combinations of these simplest types, e.g. colour centres. The mobility of lattice imperfections, which notably includes the advancing reaction interface, provides the means whereby ions or molecules, originally at sites remote from crystal imperfections and surfaces, may eventually react. 

In a more general application, thermoluminescence is used to study mechanisms of defect annealing in crystals. Electron holes and traps, crystal defects, and color-centers are generated in crystals by isotope or X-ray irradiation at low temperatures. Thermoluminescent emission during the warmup can be interpreted in terms of the microenvironments around the various radiation induced defects and the dynamics of the annealing process (117-118). ... 

Crystals with Frenkel or Schottky defects are reasonably ion-conducting only at rather high temperatures. On the other hand, there exist several crystals (sometimes called soft framework crystals ), which show surprisingly high ionic conductivities even at the room or slightly elevated temperatures. This effect was revealed by G. Bruni in 1913 two well known examples are Agl and Cul. For instance, the ar-modification of Agl (stable above 146°C, sometimes denoted also as y-modification ) exhibits at this temperature an Ag+ conductivity (t+ = 1) comparable to that of a 0.1m aqueous solution. (The solid-state Ag+ conductivity of a-Agl at the melting point is actually higher than that of the melt.) This unusual behaviour can hardly be explained by the above-discussed defect mechanism. It has been anticipated that the conductivity of ar-Agl and similar crystals is described... 

At present the iron-based alloys diffusion saturation by nitrogen is widely used in industry for the increase of strength, hardness, corrosion resistance of metal production. Inexhaustible and unrealized potentialities of nitriding are opened when applying it in combination with cold working [1-3], It is connected with one of important factors, which affects diffusion processes and phase formation and determines surface layer structure, mechanical and corrosion properties, like crystal defects and stresses [4, 5], The topical question in this direction is clarification of mechanisms of interstitial atoms diffusion and phase formation in cold worked iron and iron-based alloys under nitriding. 

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). 

Fig, 7.15 Mechanisms of ionic conduction in crystals with defect structures (a) vacancy (Schoilky defect) mechanism, (b) interstitial (Frenkel defect) mechanism, (c) inlcrsthialcy (concerted Schottky-Frenkel) mechanism. 

Table I. Intrinsic Defect Mechanisms in Nonlinear Optical Oxide Crystals...

For nonmetallic substances, the electrons cannot move as freely as in the case of metals because their energy bands are essentially completely full or empty. The electrical conductivity in nonmetallic materials is dominated by another mechanism, i.e., the defect mechanism, instead of electron conduction. In ionic crystals such as salts (e.g., sodium chloride), two types of ions, cations and anions, are driven to move by the electrical force qE once an electrical field is applied. The ions can move only by the defect mechanism that is, they exchange position with a vacancy of the same type. At the room temperature, the fraction of vacancies for salt is very small (of the order of 10-17) with low exchange frequency (of the order of 1 Hz) so that electrical conductivity is extremely low. Although impurities and high temperature can affect electrical conductivity by a large factor, nonmetallic materials generally have very low electrical conductivity and these substances are widely used as electrical insulators. 

A lattice of crystal defects 5 are formed on the surface of a CdTe substrate 1 by producing flaws in the surface mechanically by means of a needle or by the use of an electron beam. 

The introduction of crystal defects such as oxygen vacancies is also an important mechanism for doping metal oxides. In fact, many of the n-type doping processes for... 

Relaxation in Insulating Crystals. The low-frequency relaxation of ionic materials consists typically of a set of simple decays arising from the motion of defects. - Some of these have freedom of extensive motion and contribute to the conduction current others are constrained to a few ndghbouring sites. Except at high temperature, the defects are remnants of the previous thermal and mechanical history of the sample. Their movement is a thermally activated process and relaxation times normally vary with temperature as exp AjkT). The e qierimental picture may be much complicated by the motion of electrons loosely bound to crystal defects. 

Kirby, S. H., Christie, J. M. (1977). Mechanical twinning in diopside Ca(Mg, Fe)Si206 structural mechanism and associated crystal defects. Phys. Chem. Minerals, 1, 137-63. 

---