Lattices, imperfections

After some typical time, r, the electron will scatter off a lattice imperfection. This imperfection might be a lattice vibration or an impurity atom. If one assumes that no memory of the event resides after the scattering... 

Chu S N G 1993 Long wavelength laser diode reliability and lattice imperfections MRS Bull. 17 43... 

PolycrystaUine material in single crystals, it is anisotropic and affected by impurities and lattice imperfections. 

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. 

Hulbert [77] discusses the consequences of the relatively large concentrations of lattice imperfections, including, perhaps, metastable phases and structural deformations, which may be present at the commencement of reaction but later diminish in concentration and importance. If it is assumed [475] that the rate of defect removal is inversely proportional to time (the Tammann treatment) and this effect is incorporated in the Valensi [470]—Carter [474] approach it is found that eqn. (12) is modified by replacement of t by In t. This equation is obeyed [77] by many spinel formation reactions. Zuravlev et al. [476] introduced the postulate that the rate of interface advance under diffusion control was also proportional to the amount of unreacted substance present and, assuming a contracting sphere (radius r) model... 

Characteristically, the mechanisms formulated for azide decompositions involve [693,717] exciton formation and/or the participation of mobile electrons, positive holes and interstitial ions. Information concerning the energy requirements for the production, mobility and other relevant properties of these lattice imperfections can often be obtained from spectral data and electrical measurements. The interpretation of decomposition kinetics has often been profitably considered with reference to rates of photolysis. Accordingly, proposed reaction mechanisms have included consideration of trapping, transportation and interactions between possible energetic participants, and the steps involved can be characterized in greater detail than has been found possible in the decompositions of most other types of solids. 

In principle, reaction schemes similar to that given in the preceding paragraph may be developed for other comparable rate processes, for example spinel formation. However, Stone [27] has pointed out that, where the barrier phase is not an efficient ionic conductor, the overall reaction may be controlled by the movement of a single cation and anion. In addition, there is the probability that lattice imperfections (internal surfaces, cracks, leakage paths [1172], etc.) may provide the most efficient route to product formation.]... 

As with solid phase decompositions (Sect. 1), the kinetic characteristics of solid—solid interactions are controlled by the properties of lattice imperfections, though here many systems of interest involve the migration, in a crystal bulk of a mobile participant, from one interface to another. Kinetic measurements have been determined for reactions in a number of favourable systems, but there remain many possibilities for development in a field that is at present so largely unexplored. 

In this temperature range, the number of phonons is small, and their scattering is due to lattice defects or to crystal boundaries. Of the two processes of scattering, the latter is of more importance since, at low temperatures, the dominant phonon wavelength is larger than the size of the lattice imperfections. As a consequence Aph is usually temperature independent. Hence, the temperature dependence of the thermal conductivity is that of the specific heat ... 

The lattice imperfections play an important role in reactions in solid state. The reactivity of solids are due to the defect or fault in the lattice. The more perfect a crystal is, the smaller is its reactivity. The defect may be point defect, dislocations, stacking faults, bulk defects etc. 

Surface charge at the phase boundary may be caused by lattice imperfections at the solid surface and by isomorphous replacements within the lattice. For example, if in any array of solid Si02 tetrahedra an Si atom is replaced by an Al atom (Al has one electron less than Si), a negatively charged framework is established ... 

Similarly, isomorphous replacement of the A1 atom by Mg atoms in networks of aluminum oxide octahedra leads to a negatively charged lattice. Clays are representative examples where such atomic substitution causes the charge at the phase boundary. Sparingly soluble salts also carry a surface charge because of lattice imperfections. 

Namely, aluminium atoms can be inserted into the hydroxyl nests composed of four SiOH groups, and also into the lattice imperfections formed from the hydroxy nests by the dehydration. The above results and discussion have been described in detail elsewhere [11]. 

DISLOCATION. In crystallography, a type of lattice imperfection whose existence in metals is postulated in order to account for the phenomenon uf crystal growth and of slip, particularly for the low value of shear stress required lo initiate slip. One section of the crystal adjacent to the slip plane is assumed to contain one mure atomic plane that the section on the opposite side of the slip plane. Motion of the dislocation results in displacement of one of the sections with respect to another. 

A similar behavior independent of bulk composition has been found by Campbell and Emmett (91) over Ni-Au films. These alloys also have a miscibility gap. Takasu et al. suggest that the large increase in activity of the low-temperature films is due to an abundance of lattice imperfections. The absence of impurities can also be a reason, because this has been shown to increase activities (92a-92c). Furthermore, in some previous experiments (87, 91, 93a, 93b) large amounts of hydrogen may have been adsorbed by the catalysts, because they were cooled in the presence of hydrogen. This might cause an enrichment of the surface with nickel (94). 

For the same amount of sulfide formation per unit area, significant differences in the number and sizes of aggregates could occur for different emulsion preparations. The number and types of lattice imperfections could be important in determining the number and distribution. Direct experimental evidence on the distribution is lacking. The gold treatment that can make reduction centers developable does not cause the silver sulfide centers to become developable (140). 

D includes the disorder resulting from both thermal motion and lattice imperfections. Ruland showed that these two kinds of disorder could be represented approximately as one and the same function... 

The electronic properties of RGS have been under investigation since seventies [3-7] and now the overall picture of creation and trapping of electronic excitations is basically complete. Because of strong interaction with phonons the excitons and holes in RGS are self-trapped, and a wide range of electronic excitations are created in samples free excitons (FE), atomic-like (A-STE) and molecular-like self-trapped excitons (M-STE), molecular-like self-trapped holes (STH) and electrons trapped at lattice imperfections. The coexistence of free and trapped excitations and, as a result, the presence of a wide range of luminescence bands in the emission spectra enable one to reveal the energy relaxation channels and to detect the elementary steps in lattice rearrangement. 

This suggests that the subband M2 is emitted by the excitons which are self-trapped in the regular lattice (M2-centers) while the component Mi is emitted by the centers (Mi-centers) which are populated during trapping that occurs with the lattice imperfections involved. 

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