Facet, Dislocation

Among the problems facing crystal growers are dislocations, facets, striations, and nonstoichiometry. 

The discovery of perfect geodesic dome closed structures of carbon, such as C o has led to numerous studies of so-called Buckminster fullerene. Dislocations are important features of the structures of nested fullerenes also called onion skin, multilayered or Russian doll fullerenes. A recent theoretical study [118] shows that these defects serve to relieve large inherent strains in thick-walled nested fullerenes such that they can show faceted shapes. 

The anodic dissolution of metals on surfaces without defects occurs in the half-crystal positions. Similarly to nucleation, the dissolution of metals involves the formation of empty nuclei (atomic vacancies). Screw dislocations have the same significance. Dissolution often leads to the formation of continuous crystal faces with lower Miller indices on the metal. This process, termed facetting, forms the basis of metallographic etching. 

The Stonybrook group under Dudley has studied the behaviour of ice bicrystals and has shown that under certain conditions, grain boundaries can act as somces of dislocations. Grain boundary facets have been shown to act as dislocation nucleation sites and grain boundaries themselves have been observed to act as barriers to dislocation motion. 

The relevance of crystal faces to the subject of electrociystalhzation comes up as follows Each of the crystal faces just described contains all the microfeatures that have been described in previous sections, steps, kinks, etc. Further, the same phenomena of deposition—the ions crossing the electrified interface to form adions, the surface diffusion, lattice incorporation of adions, screw dislocation, growth spirals, etc.—occur on all the facets. 

Pits of the nature described here are generally desirable in etching Intended to reveal dislocations, as mentioned in section 2.1 however, these pits faceted along definite crystallographic planes are the basis for orientation of crystals by optical techniques. The accuracy of optical techniques depends, in part, on the flatness of the facets. Wolff, Wilbur, and Clark (33) have listed the accuracy achieved for silicon and germanium. The accuracy obtained thus far for metals is not as good. 

Since one desires only light reflections from etched facets, the entire surface of the sample should be covered with etch pits. This can be obtained by etching an abraded surface. Fig. 5 shows the results of the action of a preferential etch for germanium on an abraded and on a polished surface. The multitude of pits on a lapped surface raises an interesting question are these pits caused mainly by the multitude of Irregularities produced an the surface by abrasion or by the multitude of mechanically induced dislocations In an attempt to answer this... 

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