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Chapter VII Reactivity at Dislocation….
presence of the localized energy, and the lateral growth of pits may depend on
the formation of successive complexes as a result of the availability of the
common, or chemically similar-behaving, ions. Denoting the additive impurity as
MXn and an alkali halide by AX, the formation of successive complexes on the
surface at the dissolution site is as,
A
n
( n1)
M( H O)
.... .... mAX....
.... M( H O)
.... .... ( m1)
AX .... ....
2
6
2
5
The dissolution is more along the dislocation line and this type of mechanism
explains not only the change in the lateral growth of pits but also the change in
pit morphology [15-17], which has been studied by the application of activated
complex theory in conjunction with the adsorption of reacting species and
complexes on MgO crystals [17]. The role of dislocations in etching created a
great interest amongst many workers and many significant experiments were
conducted. It was the first time, the quantitative proofs regarding the reliability of
etch pit technique and the correctness of Burgers model was given by Vogel et al
[31]. Subsequently, the etch pit technique has been successfully applied to study
dislocations in plastically deformed crystals. Vogel [32] has studied the
dislocations in bent germanium crystals. Evidences of decoration of dislocations
by the etch pit techniques have been reported, as early as in 1952-53, by Gevers
et al and Gevers [33, 34]. During their study of etching on silicon carbide crystals
in fused forax at 900 o C, they observed that at the center of growth spirals, the
etch pits were developed. However, some of the pits were not located at the
center of the spirals. It was concluded that these other types of pits correspond to
edge dislocations.
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