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Chapter VII Reactivity at Dislocation….
saturation with their facets correspond to the low energy planes of KDP crystal.
The growth of etch pit was found continuous until the reservoir of water in the sol-
gel was saturated with KDP crystal. Moreover, in another study of surface
dynamics during environmental degradation of KDP crystal surfaces, Whitman et
al [57] have studied the occurrence of etch pits as a function of humidity and
under saturation. The pit formation was characterized by nucleation and growth
process. The introduction of water created a condition of under saturation at the
crystal surface. The equilibrium step directions could define the orientations of
the edges of the pits. The internal surfaces of the pits were the low index facets
of KDP crystal.
It would have been an incomplete discussion, if the present author had not
mentioned the efforts made by Sangwal and his colleagues to understand
chemical etching of crystalline faces,particularly, on KDP crystals [15,58,59].
Nelson et al [60] have studied the X-ray photoemission spectroscopic aspects
of etch pits on KDP crystals. They argued that the solution grown KDP was a
stable phase in the
K O P O H,D O ternary phase. If there was residual
2 2 5 2
P2O on the surface following the growth and knowing the hydroscopic nature of
5
P2O5 forming phosphoric acid (H3PO4), it can be surmised that the holes/etch pits
were due to the chemical etching.
In the case of dislocations, the formation of etch pits may take place either by
spontaneous opening up, by a repeated two dimensional nucleation process at
edge dislocation or by unwinding of spiral turns of screw dislocation. The strain
energy of dislocations is responsible for the spontaneous non-stationary
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