A study of atomic V-steps formed by cleavage on NaCl crystal surfaces

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Abstract
Among atomic steps formed by cleavage on the surface of ionic crystals, V-patterns, frequently linked to each other forming zig-zag patterns, are especially typical. In the present work, several features of these patterns (i.e. step orientation, step height and sign, revealed by their splitting due to slow evaporation, formation of multiple V-steps) are studied on decorated replicas of NaCl crystals, and their relation with the dynamics of screw dislocations during crack propagation is discussed. We show that, though both V-sides frequently have irrational directions, one of them sometimes changes abruptly to [100] and [010] directions. This is interpreted in terms of glide and cross-glide motions of dislocations. Therefore a mechanism of zig-zag pattern formation is proposed according to which small dislocation loops, created by the crack, would move short distances, tightly wrapped around the crack tip and would disappear quickly due to an effect of interaction with cleavage steps. In agreement with previous results, these dislocations could not be observed after cleavage and the energy absorbed by their motion would be small with respect to that absorbed by larger loops, which are revealed after cleavage by X-ray topography and chemical etching methods.