Morphological changes of the monatomic (100) NaCl cleavage structure due to edge self-diffusion: II. Numerical simulation

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Abstract

The partial differential equation describing morphological changes of monatomic steps on a crystal surface due to edge self-diffusion has been derived under the assumption of isotropy of specific edge free energy and edge self-diffusion coefficient. For the repeated integration of this partial differential equation it was necessary to develop a stable, convergent finite-difference scheme providing the numerical simulation of structural changes on any tip profile. For tips with angles of 2α>;1° club-shaped steady-state profiles are calculated, and for 2α<1° complete neckings. The excellent agreement of the numerical results with the morphological changes on monatomic NaCl cleavage tips described in part I indicates that in the temperature range of T<280°C the edge self-diffusion plays the dominating role in the molecular surface kinetics. The exact comparison of numerically and experimentally determined structures allows the activation energy of the edge self-diffusion to be established as 1.05 eV, and an estimation of the edge self-diffusion coefficient.

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    This happens, obviously, at steps of monomolecular heights. A corresponding melting-off of cleavage tips was also observed by Höche and Bethge at (100) surfaces of NaCl for temperatures below 553 K at cleavage tips with nearly parallel steps [29,30]. For these low temperatures the evaporation from the NaCl surface is also negligible.

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