(a) University of Bath
Figure1: Top and side views for the [001] screw dislocation
terminating
at the {001} surface of MgO. Mg ions are green and O red.
A major challenge in contemporary materials science is to develop a detailed understanding of the mechanisms of crystal growth. To this end we investigated the structure and stability of the [001] screw dislocation terminating at the {001} surface of MgO with energy minimisation, assuming 2-dimensional periodic boundaries using the program METADISE. We considered two dislocations, one with 2 (2V) and the other with 4 vacancies (4V) per unit length. A total of 14,000 ions were used in the simulation cell in order to ensure convergence of the surface energy with 3200 relaxed explicitly. The relaxed surface energies (energies required to cleave the crystal) were 1.32, 1.03 and 1.13 Jm-2 for the perfect crystal, dislocation terminated surfaces 2V and 4V respectively. The most stable dislocation, 2V, is shown in figure 1 with both top and side views.
Figure2: Expanded view of the [001] screw dislocation showing
the order of the MgO
unit addition dark blue, light blue, and subsequent MgO units yellow.
The adsorption energies of molecular units at the screw dislocation 2V were calculated so that we could follow the structural development of the growth spiral. The mechanism was found to be a three stage process with the first MgO unit adding parallel to the surface at the dislocation core. The second MgO unit added in parallel directly above the first unit to complete the step. The third and subsequent MgO units added perpendicular to the surface propagating the step as illustrated in figure 2.