A new type of structural unit, the ¯42m dreidl, was proposed on the basis of molecular dynamics simulations of a core model for the (001) 90º twist grain boundary. The structural unit resembled a polyhedron in which some edges, that did not correspond to bonds between atoms, were absent. The dreidl had 42m (D2d) point-group symmetry, and consisted of 14 atoms which formed 8 five-membered rings that retained tetrahedral bonding within the boundary core. Molecular dynamics simulations, using the empirical Tersoff potential, were used to estimate the energy of the (001) 90º twist boundary at 0K. The effect of rigid-body translations parallel to the grain boundary plane, and alternative reconstructions which involved conventional structural units, were investigated. The twist boundary was found to have a low energy, as compared with previous structural models for twist grain boundaries in Si; in spite of the high degree of dimerization.

A.J.Belov, D.Conrad, K.Scheerschmidt, U.Gösele: Philosophical Magazine A, 1998, 77[1], 55-65