Surface energies, antiphase boundaries

Using the constructed potentials the y-surface for the (111) plane was calculated. (For more details see Girshick and Vitek 1995). T e lowest energy minimum on this surface corresponds to the ideal Llo structure. However, there are three different metastable stacking fault type defects on (111) the antiphase boundary (APB), the complex stacking fault (CSF) and the superlattice intrinsic stacking fault (SISF). The displacements... 

In layered misfit structures of the type we are discussing, bonds at the layer surfaces (within and between the layers) will be strained periodically along a non-commensurate lattice direction parallel to the layers after a certain number of subcells there is a near match of the layers. Clapp has pointed out that, for a simple case, layer mismatch will cause tension in one layer type and compression in the other. The resulting strain energy may be relieved by the introduction of periodic antiphase boundary (apb) planes so that alternate contraction and extension occurs in all layers (Fig. 22) and hence cancels out (at the price of a small deformation of coordination polyhedra). 

It was Sato and Toth who showed that when low-energy imperfections such as antiphase domain boundaries were introduced into an ordered structure without changing the near-neighbor coordination, the positions of (some of) the Brillouin zone boundaries were altered so that they followed an expanding Fermi surface and maintained structural stability despite the increase in electron concentration due to alloying. Consider, for example, the ordered AuCu I structure. The reduced Brillouin zone is made up of 100 planes and the second extended zone is made up of (002) and (110 -type planes. Mapped back in the reduced zone, the second zone has a square cross section normal to the axis. With two electrons per primitive cell, the Fermi surface overlaps the 001 planes of the first zone and touches the 110 planes of the second zone. 

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