THE LONG-PERIOD SUPERLATTICE IN CUAU-II

We report the results of a series of calculations of the electronic st ructure of ordered, equi-atomic CuAu. We have investigated the effects of the creation of superzone boundaries on the Fermi surface of CuAu I when the long-period superlattice, the CuAu II phase, is formed. The new boundaries destroy appreciable regions of Fermi surface, thus fav ouring the formation of the latter phase. The positions of the boundar ies are related directly to the period of the long-period superlattice and we have investigated their dependence on the electron/atom ratio and pressure. We find that the results are in very good agreement with previous experimental measurements of the average domain size in CuAu II. Thus, we provide strong evidence that Fermi surface topology play s an important role in determining the period of the long-period super lattice in CuAu II.