Long-period incommensurate superstructures in Cu-Au alloys: Relation with short-period ordering

The nature of the stability of incommensurate long-period structures in all oys of the system Cu-Au is investigated on the basis of first-principles ca lculations of the electronic structure. It is shown that many structural pr operties of such formations can be explained only if the latter are treated as superstructures with respect to ordinary superstructures (L1(2) or L1(0 )): the electron spectrum of the superstructure and not that of the initial disordered alloy must serve as the initial spectrum. The observed dependen ce of the long period N on the degree eta of the "short" long-range order i s explained. The reasons why two-dimensional long-period superstructures fr om in the alloy Au3Cu are found. Arguments supporting the fact that among q uasicrystalline substances long-period superstructures fall between incomme nsurate systems and quasicrystals are presented. (C) 2000 MAIK "Nauka/Inter periodica".