Long Period Superlattice Cu3Au II

Abstract

A detailed study of the Cu-Au system between 25–50 at. % Au was made to determine the range of stability of the long period superlattices found in this system. The single-crystal thin film technique was used to make the samples, and the structure was determined using electron diffraction. It was found that a one-dimensional long period superlattice exists as an equilibrium phase from about 30% Au, substantiating Scott's original observation, to 60% Au. From about 30% to 38% Au, the antiphase structure is the ${\mathrm{Cu}}_3\mathrm{Au}$ II type, while at higher Au content, the structure is the CuAu II type. The domain size M was found to decrease continuously from a value of about 10 at 30% Au to 5 at 50% Au, regardless of the change in structure when crossing the phase region between ${\mathrm{Cu}}_3\mathrm{Au}$ and CuAu. Small amounts of Al added to a sample having the CuAu II structure and an Au content of 40%, decreased the domain size in a manner identical to that observed for stoichiometric CuAu II. The addition of Al to a sample having an Au content of 27.5%, where an antiphase structure is not observed, resulted in the formation of an antiphase structure for which the period could be predicted by our present theory. Qualitative arguments are presented to explain the appearance of these structures. Particular emphasis is placed on the role of the antiphase domain boundary energy and the electron energy in determining the formation and period of the superlattice.