On the triangular incommensurate phase in γ-brass. I

The origin of the stability of the triangular incommensurate (IC) phase of gamma -brass ( gamma -Cu 1-x Zn x ) is discussed. In a very narrow concentration range of 0.586<x<0.60, gamma -CuZn exhibits a long-period triangular inversion antiphase domain structure with the spacing as long as 2000 AA. In order to elucidate the origin of such an extremely long-period IC structure, the authors develop a phenomenological treatment based on the Landau-Ginzburg free energy functional. The order parameter is established in terms of a particular phonon mode in the prototype BCC phase. By introducing the symmetry-adapted coupling between the gradients of the order parameter and macroscopic strains, it has been shown that when the coupling is sufficiently large, a spatially modulated triangular IC phase is stabilised. The extraordinarily large periodicity is ascribed to near cancellation of two competing contributions of energy terms to the free energy functional.