Lattice Modulations in the CuAu Alloy

The lattice modulation in the long-period superlattice CuAu II is investigated on the basis of a theory recently developed by Teramoto and the author to explain the origin of the long-period superlattice and the phase transitions in the CuAu alloy. From a symmetry consideration, we find that there are two types of lattice modulations: one is uniform along the $c$ axis and the other changes its sign alternately layer by layer along the $c$ axis. The calculated atomic displacements are in semiquantitative agreement with the experimental results, if the repulsive interaction between atomic cores is considered as causing the lattice modulations. In addition, these lattice modulations explain some peculiar properties of the x-ray diffraction patterns: the asymmetry in the intensities of the split spots of the same order, and the intensities of the satellites around the ordinary Bragg diffraction spots.