Lattice modulation in the hexagonal-type antiphase domain structure of Au–33at.%Cd alloy

Abstract

The crystal structure of an ordered Au–33at.%Cd alloy has been reinvestigated by X-ray diffraction. Least-squares refinement using single-crystal intensity data collected by photographic methods has shown that mixed occupation by the two kinds of atoms preferentially occurs in the atomic sites located near the boundaries of the hexagonal antiphase domains, confirming the results obtained by electron diffraction [Hirabayashi, Yamaguchi, Hiraga, Ino, Sato & Toth (1970). d. Phys. Chem. Solids, 31, 77–94]. The refinement has also shown that many of the atoms are periodically displaced from the normal positions of the fundamental h.c.p. lattice. The physical significance of the occupancy and displacement modulations is discussed. The latter bears a resemblance to the motion of cations in the transition-metal dichalcogenides and the direction of the displacements in the alloy can be explained if charge-density waves synchronizing with the occupancy waves are assumed to exist.