SUPERLATTICE STUDIES IN THE SYSTEMS SILVER–MAGNESIUM–TIN AND SILVER–MAGNESIUM–ZINC

The 3/2 electron compound in the system silver–magnesium, in which tin and zinc are soluble, has an ordered structure of the cesium chloride type (second zone superlattice). The first additions of tin to this phase do not affect the ordered structure. Above a certain limit, a third zone superlattice is formed, and the tin atoms themselves occupy ordered positions. X-ray examination at high temperatures shows that the third zone superlattice disorders independently of the main, second zone superlattice. The transformation temperature increases with increasing tin content; that of the cesium chloride superlattice, however, decreases with increasing tin content. In the silver–magnesium–zinc system, the introduction of zinc into the binary silver–magnesium 3/2 electron compound causes no development of a third zone superlattice. The temperature at which the second zone superlattice disorders decreases with increasing zinc content.It is suggested that the reason for the existence of the second superlattice depends upon a combination of the following factors:(i) The lattice distortion due to the high valency of tin creates a long-range distortion which is more easily distributed, leading to a minimum interaction strain energy, by the formation of a doubly ordered structure.(ii) The electrochemical effect between magnesium and tin atoms at a distance "a" apart (where "a" is the lattice spacing of the body-centered cubic cell) contributes to the lowered free energy of a doubly ordered structure.