The γ-β transition in a superionic conductor Ag 3 SI is studied on the basis of the order-disorder transition for Ag ions. The Ag ion, taking one of the four equivalent sites which are slightly apart from the face centered position of cube, is represented by the orientational rotator with four components. The dielectric properties are described by the ordered parameter appearing in [111] direction, and the transition temperature is estimated by assuming three kinds of the interactions: Coulomb potential between Ag ions, dipole-dipole interaction and repulsive potential between electron clouds.
