PHASE-TRANSITIONS IN AGI

The ordering of silver ions in the superionic Conductor alpha-AgI has been studied using a discrete version of density-functional theory. We obtained a transition to a low-temperature phase different from that found by Madden et al. using computer simulations [Phys. Rev. B 45, 10 206 (1992)]. We suggest that the difference originates from the fact that the I- lattice is fixed in bur study, while the periodic boundary conditions in the simulation allow certain deformations of the I- lat tice on the basis of calculated energies for deformed structures. We t herefore argue that the structural change of the I- lattice and the or dering of Ag+ ions onto particular sublattices are correlated processe s, rather than the ordering of Ag+ ions driving the alpha-beta transit ion, as suggested by Madden et al. The controversial order-disorder tr ansition within the alpha phase is also discussed in terms of a deform ation of the I- lattice. [S0163-1829(97)01042-4].