UNSTABLE MODES IN IONIC MELTS

Imaginary frequency instantaneous normal modes (INM) of a number of io nic melts are examined in the light of recent proposals which connect them to diffusion and to the strong/liquid character of the fluid. Sep arate INM density-of-states may be found for each ionic species. A lin ear relationship between the temperature dependence of the fraction of INMs which are unstable (f(u)) and that of the diffusion coefficient of each species has been found. The shapes of the single-mode potentia l energy curves of the fluid along INM eigenvectors are examined. The results do not support a general significance for ''double-well'' mode s as the only ones involved in diffusion. An alternative explanation o f how single or double well features develop on the single-mode potent ial curves is proposed. The shape of the unstable INM density-of-state s reflects the character of the ionic motion: a crossover from \omega\ exp(-B omega(2)) to \omega\exp(-B omega(4)) occurs as the ionic motion becomes increasingly hindered. Both behaviors, which have been report ed previously for the Lennard-Jones atomic fluid at normal liquid and supercooled conditions, respectively, can occur simultaneously in a si ngle melt for each ionic species. (C) 1998 American Institute of Physi cs.