INSTANTANEOUS NORMAL-MODE PREDICTION FOR CATION AND ANION DIFFUSION IN IONIC MELTS

Instantaneous normal mode (]NM) analysis was undertaken for several io nic melts: NaCl at six distinct thermodynamic states, and for a partic ular state of liquid LiCl, LiF, KF, KI, NaI, ZnCl2, and CuCl. In this Communication, we show that, in most cases, the ratio between the diff usion constants for cations (D-ca) and anions (D-an) is predicted from the average frequency of the real (''stable'' [omega(s)] and imaginar y (''unstable'' [omega(u)]) frequency modes of the projection of the t otal density of states on cations and anions, respectively. The propos ed relationship, D-ca/D-an ).(m(an)(-1)[omega(u)](an)[omega(s)](an)(-2 ))(-1), where m(i) is the mass of a particular species, is suggested b y Keyes' INM theory for diffusion [J. Chem. Phys. 101, 5081 (1994)], w ith the further assumption that the parameters which are related to th e topology of the multidimensional potential surface are equal for cat ions and anions. The above equation is shown to be valid for the simpl e melts NaC, LiCl, LiF, KF, KI, and NaI, but to fail for the network f orming melt ZnCl2 and for CuCl, which shows fast ionic diffusion chara cteristics. (C) 1997 American Institute of Physics.