Instantaneous normal mode prediction for cation and anion diffusion in ionic melts

Instantaneous normal mode (]NM) analysis was undertaken for several ionic melts: NaCl at six distinct thermodynamic states, and for a particular state of liquid LiCl, LiF, KF, KI, NaI, ZnCl2, and CuCl. In this Communication, we show that, in most cases, the ratio between the diffusion constants for cations (D-ca) and anions (D-an) is predicted from the average frequency of the real (''stable'' [omega(s)] and imaginary (''unstable'' [omega(u)]) frequency modes of the projection of the total density of states on cations and anions, respectively. The proposed relationship, D-ca/D-an (m(ca)(-1)[omega(u)](ca)[omega(s)](ca)(-2)).(m(an)(-1)[omega(u)](an)[omega(s)](an)(-2))(-1), where m(i) is the mass of a particular species, is suggested by Keyes' INM theory for diffusion [J. Chem. Phys. 101, 5081 (1994)], with the further assumption that the parameters which are related to the topology of the multidimensional potential surface are equal for cations and anions. The above equation is shown to be valid for the simple melts NaC, LiCl, LiF, KF, KI, and NaI, but to fail for the network forming melt ZnCl2 and for CuCl, which shows fast ionic diffusion characteristics. (C) 1997 American Institute of Physics.