CATION DEPENDENCE OF THE IONIC DYNAMICS IN COMPUTER-SIMULATED MOLTEN NITRATES

In order to study the cation dependence of the ionic dynamics in molte n nitrates, molecular dynamics simulations including vibrational degre es of freedom were carried out for molten LiNO3, NaNO3, and RbNO3. Cou lomb pair potential with Born-type repulsion was adopted for the inter ionic interaction. The simulated diffusion coefficient was smaller for a larger cation, and that of nitrate ions did not change with changin g cation species. The mean squared charge displacements showed that th e static conductivity decreased considerably as the cation size increa sed from Li+ to Rb+. The simulated orientational correlation function of nitrate ions decayed more quickly as the cation size increased. Far infrared absorption spectrum simulated from the time evolution of the dipole moment (or the current) of the system showed that the peak shi fted to the low energy side and the intensity decreased as the cation size increased. Results of the simulation were compared with the exper imental diffusion constants, static and dynamic conductivities, and ro tational behavior revealed by Raman spectroscopy. The simulated vibrat ional correlation functions and the power spectra of NO3- could reprod uce the observed cation dependence of the peak frequencies of the nu1( A1') and nu2(A2'') modes. However, the assumed interionic potentials i n the present simulation were found to result in too slow vibrational dephasing of the nu1 mode and too fast dephasing of the v2 mode as com pared with the infrared and isotropic Raman spectra. Strong correlatio n between radial and angular distributions of cations was found in the first coordination spheres of nitrate ions in the simulated molten ni trates.