Cation dependence of the ionic dynamics in computer simulated molten nitrates

Abstract

In order to study the cation dependence of the ionic dynamics in molten nitrates, molecular dynamics simulations including vibrational degrees of freedom were carried out for molten LiNO3, NaNO3, and RbNO3. Coulomb pair potential with Born-type repulsion was adopted for the interionic interaction. The simulated diffusion coefficient was smaller for a larger cation, and that of nitrate ions did not change with changing cation species. The mean squared charge displacements showed that the static conductivity decreased considerably as the cation size increased 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 shifted to the low energy side and the intensity decreased as the cation size increased. Results of the simulation were compared with the experimental diffusion constants, static and dynamic conductivities, and rotational behavior revealed by Raman spectroscopy. The simulated vibrational correlation functions and the power spectra of NO3− could reproduce the observed cation dependence of the peak frequencies of the ν1(A′1) and ν2(A″2) modes. However, the assumed interionic potentials in the present simulation were found to result in too slow vibrational dephasing of the ν1 mode and too fast dephasing of the ν2 mode as compared with the infrared and isotropic Raman spectra. Strong correlation between radial and angular distributions of cations was found in the first coordination spheres of nitrate ions in the simulated molten nitrates.