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.

Structural Analysis of Molten NaNO3by Molecular Dynamics Simulation

“…The N +-value (= 5.6) for molten NaNO 3 is approximately intermediate between that of molten NaCl (N +-= 4.85) and CsCl (N +-= 6.1) [14]. This N +-value is consistent with that reported by Kato et al (N +-= 5.7), but larger than that for molten LiNO 3 (5.0) [11]. The N ++ value for molten NaNO 3 is smaller than the N --value because the first correlation length of Na-Na correlation defined in Sec.…”

“…The difference in the peak positions of B +++ and B ---for molten NaNO 3 reflects the difference in the size of the cation and anion. According to Kato et al [11], B +-+ for molten NaNO 3 can be estimated to be 60-90°, which is consistent with our analysis results. Smaller B +-+ has been observed for molten LiNO 3 [11] due to the smaller size of the Li ion than the Na ion.…”

“…Adya et al for the first time investigated the microscopic atomic configuration and diffusion in molten NaNO 3 by means of X-ray diffraction measurement and molecular dynamics (MD) simulation, and compared its partial structure factors and partial pair distribution functions with those of molten NaNO 2 and the eutectic mixture of the NaNO 3 -NaNO 2 system showed good additivity for the intermolecular structure because the ionic radius of NO 3 and NO 2 ions are similar owing to the presence of a lone pair on the nitrite ion [6]. After Adya et al [6], several researchers have reported the MD simulation results of molten alkali nitrates [7][8][9][10][11][12]. For example, Kato et al have reported cation dependence of the detailed structural features and ionic dynamics in molten alkali nitrates [11].…”

“…After Adya et al [6], several researchers have reported the MD simulation results of molten alkali nitrates [7][8][9][10][11][12]. For example, Kato et al have reported cation dependence of the detailed structural features and ionic dynamics in molten alkali nitrates [11]. They have pointed out that the diffusion constant of small cation is larger than that of large anion.…”

A Combined QM/MM Molecular Dynamics Simulations Study of Nitrate Anion (NO3-) in Aqueous Solution

“…In conjunction with experiments, computer simulations of aqueous solutions containing NO 3 -have provided structural and dynamical details of this solvated ion at molecular level. 9,[12][13][14][15][16][17] However, most of the early simulation works had relied on classical molecular mechanical models. It has been demonstrated that the hydration shell structure as well as the orientations of water molecules surrounding the ion depend quite sensitively on the interaction potentials employed in the simulations.…”