서강대학교

초록

Organic ionic plastic crystals (OIPCs), which consist of pairs of organic molecular ions and undergo multiple solid-to-solid phase transitions, are considered as excellent candidates for solid electrolytes due to high ionic conductivity in solid phases. Molecular ions are allowed to undergo either rotational or conformational relaxations at certain temperatures in OIPCs, which enhance the ionic conductivity. There have been molecular simulations to understand the rotational motion of ions and the high conductivity at an atomistic scale. The polarizability of ions was, however, often ignored in simulations due to a high computational cost. Since it is well-known that the polarizability may affect the translational diffusion, the ionic conductivity, and the phase transition of ionic liquids significantly, it should be of interest to investigate whether and how the polarizability would affect the rotational diffusion of ions in solid phases. In this work, we perform extensive atomistic molecular dynamics simulations for two different kinds of OIPCs, 1-methyl-3-methylimidazolium hexafluorophosphate ([MMIM][PF6]) and 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]). We employ two different simulation models for ions by turning on and off the polarization in interaction potentials in order to evaluate the effect of the polarization on the rotational diffusion. We find that the polarizability hardly affects the density and the phase transition of both [MMIM][PF6] and [BMIM][PF6] OIPCs. Even the crystalline structure (that we monitor from the pair correlation functions between ions) is hardly affected by the polarizability. However, a certain rotational motion, especially the rotational diffusion of PF6 ions in [MMIM][PF6] OIPCs, is enhanced by a factor of up to four at low temperatures when the polarizability is turned on. The PF6 ions in [MMIM][PF6] OIPCs undergo the rotational hopping motions more significantly due to the polarizability. We find that the rotational diffusion of a certain ion can be enhanced only when the polarization results in a significant change in the dipole moment of the neighbor ions around the ion.