Atomistic simulation of structure and dynamics of the plastic crystal diethyl(methyl)(isobutyl)phosphonium hexafluorophosphate

Abstract

Molecular dynamics simulations have been performed to investigate the interrelations between structures, transport mechanisms, and phase transitions of an organic ionic plastic crystal material, diethyl(methyl)(isobutyl)phosphonium hexafluorophosphate ([P1,2,2,4][PF6]), in both solid and liquid phases. Examination of the temperature dependence of supercell parameters and radial distribution functions provides evidence of plastic phase transitions. Nonlinear increments of cell size within the temperature range 123-413 K are consistent with the plastic phase transitions identified from experimental analysis. The time- and temperature-dependent microstructure and dynamics have been intensively studied through analysis of trajectory files. The rotational motion and diffusion of the matrix ions are quantitatively analysed via rotational correlation functions and mean square displacements. We present new information on the evolution of molecular motions in different phases, and compare and contrast our findings with previously reported hypotheses based on nuclear magnetic resonance results. This work provides valuable information at an atomistic level to explain the experimental observations, which helps further understanding of the molecular motions underlying the plastic phase transitions.

New insights into ordering and dynamics in organic ionic plastic crystal electrolytes

“…3a shows the radial distribution functions (RDFs) for the anioncation pair in the P 1224 PF 6 OIPC system determined from the MD simulations at various temperatures with and without the inclusion of a vacancy pair defect within the structure. The structural differences between the two systems at all temperatures are insignificant, with the lowest temperature (313 K) corresponding to the material in its low temperature phase III as previously reported based on thermal analysis measurements [9,31]. The thermal and NMR analysis of this system have previously suggested some rotational disorder is present in phase III with isotropic tumbling of the PF 6 anion and a crankshaft rotational motion of the alkyl groups around the P-C bonds, however, the structure remains positionally ordered over a long distance as indicated by the RDFs.…”

Modelling Ion‐Pair Geometries and Dynamics in a 1‐Ethyl‐1‐methylpyrrolidinium‐Based Ion‐Conductive Crystal

“…They contribute to the structural disorders, which enable the formation of free volume that could become large enough to facilitate the ion’s diffusive motion at higher temperatures, and thus enhance the ionic conductivity 6. 15…”

Computational approaches to understanding reaction outcomes of organic processes in ionic liquids

“…While this method is not ideal, it has been shown to improve the calculated dynamic properties substantially without any additional computational cost, and is therefore becoming a very popular approach. 66,76,105,[139][140][141][142][143][144][145][146][147][148][149][150][151] For example, ionic liquid densities can be calculated quite accurately using either unscaled (i.e. net ion charge ¼ 1) or scaled charges (Table 5), yet generally other dynamic properties such as the self-diffusion coefficient ( Table 6) and heat of vaporisation (Table 7) are better predicted when scaled charges are used.…”