Rotational Dynamics and Li+ Transport Pathways in [PF6][P1,2,2,4]

Diethyl(methyl)(isobutyl)phosphonium hexafluorophosphate, [PF₆][P_1,2,2,4], is an organic ionic plastic crystal with potential uses as a solid electrolyte in Li-ion cells. Using molecular dynamics simulations we perform an exhaustive study of this material at temperatures spanning from 175 K to 500 K. The simulations predict the stability of the crystalline phase for temperatures lower than 197 K where a transition to a semi-plastic phase with rotations of the PF₆ anion is observed.  At 280 K the larger cations start to undergo jump-like rotations. A transition to a fully plastic phase is observed at 340 K, and melting occurs at 450 K. We have also studied the most favorable pathways for a single Li ion to move in the bulk of the material at each one of the different phases. Overall the results agree with previously reported experimental data and the simulations provide exquisite details on the atomistic rotational dynamics. Nevertheless, the simulations also reveal the shortcomings of current force fields to describe material properties in an extended temperature range and highlight the needs for further theoretical efforts in order to achieve predictive power in general systems.