# Crystal Structure Prediction for Aprotic Ionic Liquids – Searching for the Unknown

## TL;DR

This paper introduces a new method to predict crystal structures of aprotic ionic liquids, helping explain why some materials resist crystallization.

## Contribution

A novel crystal structure prediction protocol for aprotic ionic liquids is developed and validated.

## Key findings

- The CSP protocol successfully identifies the thermodynamically stable polymorph of [emIm][MeSO3].
- The method explains the crystallization reluctance of [emIm][EtSO4] due to its glassy polymorph landscape.
- Low-energy, high-entropy crystal structures are proposed for unresolved polymorphs of [emIm][MeSO3].

## Abstract

Ionic liquids (ILs)
represent an extensively studied
class of materials.
Nevertheless, their solid state has often been overlooked, leading
to frequent knowledge gaps about their phase behavior or crystal structures
that such materials may form. This work focuses on the development
of a crystal structure prediction (CSP) scheme suitable for aprotic
ILs, relying on quasi-random crystal structure generation, dispersion-corrected
density functional theory (DFT-D)-based energy reranking, and quasi-harmonic
phonon treatment. The interpretation of peculiar differences in the
crystallizability of very similar ILs upon cooling of their melts
is presented. The versatility of the computational protocol is validated
for [emIm]­[MeSO3], an IL known to be polymorphic. The current
CSP identifies the [emIm]­[MeSO3] polymorph that is thermodynamically
stable in reality at the top of the stability ranking, both in terms
of DFT-D refined lattice energies and quasi-harmonic Gibbs free energies.
Several low-energy, high-entropy crystal structures are also proposed
for [emIm]­[MeSO3] as candidates for the remaining known
polymorphs with yet unresolved crystal structures. Our CSP modeling
explains the extraordinary reluctance of [emIm]­[EtSO4]
to crystallize due to its glassy shape of the polymorph landscape
with no distinct global energy minimum crystal structure.