PLASTIC CRYSTALS

1. Introduction

In plastic crystals the centers of mass of the molecules form a regular crystalline lattice, but the molecules are dynamically disordered with respect to the orientational degrees of freedom (see figure below). Orientationally disordered crystals are considered as model systems for structural glasses as the reorientation of the molecules often exhibits glassy freezing upon cooling, forming a so-called glassy crystal. We have investigated the α and high-frequency relaxation dynamics [1-7,13] and the nonlinear dielectric response [8,10] in this class of glass-like systems.

Recently, plastic crystals attracted additional interest because they are promising candidates for solid-state electrolytes to be used in advanced solid-state based batteries. One focus of our research is the clarification of the ionic charge-transport mechanisms in plastic crystals [9,11,12], for which the molecuar reorientational motions and their glassforming properties seem to play a significant but often neglected role.

Schematic representation of the possible transitions of a liquid of dipolar molecules (represented by asymmetric dumbbells) into a structural glass, an ordered crystal, or a glassy crystal. [from: P. Lunkenheimer, M. Michl, and A. Loidl, Nonlinear dielectric response of plastic crystals, in Nonlinear Dielectric Spectroscopy, edited by R. Richert (Springer, Cham, 2018), p. 277.]

An overview of many of our broadband dielectric investigations on plastic crystals can be found in:

Our nonlinear measurements on plastic crystals are summarized in:

2. Examples:

a) Broadband spectra of ortho-carborane:

The carborane molecule B₁₀C₂H₁₂ forms a nearly perfect icosahedron, whose corners are occupied by ten boron and two carbon atoms. For ortho-carborane the two carbon atoms occupy adjacent positions. There is little steric hindrance for reorientational processes and, thus, a plastic-crystalline phase is formed. Below we present the broadband loss spectra of ortho-carborane [3,5]. If compared to corresponding spectra of glassforming liquids, it is astonishing that there is no indication of an excess wing or a beta-relaxation [3]. Note the presence of a high-frequency minimum as also observed in glassforming liquids. In the infrared region, a peak shows up which resembles the boson peak known from canonical glass formers. Its spectral form gives some hints concerning the microscopic origin of the boson peak [5].

Dielectric loss of ortho-carborane for various temperatures. [from: P. Lunkenheimer and A. Loidl, Glassy dynamics beyond the α-relaxation, in Broadband Dielectric Spectroscopy, edited by F. Kremer and A. Schönhals (Springer, Berlin, 2002), chapter 5; see also refs. 3 and 5.]

b) Revolving doors and ionic conductivity in plastic crystals:

When ions are added to plastic crystals, some of them can exhibit sizable ionic conductivity reaching technically relevant conductivity values. Such materials represent solid-state electrolytes, which could be of high relevance for the development of batteries, fuels cells, etc. They avoid shortcomings of liquid electrolytes (leakage, volatility, flammability, toxicity) and are considered as alternatives to polymer electrolytes (polymer/salt mixtures).

For a possible explanation of their high ionic mobility, one may consider that the typical molecular reorientations in the plastic-crystal phase enhance the conductivity via a paddle-wheel or revolving-door mechanism as schematically indicated to the right.

Dielectric spectroscopy can provide information on both, the translational dynamics of the ions and the reorientational motions of the dipoles in plastic crystals. From our investigations, we conclude that the revolving-door effect indeed can enhance the ionic mobility [9,11,12]. Moreover, we find that adding large molecules to plastic-crystalline materials can enhance the conductivity by several decades, most likely via an optimization of the revolving-door mechanism (see figure below) [9].

Dc conductivity of glutaronitrile/succinonitrile (SN/GN) mixtures with 1% LiPF6. Adding larger GN molecules to SN strongly enhances the ionic conductivity. The plusses show the results for pure SN, doped with 5% LiPF6 as published by Alarco et al., Nat. Mater. 3, 476 (2004). The inset shows the dependence of the conductivity on the SN concentration. [from: Communication: Conductivity enhancement in plastic-crystalline solid-state electrolytes, K. Geirhos, P. Lunkenheimer, M. Michl, D. Reuter, and A. Loidl, J. Chem. Phys. 143, 081101 (2015).]

3. Materials:

• Cyclo-octanol [2,8]
• Meta- and ortho-carborane [1,3,5,8]
• 1-cyanoadamantane [3,12]
• Freon112 [3]
• Nitrile mixtures [7,9-11]
• AgCN [13]

and many more...

4. Some relevant publications: