Ionic conductivity and relaxation dynamics in plastic crystals with nearly globular molecules

Abstract

We have performed a dielectric investigation of the ionic charge transport and the relaxation dynamics in plastic-crystalline 1cyano-adamantane (CNA) and in two mixtures of CNA with the related plastic crystals adamantane or 2-adamantanon. Ionic charge carriers were provided by adding 1% of Li salt. The molecules of these compounds have nearly globular shape and, thus, the so-called revolving-door mechanism assumed to promote ionic charge transport via molecular reorientations in other PC electrolytes, should not be active here. Indeed, a comparison of the dc resistivity and the reorientational -relaxation times in the investigated PCs, reveals complete decoupling of both dynamics. Similar to other PCs, we find a significant mixinginduced enhancement of the ionic conductivity. Finally, these solid-state electrolytes reveal a second relaxation process, slower than the -relaxation, which is related to ionic hopping. Due to the mentioned decoupling, it can be unequivocally detected and is not superimposed by the reorientational contributions as found for most other ionic conductors.

Cation and Anion Transfer in Quinuclidinium Hexafluorophosphate Plastic Crystal: Role of Constituent Ions and the Crystalline Structure

“…On the other hand, more defects (mainly vacancies) would appear simultaneously in the loose molecular structure due to mixing. Similar phenomena were found in molecular plastic crystals, for example, the adamantane-Li and succinonitrile-Li systems, where large globular dopants were proved to increase the ionic conductivity due to the appropriate space formed after mixing . Transport paths seem to increase, and chloride ion transfer should be detectable.…”

“…Similar phenomena found in molecular crystals, for example, the adamantane-Li and succinonitrile-Li systems, where large globular dopants were proved to increase the ionic conductivity due to the appropriate space formed after mixing. 27 Transport paths seem to increase, and chloride ion transfer should be detectable. However, the large volume of Cl − may be more affected by steric hindrance than Li + , which would offset the increase of ion conduction.…”

“…As a result, ion transport is hindered through lattice sites, much less through interstitial sites, along any axis. Moreover, the [HQ] + cation and PF 6 – anion could hardly introduce more space when rotating due to the high symmetry and specific spatial structure, in contrast to those disk-shaped cations (e.g., cyclo-alcohols) in molecular plastic crystals which could make a transfer path in parallel orientations of adjacent molecules . The insufficient transport space (or the steric hindrance provided by molecular packing) is unfavorable for the conduction of [HQ] + and PF 6 – through a paddle-wheel mechanism in the crystal lattice; hence, pure plastic crystal exhibits relatively lower ionic conductivity.…”

Lithium-Ion and Chloride-Ion Transfer in Chiral and Racemic 3-Hydroxyquinuclidinium Perchlorate: Influence of Defects and a Mixed Plastic Crystal Matrix

“…The results are reasonable considering the existence of the same dominant cubic crystal phase throughout the whole temperature range, as evidenced from the XRD analysis. As demonstrated in previous studies, the appearance of a cubic crystal results from the orientational disorder of the constituent ions. ,,, Accordingly, the cations of the racemic mixture should gain local orientational freedom starting from low temperatures.…”

Phase-dependent dielectric properties and proton conduction of neopentyl glycol

“…1 Plastic crystals have the long-range center-ofmass crystalline order (positional order) and short-range dynamic disorder of molecular orientations, and thus are also known as the orientationally disordered crystals or rotator phases. 2,3 Normally, they have a very low entropy of fusion and a relatively large entropy of solid-solid phase transition thus enabling high ionic conductivities, which makes them promising candidates for applications as protonic or ionic conductors in fuel cells, batteries, and supercapacitors. [4][5][6][7][8][9][10] Neopentylglycol (NPG) or 2,2-dimethyl-1,3-propanediol, an important representative of molecular plastic crystals, has been widely studied in various aspects.…”