Insights from experiment and $ab\,initio$ calculations into the glass-like transition in the molecular conductor $\kappa$-(BEDT-TTF)$_2$Hg(SCN)$_2$Cl

TL;DR

This study combines high-resolution thermal expansion measurements and $ab$ initio calculations to investigate a glass-like transition in a molecular conductor, revealing unique ethylene endgroup ordering behavior and its energetic origins.

Contribution

The paper provides the first detailed experimental and theoretical analysis of ethylene endgroup ordering in ${ ext{kappa}}$-(BEDT-TTF)$_2$Hg(SCN)$_2$Cl, highlighting a peculiar selective glass-like transition.

Findings

Identification of a glass-like transition at 63 K linked to ethylene endgroup dynamics.

Unique ordering where only one ethylene endgroup type undergoes glass-like freezing.

Correlation between experimental activation energies and $ab$ initio$ predictions, emphasizing the role of interaction energetics.

Abstract

We present high-resolution measurements of the relative length change as a function of temperature of the organic charge-transfer salt $\kappa$-(BEDT-TTF)$_2$Hg(SCN)$_2$Cl. We identify anomalous features at $T_g \approx\,63$ K which can be assigned to a kinetic glass-like ordering transition. By determining the activation energy $E_A$, this glass-like transition can be related to conformational degrees of freedom of the ethylene endgroups of the organic building block BEDT-TTF. As opposed to other $\kappa$-(BEDT-TTF)$_2X$ salts, we identify a peculiar ethylene endgroup ordering in the present material in which only one of the two crystallographically inequivalent ethylene endgroups is subject to glass-like ordering. This experimental finding is fully consistent with our predictions from $ab\,initio$ calculations from which we estimate the energy differences $\Delta E$ and the activation energies $E_A$ between different conformations. The present results indicate that the specific interaction between the ethylene endgroups and the nearby anion layers leads to different energetics of the inequivalent ethylene endgroups, as evidenced by different ratios $E_A/\Delta E$. We infer that the ratio $E_A/\Delta E$ is a suitable parameter to identify the tendency of ethylene endgroups towards glass-like freezing.