Determining ethylene group disorder levels in $\kappa$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Br

TL;DR

This study investigates the structural behavior of the organic superconductor $$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Br across a temperature range, focusing on ethylene group disorder and its relation to a glass-like transition at 77 K.

Contribution

The paper provides detailed structural data and quantifies ethylene group disorder levels, clarifying that the glass transition is not mainly due to ethylene conformational freezing.

Findings

Orthorhombic symmetry confirmed at all temperatures.

Ethylene group occupancy increases from 67% at 300 K to 97% at 9 K.

Glass transition not primarily caused by ethylene conformational freezing.

Abstract

We present a detailed structural investigation of the organic superconductor $\kappa$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Br at temperatures $T$ from 9 to 300 K. Anomalies in the $T$ dependence of the lattice parameters are associated with a glass-like transition previously reported at $T_g$ = 77 K. From structure refinements at 9, 100 and 300 K, the orthorhombic crystalline symmetry, space group {\it Pnma}, is established at all temperatures. Further, we extract the $T$ dependence of the occupation factor of the eclipsed conformation of the terminal ethylene groups of the BEDT-TTF molecule. At 300 K, we find 67(2) %, with an increase to 97(3) % at 9 K. We conclude that the glass-like transition is not primarily caused by configurational freezing-out of the ethylene groups.