The layered organic conductor $\kappa \text{−}{\left(\mathrm{BETS}\right)}_2\mathrm{Mn}{\left[\mathrm{N}{\left(\mathrm{CN}\right)}_2\right]}_3$ is a strongly correlated electron system exhibiting a metal-insulator transition at $T_{\text{MI}}\approx 25\mathrm{K}$ and unusual equilibrium and nonequilibrium dynamics above and below $T_{\text{MI}}$, respectively, caused by electronic ferroelectricity. However, at higher temperatures ($T^{*}=102\mathrm{K}$), another transition, associated with the formation of an incommensurate superstructure, has been reported, but its origin has not been clarified. By using resistance noise spectroscopy to investigate the low-frequency charge carrier dynamics as well as thermal expansion measurements, we find strong indications for a structural order-disorder transition of the BETS molecules' terminal ethylene groups, which manifests itself in a pronounced maximum in the noise magnitude around $T^{*}\approx 100\mathrm{K}$. Performing computational studies of the energy landscapes of the BETS cations and the Mn[N(CN)${}_2{]}_3$ polymeric anion, we find evidence suggesting that the observed structural transition is due to a possibly glassy freezing of the dicyanamide ligands in the anion layer.

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• Fluctuations & noise
• Metal-insulator transition
• Noise
• Structural properties