We present a combined study of thermal expansion and resistance fluctuation spectroscopy measurements exploring the static and dynamic aspects of the charge-glass formation in the quasi-two-dimensional organic conductors $\theta \text{−}({\mathrm{BEDT-TTF})}_{2}M{M}^{\prime }{\left(\mathrm{SCN}\right)}_4$ with $M$ = Cs and $M^{\prime }$ = Co,Zn. In these materials, the emergence of a novel charge-glass state so far has been interpreted in purely electronic terms by considering the strong frustration of the Coulomb interactions on a triangular lattice. Contrary to this view, we provide comprehensive evidence for the involvement of a structural glasslike transition at $T_{\text{g}}\sim 90-100\mathrm{K}$. This glassy transition can be assigned to the freezing of structural conformations of the ethylene endgroups in the donor molecule with an activation energy of $E_{\mathrm{a}}\approx 0.32\mathrm{eV}$, and the concomitant slowing down of the charge-carrier dynamics is well described by a model of nonexponential kinetics. These findings disclose an important aspect of the phase diagram and calls for revisiting the present views of the glassy dynamics in the whole family of $\theta \text{−}({\mathrm{BEDT-TTF})}_{2}M{M}^{\prime }{\left(\mathrm{SCN}\right)}_4$. Our results suggest that the entanglement of slow structural and charge-cluster dynamics due to the intimate coupling of lattice and electronic degrees of freedom determine the charge-glass formation under geometric frustration.

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• Charge order
• Fluctuations & noise
• Glass transition
• Thermal expansion
• Molecular solids
• Strongly correlated systems