Structureless transition and strong localization effects in bis-tetramethyltetrathiafulvalenium salts [(TMTTF)2X]

We find a new ‘‘structureless anomaly’’ in several salts in the family of compounds of the form (TMTTF${)}_2$X (for X=${\mathrm{ReO}}_4$, ${\mathrm{SbF}}_6$, ${\mathrm{AsF}}_6$, and alloys of the latter anions). This anomaly is seen very clearly through thermopower studies and less clearly in conductivity data: In both cases it appears as a change in derivative of the corresponding transport property. There is no change in the magnetic susceptibility of the material at the anomaly, and no change in structure has been observed. We suggest that the presence of an anomaly of such description indicates that the electronic system is highly correlated and that this must be taken into account when modeling these materials, making such models rather complicated.