Physical properties and phase transitions in the (tTTF)$\mathsf{_{2}}$X series of organic conductors

The electrical, magnetic and structural properties of several (tTTF)$_{2}$X salts with X $=$ PF$_{6}$, AsF$_{6}$, SbF$_{6}$, CIO$_{4}$, ReO$_{4}$, Br have been investigated. In each case, low temperature phase transitions are found corresponding to the stabilization of various ground states. In the case of salts with the octahedral anions , the antiferromagnetic ground states observed below $T_{\rm N}=25$ K and 19 K respectively has been characterized by resonance measurements. In the case of the CIO$_{4}$ salt a structural distortion, probably relaxing the Coulomb repulsions between localized charges, is observed at 137 K. An unusual phase transition affecting both the charge and spin degrees of freedom is observed in the PF$_{6}$ salt around 50 K. A comparison is made with the properties of the TMTTF salts and related series in order to rationalize these results. Finally the role of the interchain coupling in the competition between the spin-Peierls and antiferromagnetic ground states is emphasized