Magnetic properties of an organic Mott insulator. Separate donor and acceptor phase transitions in hexamethylenetetrathiafulvalene tetrafluorotetracyano-quinodimethane (HMTTF-TCNQF4)

In the Mott insulator hexamethylenetetrathiafulvalene tetrafluorotetracyano-$p$-quinodimethane (HMTTF-TCNQ${\mathrm{F}}_4$), the magnetic spin susceptibility has a contribution from both the donor stacks and the acceptor stacks. These separate contributions have been determined by measuring the total susceptibility and using the temperature dependence of the $g$ value. It is found that each stack undergoes an apparently separate phase transition, but at very different temperatures: the acceptor stack $T_c^A\sim 145$ K, while for the donor $T_c^D\sim 95$ K. Independent measurements of the intensities of infrared vibronic absorption bands for the acceptors and the donors confirm the magnetic results; they indicate a $2k_F$ distortion on the TCNQ${\mathrm{F}}_4$ stack near 145 K and one near 95 K for HMTTF. These phase transitions appear to be quasi one dimensional, with evidence of large fluctuations above $T_c$. The transitions are probably driven largely by a gain in magnetic energy, i.e., a spin-Peierls mechanism. Furthermore, the magnitude and temperature dependence of $\chi \mathrm{}\left(T\right)\mathrm{}$ of this Mott insulator are strikingly to those of related organic metals.