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

Abstract

In the Mott insulator hexamethylenetetrathiafulvalene tetrafluorotetracyano-p-quinodimethane (HMTTF-TCNQF4), 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 TcA145 K, while for the donor TcD95 K. Independent measurements of the intensities of infrared vibronic absorption bands for the acceptors and the donors confirm the magnetic results; they indicate a 2kF distortion on the TCNQF4 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 Tc. 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 (T) of this Mott insulator are strikingly to those of related organic metals. © 1982 The American Physical Society.