An unusual metal-insulator transition: bis(tetramethyltetraselenafulvalenium)-perrhenate (TMTSF2ReO4)

An experimental study of the 2:1 charge transfer state of Delta 2,2' -bi-4,5-dimethyl=1,3-diselenolylidene (TMTSF) and ReO 4 - is presented. The crystal structure is isomorphous with that of other (TMTSF) 2 X salts. Physical properties including DC and microwave conductivity, thermoelectric power, optical properties, and magnetic properties, all show a sharp metal-insulator transition at T c =182 K. Below T c , (TMTSF) 2 ReO 4 is found to be a non-magnetic semiconductor with a large energy gap of 200 meV. Based on these results and on previously reported changes in the crystal structure at T c , it is suggested that (TMTSF) 2 ReO 4 undergoes a Peierls transition, driven by the interaction of the electrons with the ReO 4 - anions. The aspects of an order-disorder transformation in the anionic system are included in a simple model for the material.