Metal—insulator transitions in double-column salts: (DMET)2BF4, (DMET)2ClO4 and (DIMET)2BF4

Organic conductors, (DMET)2BF4, (DMET)2ClO4 and (DIMET)2BF4 (DMET = dimethyl(ethylenedithio)diselenadithiafulvalene), have two types of donor columns, which are almost perpendicular to each other. Resistivity and thermopower were measured along the two types of columns under ambient pressure between 4.2 and 300 K. All the salts show metal—insulator transition at about 30 K. Unusual stepwise increases in resistivity and corresponding changes in thermopower were found for the salts below 30 K. The stepwise changes suggest that a few transitions occur in the semiconducting state.