Electronic properties of isostructural organic conductors (ET)3(HSO4)2 and [Ni(dddt)2]3(HSO4)2. Thermopower and tight-binding calculations

The temperature dependence of thermopower of isostructural molecular conductors [Ni(dddt)2]3(HSO4)2 and (ET)3(HSO4)2 has been investigated. Drastic difference in S(T) dependences is qualitatively explained on basis of tight binding band structure calculations. A peak at 25 K in [Ni(dddt)2]3(HSO4)2 was shown to be manifestation of metal to semimetal phase transition, and the abrupt decrease at 125 K in (ET)3(HSO4)2 is concerned with metal to insulator phase transition. Rather complex semimetal band structure was found in both cases.