(Nb//1// minus //xTa//x)Se//4.

Abstract

(NbSe//4)//3I has a quasi 1D structure consisting of NbSe//4 chains parallel to the c axis. The Nb-Nb distance within the chain is approx. 3 A whereas the interchain distance is much larger (6. 7 A). Electrical resistivity and thermoelectric power measurements were carried out in the temperature range 130 K-350 K. for some crystals the low activation energy persists up to the lowest temperature investigated. This was designated type I. For some crystals the activation energy increases once again to a higher value (0. 11 ev) at lower temperatures. These crystals were designated type II although X-ray diffraction is unable to distinguish the two types of crystals. The effect of introducing disorder in the chain by introduction of Ta is investigated in the present work.