Electrical Properties of the Organic Metallic Compound bis (tetrathiotetracene)‐triiodide, (TTT) 2 ‐I 3

The maximum of the d.c. conductivity of single crystals of the title compound ((TTT) 2 ‐I 3 ), grown by cosublimation, is shown to be at about 35K. The ratio of the b ‐axis (needle axis) conductivity at the temperature of its maximum (T m ≅ 35 K) and its value at T = 300 K (normalized conductivity σ   b ″ (T m )/σ   b ″ (300)) is strongly sample dependent. Values for σ   b ″ (T m )/σ   b ″ (300) as high as 8 have been obtained. From 300 K to about 120 K the conductivity σ   b ″is proportional to T −1.3 . in the temperature interval 40K⩽T⩽120K we found: σ   b ″ ∝T −1 . On the low temperature side of the maximum (6K⩽T⩽20K) the conductivity is proportional to T α , with α ranging from 2 to 2,5. It is shown that impurities, misfit of the stoichiometry and/or physical sample quality (crystal perfection) drastically shift the minimum of the resistivity to higher temperatures ( Fig . 5), thereby altering also the temperature dependence of the resistivity on both sides of the minimum. Our crystallographic data difference from the results published by Isett et al . and Buravov et al . (Table 1) with regard to the iodine spacings. We argue that the polarisability of the I   3 −1ions in their chain direction contributes significantly to the high conductivity (σ   b ″ =1000 to 1000 Ω −1 cm‐1 ) along the crystallographic b ‐axis (needle axis).