Measurements of the thermal diffusivity D on crystals of the highly anisotropic organic semiconductor TEA(TCNQ)? are reported. These measurements are made using a heat pulse method. For the crystal axis 1 parallel to the TCNQ columns and associated with the largest component a, of the electrical conductivity, D, is 1.6 x om2 8-l a t 200 R. The thermal conductivity k, is evaluated to 4 mW cm-' K-I a t 300 K with an estimated electronic contribution of only 1%. The phonon mean free path at 300K is extremely small I, e 2 A, of the order of the shortest distance between the TCNQ molecules in a column. It is thus expcctcd to be almost constant a t this temperature, a result, which is also suggested from the observed T dependence of D,.
Recently, we have reported the existence of remarkable anomalies in the electrical conductivity dl, systematically observed in small needle-like crystals of the salt MelNEtBz (TCNQ)2 formed from the cation methyl-1-Nethyl benzimidazolium /1, 2/. On cooling the needles from ambient temperature, dl undergoes strong discontinuities in which i t decreases typically by two o r d e r s of magnitude either in a single s t e p or in two or more closely spaced steps. The temperature a t which the anomalies occur vary from needle to needle. By re-heating the needles just after these transitions are achieved, it is possible to gradually recover the initial room temperature value of cfl, generally to within better than 5 to 10%. We have also demonstrated that such characteristic dl discontinuities are never observable with the l a r g e r crystals obtained from recrystallization of needles. In this latter case, however, one observes as a residual effect strong but continuous slope anomalies in the 6 versus T behaviour of large crystals.
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