Observation of conductivity anomalies in small needle-like crystals of TEA(TCNQ)2

Abstract

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.

Structural evolution of the one-dimensional organic conductor triethylammonium–7,7,8,8-tetracyano-p-quinodimethane (1:2) [TEA–(TCNQ)2] in the temperature range 40 to 345 K

“…However, the temperature dependence of the longitudinal conductivity of this compound shows an anomaly at 200-220 K (see e.g. Brau & Farges, 1974) which is in fact a jump of one order of magnitude if the samples are small, needle-shaped single crystals (Grassi, Brau & Farges 1979;Farges, 1979). Furthermore, some authors (e.g.…”

Order‐Disorder Phase Transition in TEA+(TCNQ)2‐

“…This is in fact the consequence of a number of conductivity jumps which are individually observed only when small single crystals are investigated [2]. I n this latter case if To is the temperature a t which the first step occurs on the first thermal cycle, it varies a t random from one thermal cycle to the next and from one needle to another.…”

“…TEA + TCNQt3) is a good one-dimensional (1D) semiconductor in which the temperature dependence of the longitudinal dc conductivity observed in a large crystal shows an anomaly in the slope of Iog versus T-1 a t 220 K [I]. This is in fact the consequence of a number of conductivity jumps which are individually observed only when small single crystals are investigated [2]. I n this latter case if To is the temperature a t which the first step occurs on the first thermal cycle, it varies a t random from one thermal cycle to the next and from one needle to another.…”

Temperature Study of the Polarized Reflectivity of TEA(TCNQ)2Single Crystal

“…On the other hand, a number of conductivity jumps have been observed only when small single crystals have been investigated [8]. The temperature at which the first conductivity step occurs varies randomly from one thermal cycle to the next, and from one single crystal to another.…”

“…The temperature at which the first conductivity step occurs varies randomly from one thermal cycle to the next, and from one single crystal to another. It is scattered in the T-range from 268 to 196 K [8,16]. Furthermore, some authors have postulated that this transition is of the metal-insulator type, while others tend to see a semiconductor-semiconductor mechanism.…”