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

Abstract

The infrared reflection spectra of the organic quasi-one-dimensional semiconductor triethylammonium (TCNQ)2 between 80 and 300 K are reported. Characteristic changes in the temperature dependencies of the bands assigned to the donor vibrations are observed. An interpretation of the electron-molecular vibration coupling in terms of the dimer theory is discussed. It is concluded that the thermal evolution of the spectrum corroborates the semiconductor-semiconductor type of phase transition attributed to the cation disorder.

Application of the Dimer and Tetramer Models for Interpretation of the IR Spectra of TEA(TCNQ)2

“…Thus the crystal structure shows typical irregular tetradic stacking of the TCNQ molecules. TEA(TCNQ)2 single crystals undergo a first order phase transition at 220 K, which was discussed in our previous paper [1].…”

“…These peaks are shown to have a fine structure if the charge density is not shared equally by the constituent molecules [5]. The fine structure of the activated ag modes was observed experimentally in TEA(TCNQ)2 [6][7][8]1].…”

“…The data extrapolated down to O K indicate that, at this temperature, each column is a pile of tetrads [10]. The dependence of totally symmetric (a g ) vibrations of the TCNQ molecule on its charge [11] leads to fine structure doublets becoming more pronounced with the decrease of temperature [1]. Considering its assumptions, the dimer model neglects the doublet structure of the activated a g modes and, thus, one can anticipate rather good agreement with the experiment for TEA(TCNQ)2 at high temperatures.…”

“…This study is concerned with the ion-radical salt triethylammonium-tetracyanoquinodimethane (TEA(TCNQ)2), the material whose physical properties were quite extensively investigated in the past (see [1]). However, in spite of an abundance of various experimental data, the problem of the phase transition in the salt remains open.…”

“…The purpose of the paper [1] was to investigate the evolution of the polarized IR reflectivity within the phase transition region. We drew the following conclusions: TEA(TCNQ)2 undergoes a semiconductor-semiconductor phase transition (497) at about 220 K, which occurs mainly in the cation sublattice; the bands assigned to the cation vibrations exhibit distinct changes whereas totally symmetric (a g ) modes of the anions, coupled with molecular vibrations of TCNQ molecules or clusters, show to be changed much smaller.…”