Temperature and pressure dependence of the electronic structure of tetracyanoquinodimethane crystals and the effect of impurities
ABSTRACT The absorption spectrum (12 500–32 260 cm−1) of tetracyanoquinodimethane crystals has been obtained at 0 bars in the  and  direction at 84 and 300 K. The peak of the Ag → Bu transition has been identified at 21 370 cm−1 and that of the Ag → Au transition at 22 000 cm−1. The change with temperature of both transitions was −3.4 ± 0.1 cm−1∙K−1. The  absorption was also obtained at room temperature as a function of the pressure, up to 5.0 × 104 bar, for crystals grown in two different laboratories, giving the change with pressure as −0.037 ± 0.003 and −0.092 ± 0.010 cm−1∙bar−1, respectively. At ambient temperature the explicit contribution, which is a measure of the electron–phonon interaction, was negative and dominated the temperature dependence. The implicit contribution, which is a measure of the volume dilatation, contributed in the opposite way, i.e., positively. Working at room temperature, we observed on both samples irreversible effects at higher pressures. In the first case, a discontinuous change occurred at (12 ± 1) × 103 bar, with new peaks appearing both at higher energy (25 600 cm−1) and lower energy (12 500 cm−1). In the second case, the absorption peak shifted continuously towards lower energies, but it broadened abruptly above 1.3 × 104 bar. We believe that the differences in the pressure dependence of the optical properties are due to the presence of small amounts of impurities in the samples causing subtle structural differences and that the irreversible effects are due to pressure-induced chemical changes.
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ABSTRACT: This work reports the pressure dependence of the Raman spectra of TCNQ0, TTF0, K(TCNQ), Cs2(TCNQ)3, perylene–TCNQ, and phenazine–TCNQ in the range from 0 to 70 kbar. By using the average compressibility of ∼0.01 kbar−1 of TTF–TCNQ, approximate mode Grüneisen parameters are derived spanning a range of several orders of magnitude. In the gas phase spectrum of TCNQ, low lying internal modes are observed at 40, 76, and 145 cm−1, lowering the so far assumed demarcation between internal and external modes from about 200 cm−1 down to less than 50 cm−1. From the pressure and gas phase data it is concluded that all of the observed low frequency modes in this class of compounds actually are modes of mixed internal and external character. In TTF0 and TCNQ0 a continuous change in color from yellow to dark red is observed when pressure is applied until the samples become opaque at about 80 kbar. Furthermore, TCNQ shows a phase transition at 12 kbar.The Journal of Chemical Physics 01/1981; 75(7):3220-3225. DOI:10.1063/1.442495 · 3.12 Impact Factor
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ABSTRACT: A systematic study of mixed stacked organic charge-transfer compounds shows that these materials are either neutral or ionic and that they range from being near the neutral-ionic boundary to being far away. Under pressure, several neutral compounds near this boundary are found to undergo a reversible phase transition to an ionic ground state. This is the first observation of a neutral-to-ionic transition in any kind of material.Physical Review Letters 01/1981; 46(4):253-257. DOI:10.1103/PhysRevLett.46.253 · 7.73 Impact Factor