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.