High pressure optical absorption in organic semiconductors ( NMe 4 ) 2 TCNQ 3 and Cs 2 TCNQ

The pressure dependence of the optical absorption spectrum in (NMe4)2TCNQ3 and Cs2TCNQ3 has been measured at room temperature in the spectral range from mid-infrared to visible. At the ambient pressure the optical energy gap is observed at around 0.5 and 0.3 eV in (NMe4)2TCNQ3 and Cs2TCNQ3, respectively. The energy gap in (NMe4)2TCNQ3 decreases by 0.05 eV with increasing pressure up to 2 GPa, followed by an abrupt decrease towards 0.2 eV between 2 and 3 GPa. At higher pressures up to 5.5 GPa the gap remains almost unchanged. The intraand inter-molecular optical absorption bands in the near-infrared to visible region are intensified by pressure and broadened out at 2.5 GPa. The absorption spectrum in Cs2TCNQ3 exhibits a similar behavior under pressure. These results suggest that an electronic phase transition is induced by pressure at around 2.5 GPa in both substances. This phase transition resembles the charge-order-disorder (Verwey) transition in Fe3O4 in the electronic mechanism.