Pressure dependence of charge distribution along TCNQ columns in Cs2TCNQ3

Pressure dependence of the charge distribution along the columns of tetracyanoquinodimethane (TCNQ) molecules in Cs2TCNQ3, which is a semiconducting 1/3-filled Hubbard system with the optical energy gap of 0.3 eV at ambient conditions, has been studied by the infrared and visible absorption experiments. An electronic phase transition is found to occur at around 3.5 GPa, in accordance with the previously reported insulator-to-metal transition. Our result suggests that this phase transition accompanies a symmetry change in the relative position of the TCNQ− radical molecules sandwiching the TCNQ0 neutral molecule. In addition, the π* electrons, which are localized on the TCNQ− molecules in the low-pressure phase, are delocalized significantly in the high-pressure phase. However, in the high-pressure phase the π* electrons still maintain the localizing character. The charge-transfer degree of the neutral-like and radical-like molecules of the high-pressure phase is estimated to be ρ = 0.46-0.50 and 0.75-0.77, respectively