Mid-infrared response of charge-ordered quasi-1D organic conductors (TMTTF)2X

We investigated the temperature dependence of the charge disproportionation in the charge ordered quasi one-dimensional conductors (TMTTF)(2)AsF6 and (TMTTF)(2)PF6 by infrared spectroscopy. In the mid-infrared, we observed a splitting of characteristic vibronic modes below the charge-order transition which call be directly linked to the charge disproportionation oil the molecules. From our analysis of these modes we extracted the temperature dependence of the charge disproportionation which is much stronger in (TMTTF)(2)AsF6 compared to (TMTTF)(2)PF6. The charge disproportionation is only weakly affected by the spin-Peierls transition in the AsF6 salts and remains basically unchanged in the PF6 salts. Therefore, our data confirm that the spin-Peierls and charge-ordered ground state coexist at low temperatures.