Figure 1: Electronic, magnetic, and optical characterization of the three compounds , , and . (a)–(c) The open symbols correspond to the spin susceptibility along the direction evaluated at constant volume (left axis). The dashed blue lines correspond to fits using the model  of a Heisenberg chain with as indicated. For and , drops exponentially at and 13 K, respectively, indicating a transition to a nonmagnetic spin-Peierls ground state. In contrast, orders antiferromagnetically at K. The solid red dots show the temperature-dependent dc resistivity measured along the -axis (right scale). The solid arrows indicate the charge localization at and the charge-order transition (d)–(f) Temperature development of the asymmetric vibration of the TMTTF molecule involving the C=C bonds obtained with light polarized perpendicular to the plane. For the mode splits due to charge imbalance.