2k_F and 4k_F charge-density waves in TTF_0.4TSeF_0.6-TCNQ - An X-ray study

In the quasi one-dimensional conductor TTF0.4TSeF0.6-TCNQ, it is found that the 2kF CDW has three-dimensional correlations of relatively long range below the metal-insulator transition temperature ∼40K, although the 4kF CDWs are only one-dimensional in character at all temperatures. This result supports the speculation that the 2kF CDWs and the 4kF CDWs are present predominantly on the TCNQ and the fulvalene stacks, respectively. The 2kF-value of the present material is similar to that of TTF-TCNQ but far from that of TSeF-TCNQ. The change of the value with the concentration of TSeF (or TTF) is nonlinear. The transverse period of the ordered 2kF CDW along the a-axis is a just below the transition temperature and becomes 4a below 34.5K. A discussion is given based on the two-chain model. © 1980.