X-Ray Scattering Study of Phonon Anomalies and Superstructures in TTF-TCNQ

By the X-ray scattering measurements of TTF-TCNQ, two anomalies at q b =0.29 b * and 0.41 b * are studied, where q b is the component of the wave vectors of phonons and superstructures along the b * -axis. The anomaly at 0.29 b * appears in the TA phonon mode below 150 K and becomes the superstructure below 54 K. The anomaly at 0.41 b * appears in the LA phonon mode below room temperature and it also forms the superstructure below 54 K. It is concluded that the wave vector of the modulation of the superstructure at 0.41 b * (which is equivalent to 0.59 b * in the reduced zone scheme) is twice the wave vector of that at 0.29 b * (including the temperature dependent a * -component). Further, it was found that the wave number of the phonon anomaly at 0.41 b * varies from 0.41 b * to 0.45 b * with increasing temperature from 150 K to room temperature.