LANDAU THEORY OF PHASE-TRANSITIONS IN TTF-TCNQ

Several parameters of the Landau Free Energy Function of TTF-TCNQ are calculated from the crystal structure taking into account the atomic positions and realistic charge distributions. In particular, the parameters that determine the coupling between charge density waves and rotations (librons) on neighbouring chains are evaluated. The Free Energy Function is solved, and good agreement with the experimental observation of the 2 a period between 53 K and 48 K and its variation below this temperature, is obtained. The CDW-libron interaction seems to be responsible for this transverse period. The behavior of C//p in the vicinity of the two close phase transitions is studied. The Free Energy should possess several (up to four) Lifshitz points, for appropriate values of the parameters.