J. Phys. I France 1 (1991) 703-720
Successive phase transitions in the organic conductor (TMTSF) PF OS. Ravy1, J. P. Pouget1, R. Moret1 and F. Wudl2
1 Laboratoire de Physique des Solides CNRS URA 2, Bât. 510, Université Paris-Sud, 91405 Orsay Cedex, France
2 Institute for Polymers and Organic Solids and Department of Physics, University of California, Santa Barbara, CA 93106, U.S.A.
(Received 20 December 1990, accepted 5 February 1991)
We present an X-ray study of the organic conductor (TMTSF) 2PF 2O 2. At ambient pressure this compound undergoes a metal-insulator transition at about 137 K but, at variance with similar materials, a metallic state is not restored in the whole temperature range under pressure lower than 14.5 kbar. We show that (TMTSF) 2PF 2O 2 undergoes two successive structural phase transitions at K and K. Between and , we observe a phase characterized by the presence in the diffraction pattern of superstructure reflections of reduced wave vector . At , a first order phase transition suppresses this phase and a superstructure with is stabilized as for most (TMTSF) 2X salts built with tetrahedral anions. We have studied the behaviors of the associated thermodynamical quantities (order parameters, susceptibilities) that a Landau model with two biquadratically coupled order parameters qualitatively explains. We discuss the nature and the competition of these two phases.
© Les Editions de Physique 1991