J. Phys. I France
Volume 1, Numéro 5, May 1991
Page(s) 703 - 720
DOI: 10.1051/jp1:1991164
J. Phys. I France 1 (1991) 703-720

Successive phase transitions in the organic conductor (TMTSF) $\mathsf{_{2}}$PF $\mathsf{_{2}}$O $\mathsf{_{2}}$

S. 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 $T_{\rm c1/4}=136.3$ K and $T_{\rm c1/2}=135.3$ K. Between $T_{\rm c1/2}$ and $T_{\rm c1/4}$, we observe a phase characterized by the presence in the diffraction pattern of superstructure reflections of reduced wave vector ${\bf q}_{1/4}=(1/2, \pm 1/4, 0)$. At $T_{\rm c1/2}$, a first order phase transition suppresses this phase and a superstructure with ${\bf q}_{1/2}=(1/2, 1/2, 1/2)$ 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.

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