Successive phase transitions in the organic conductor (TMTSF)₂PF₂O₂

We present an X-ray study of the organic conductor (TMTSF)2PF2O2. 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)2PF2O2 undergoes two successive structural phase transitions at Tc1/4=136.3 K and Tc1/2=135.3 K. Between Tc1/2 and Tc1/4, 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 Tc1/2, 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