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

Abstract

We present an X-ray study of the organic conductor (TMTSF~~PF~O~. At ambient pressure this compound undergoes a metal-insulator transition at about 137 K but, at variance with sHflar materials, a metallic state is not restored in the whole temperature range under pressure lower than 14.skbar. We show that (TMTSF~~PF~02 undergoes two successive structural phase transitions at T~,j4 =136.3 K and T~,j~ = 135.3 K. Between 2~jj~ and T~j, we observe a phase characterized by the presence in the diffraction pattem of superstructure reflections of reduced wave vector qj~ = (1/2, ± JR, 0). At T~,j~ a first order phase transition suppresses this phase and a superstructure with qjj~ = (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.