Behavior of the p-T phase diagram of the organic conductor (ET)(4)Hg3I8

The conductivity of single crystals of the organic conductor (ET)(4)Hg3I8 [ ET-bis(ethylendithio)tetrathiafulvalene] has been investigated at temperatu res from 4.2 to 360 K and pressures of up to 75 kbar. Two first-order phase transitions have been detected at room temperature at pressures of 2.75 an d 6.7 kbar. On the basis of the experimental data, the p-T phase diagram fo r the first-order phase transitions has been plotted. The unusual shape of the phase diagram (a slow monotonic growth of the transition temperature wi th a slope dT/dp=4 deg/kbar followed by a sharp drop around the point p(0)= 6.5 kbar, T-0=324 K) has been analyzed using the Landau theory of second-or der phase transitions. Our analysis supports the hypothesis of a second-ord er phase transition around this point and also exhibits satisfactory agreem ent between calculations and the experimental curves of the first-order pha se transitions. (C) 1999 American Institute of Physics. [S1063-7761(99)0210 6-X].