Synthetic Metals, 27 (1988) A367-A373 A367 UNUSUAL FEATURES OF THE STRUCTURAL PHASE DIAGRAM OF ~-(BEDT-TTF) 2 13 : AN EXPERIMENTAL AND THEORETICAL STUDY S. RAVY, R. MORET and J.P. POUGET Laboratoire de Physique des Solides (U.A. 2), Universit~ Paris-Sud B~timent 510, 91405 ORSAY, France. ABSTRACT We present an X-ray study of the incommensurate modulation of the organic superconductor ~-(BEDT-TTF)2I 3. This modulated phase aDDears at 175 K, througha second order phase transition. We shall pay special attention to three original aspects of this phase : anharmonicity of the displacement wave, kinetics and effects of the irradiation induced defects. We propose then a simple model, based on the two configurations of the BEDT-TTF molecule, which satisfactorily explains the pressure-temperature phase diagram of the compound. This model also enables us to understand the original features of the modulated phase, previously mentioned. INTRODUCTION The organic superconductor ~-(BEDT-TTF)2I 3 shows an unusual phase diaqram in temperature and pressure with large metastability domains,for the stabilization of an incommensurate modulation,and the presence of a tricritical point (PT % 350 bar, T T % 150 K)[I]. By cooling this compound at pressures lower than , state) (T~ = 175 K, PT one observes the appearance of a modulated phase (~L 1 bar [2])and the system becomes superconductor at T S = 1.5 K [3]. This modu- lated phase is not stabilized by cooling at pressures greater than PT' and a superconducting phase (~H state) with a higher T S : 7.4 K is obtained [4]. Depressurization at low temperature enhances this critical temperature to 8.1 K [5], showing clearly a link between the T Sand the presence of the modulatio~ At 295K, I bar , the space groupof thecrystal is PTand the unit cell contains two. molecules and one anion . The outer ethylene groups of the BEDT-TTF molecules in closer contact with the anions are disordered with two non equivalent positions 0379-6779/88/$3.50 © Elsevier Sequoia/Printed in The Netherlands