Study of phase stability of β-(BEDT-TTF)2I3 by differential thermal analysis

We report a study of the pressure-temperature phase diagram of β-di[bis(ethyleneditio)tetrathiafulvalene] triiodide, β-(BEDT-TTF) 2I3, by a differential thermal analysis technique. We show that the nature of the state of the compound at low temperature depends on how cooling is achieved. A critical point (Pc ~ 345 bar, Tc ~ 150 K) plays a crucial role in this cooling process. The β- H phase displaying superconductivity at 8.1 K at ambient pressure is obtained by a clockwise pressure-temperature cycling around the critical point. We also show that an anticlockwise P — T cycling with P < Pc allows the stabilization of the low T c superconducting phase (β-L) at low temperature, very likely the phase displaying an incommensurate lattice distortion. The limit of stability of both β- Land β-H phases in the P — T diagram has been determined. The properties of metastability of the various phases of β-(BEDT-TTF)2I3 can be understood in terms of a Landau expansion of the free-energy with the appropriate pressure and temperature dependence of the second and fourth order terms of the expansion.