ANISOTROPIC THERMAL-EXPANSION OF THE ORGANIC CONDUCTOR ALPHA-(BEDT-TTF)2I3 - NEW ASPECTS OF THE METAL-INSULATOR-TRANSITION

We have measured for the first time the anisotropic thermal expansion of alpha-(BEDT-TTF)2I3 in the vicinity of its metal-insulator transition at 135 K using two novel capacitive dilatometric cells as well as X-ray diffraction. The observed volume change of DELTAV/V=(+0.84+/-0.3)x10(-3) confirms the transition to be of first order and leads to a change of entropy of DELTAS=3.1 mJ/(g.K) and a latent heat of L=0.42 J/g, consistent with published calorimetric data. At the transition the crystal contracts along the donor stacks (a direction) and expands in the perpendicular (b) direction indicating the appearance of additional force components. Supported by an analysis of literature data we suggest that these may be coulomb forces associated with a redistribution of charge in the unit cell.