On the basis of the crystal structures of β-(ET) 2 X (X“ = IBr 2 “, Aul 2 ”, I 3 ”) determined at low temperature under ambient and applied pressure, we examined the structural factors responsible for the occurrence of the structural modulation in β-(ET) 2 I 3 under ambient pressure, its disappearance under pressure greater than —0.5 kbar, and its effect on the superconducting transition temperature T c and the dependence of T c on anion X” in β-(ET) 2 X. ET is an abbreviation for BEDT-TTF, i.e., bis(ethylenedithio)tetrathiafulvalene. The electron-phonon coupling constants, X, calculated for β-(ET) 2 X on the basis of the McMillan equation suggest that the lattice softness with respect to translational and/or librational modes of vibration increases in the order β-(ET) 2 I 3 < 0-(ET) 2 IBr 2 < β-(ET) 2 AuI 2 < β*-(ET) 2 I 3 , where β*-(ET) 2 I 3 refers to the β-(ET) 2 I 3 salt under an applied pressure greater than —0.5 kbar with T c a; 8 K. An explanation for this trend in the lattice softness is based on the observation that the principal forces providing the overall lattice cohesiveness of the /3-(ET) 2 X salts are attractive Coulombic interactions of oxidized ET molecule layers with anion X - layers and that each anion X - of β-(ET) 2 X is enclosed in a hydrogen pocket made up of the ethylene groups of ET molecules, which leads to short intermolecular hydrogen—hydrogen (H—H) and hydrogen-anion (H—X“) contacts. Thus, ET molecules are anchored around anions, X”, through short H—X” contacts, so that the softness of the ET molecules with respect to translational and/or librational modes of vibration can be correlated with the lengths of their shortest H—H and H—X” contacts. Such short contacts, which depend sensitively upon the nature and size of the anion X”, also govern whether the two ethylene groups of an ET are eclipsed or staggered in conformation, when viewed along the long molecular axis of an ET. For the ions X” = IBr 2 ” and Aul 2 ”, both of which are smaller than I 3 ”, all ETs of β-(ET) 2 X have eclipsed conformations. In β-(ET) 2 I 3 under ambient pressure, however, both eclipsed and staggered ET molecules are found in nearly equal proportions. Under an applied pressure greater than —0.5 kbar, each eclipsed ET molecule of 0-(ET) 2 I 3 undergoes a conformational flipping at one of the two ethylene groups, so that all ETs of β*-(ET) 2 I 3 have staggered conformations. It is this conformational change that makes the lattice of β*-(ET) 2 I 3 softer than that of (β-(ET) 2 AuI 2 or β-(ET) 2 I 3 , resulting in an increase of T c from — 1.4 to —8 K. Finally, we discuss the importance of hydrogen-anion interactions in understanding the lower T c values (1-1.5 K) of the other class of organic superconductors (TMTSF) 2 X (X“ = C10 4 ” PF 6 ”) compared to the (β-(ET) 2 X systems and the implications concerning the synthesis of new organic superconductors.
All Science Journal Classification (ASJC) codes
- Colloid and Surface Chemistry