Temperature and pressure dependencies of the crystal structure of the organic superconductor (TMTSF)(2)ClO4

The crystal structure of (TMTSF)(2)ClO4 has been determined at (7 K, 1 bar) and at (7 K, 5 kbar) with a high accuracy. For the latter, low temperature and pressure were applied simultaneously using a X-ray diffraction instrum entation designed in our laboratory, these results are the first for molecu lar. compounds. The effects of lowering the temperature are not the same as those produced by increasing the pressure. At (7 K, 1 bar) the anion order ing which occurs in this compound, and which is characterised by the appear ance of b*/2 superlattice reflections, is well observed. This anion orderin g leads to the presence of two independent stacks of TMTSF cations which is the only case found in the Bechgaard salts family. The comparison of the l ow temperature crystal structures under atmospheric pressure and at 5 kbar shows that the centres of mass are nearly the same, independent of the pres sure: the interchain interactions do not depend on the doubling of the unit cell. Under pressure, the ordering (0, 1/2, 0) does not occur at any tempe rature. These structural data are confirmed by the quantum chemical calcula tions which show that the difference in the site energy of the two independ ent cations is 100 meV.