Eur. Phys. J. B 19, 363-373 (2001)
https://doi.org/10.1007/s100510170312

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

¹ Groupe des Sciences Moléculaires (UPR CNRS 9048) , Institut de Chimie de la Matière Condensée de Bordeaux, 33608 Pessac Cedex, France
² Laboratoire de Physico-Chimie Moléculaire (UMR 5803) , Université Bordeaux I, 33405 Talence Cedex, France

Corresponding author: ^a chasseau@icmcb.u-bordeaux.fr

Received: 10 April 2000
Revised: 27 September 2000
Published online: 15 February 2001

Abstract

The crystal structure of (TMTSF)₂ClO₄ 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 instrumentation designed in our laboratory, these results are the first for molecular 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 ordering which occurs in this compound, and which is characterised by the appearance of superlattice reflections, is well observed. This anion ordering 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 low temperature crystal structures under atmospheric pressure and at 5 kbar shows that the centres of mass are nearly the same, independent of the pressure: 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 temperature. These structural data are confirmed by the quantum chemical calculations which show that the difference in the site energy of the two independent cations is 100 meV.