Pressure Dependence of the Structural and Electronic Properties of Organic Superconductors
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PRESSURE DEPENDENCE OF OF ORGANIC SUPERCONDUCTORS. CHASSEAU
D.*,
GAULTIER
KURMOO M.*** and DAY P.***
THE J.*,
STRUCTURAL HOUBIB
H.*,
AND
ELECTRONIC
RAHAL
M.*,
PROPERTIES
DUCASSE
L.**,
Laboratoire de Cristallographie et Physique Cristalline, URA 144 CNRS ** Laboratoire de Physico-Chimie Thdorique, URA 503 CNRS Domaine Universitaire de Bordeaux I, 33405 - TALENCE C~dex (FRANCE). *** Inorganic Chemistry Laboratory, University of Oxford, South Parks Road, OXFORD OXI 3QR (U. K.) *
Abstract The pressure dependence (TMTSF)
- C10 4 and x -
(s12 x 102 MPa)
(BEDT-TTF) 2
- Cu(NCS)
of the cell 2
parameters
of
has been measured at room
temperafure. The magnitudes of the principal compressibilities calculated and the bulk modulus (B) do not vary all over the experimental pressure - ClO4, while ki decrease slightly for i - (ET) range for (TMTSF) Cu(NCS) 2 and its %ulk modulus strongly increases. The directions of ?he principal compressibilities strongly vary; with pressure for the former salt; in the latter case, the projections of the directions of the smallest and of the largest principal compressibilities remain parallel and perpendicular to the long molecular axis respectively whereas the intermediate is along the monoclinic b axis. The crystal structures have been refined at 4 x 102 ,6 x 102 and 9.5 x 102 MPa (R - 0.05) for (TMTSF) 2 - C10 4 and at 7 x 102 MPa (R = 0.095) for (ET) 2 - Cu(NCS) . The transfer integrals have been calculated for the two compounds. In (TWTSF) 2 - ClO4, both normal and transverse overlaps strongly increase with pressure; the dimerization is reduced under pressure and the two largest transverse interactions become identical. In x - (ET) 2 Cu(NCS) 2 , the intra-dimer interaction strongly increases whereas the variations of the inter-dimer interactions may be very different.
Introduction X Since the first observation [1] of superconductivity in (TMTSF) salts (TMTSF = tetramethyltetraselenafulvalene; X- = PF6 ", Cl04", etc), ihe physical properties of these compounds have been the subject of extensive studies over a wide temperature and/or pressure range. It has been the same for the organic superconductors based on the BEDT TTF donor molecule (BEDT TTF = ET = bis (ethylenedithio) tetrathiafulvalene) since the discovery of the first ambient pressure superconductor of this family in 1984 [2]. The structural properties have been extensively studied in normal conditions, sometimes at very low temperatures [3] but very rarely under high pressure. However, the high pressure (6.5 x 102 and 9.8 102 MPa) X-ray and the low temperature and high pressure (1.7K, 7 x 102 MPa) neutron crystal structures of (TMTSF) 2 - PF6 [4,5], and high pressure (9.5 x 10 MPa) and high pressure (4.5K, 1.5 x 102 MPa) X-ray and the low temperature neutron structures of p (BEDT-TTF) 2 - I [6 ,7] have been determined. In the present paper, we discuss tRe compressibilities and the pressure dependence of the crystal structures of the first ambient pressure superconductor (TMTSF)J-C10 4 [8] and of the x - (ET) -
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