Neutron low-temperature (4 and 20 K) and X-ray high-pressure (6.5 x 10² and 9.8 x 10² MPa) structures of the organic superconductor di(2,3,6,7-tetramethyl-1,4,5,8-tetraselenafulvalenium) hexafluorophosphate, (TMTSF)₂PF₆

Abstract

The Bechgaard salts (TMTSF)2X (with X a small centro-or non-centrosymmetric anion) show several kinds of ground states, in particular a superconducting one at low temperatures (T<l.5 K), under a pressure which is strongly dependent on the nature of the anion X. Since intrachain and interchain interactions and anion disorder are key parameters in the understanding of this behaviour, single

Low-temperature structural effects in the (TMTSF)2PF6and AsF6Bechgaard salts

“…It shows that the PF 6 -D12 lattice parameters are larger than those for PF 6 -H12. The 4 K lattice parameters of PF 6 -D12 are comparable to those of PF 6 -H12 at 120 K. 23 Using the low-temperature compressibility of PF 6 -H12, 37 one deduces that the increase of volume of PF 6 -D12 corresponds to the application of a negative pressure of 500 MPa to PF 6 -H12 at 4 K. This is a much larger effect than the one revealed for κ-(BEDT-TTF) 2 Cu[N(CN) 2 ]Br where deuteration corresponds to the application of a negative pressure of about 40 MPa to the hydrogenated salt. 3 As for the PF 6 -H12 salt, the structural refinement of PF 6 -D12 shows that both CD 3 methyl groups and the PF 6 anions are ordered at 4 K. The thermal parameters [U(eq) in Table III] of the P, F, Se, and C atoms (nuclei) of the PF 6 -D12 salt are comparable to those previously determined in PF 6 -H12.…”

“…3 As for the PF 6 -H12 salt, the structural refinement of PF 6 -D12 shows that both CD 3 methyl groups and the PF 6 anions are ordered at 4 K. The thermal parameters [U(eq) in Table III] of the P, F, Se, and C atoms (nuclei) of the PF 6 -D12 salt are comparable to those previously determined in PF 6 -H12. 23 U(eq) of D is however 50% larger than U(eq) of H, which means that D undergoes larger quantum fluctuations than H. This result is surprising because D has a mass twice larger than that of H. It is tempting to assign this effect to the presence of smaller potential barriers for quantum tunneling in PF 6 -D12 than in PF 6 -H12. In addition, as for PF 6 -H12 and despite the fact that the PF 6 anion is well localized in the 4 K structure, Table III shows that U(eq) for F is three times larger than the U(eq) for the central P atom of the anion.…”

“…Both anion and methyl-group disorders are progressively removed upon cooling. In particular, the refinement of the 20 K and 4 K neutron-scattering structural data of hydrogenated (TMTSF) 2 PF 6 (named PF 6 -H12 below) 23 show that each anion locks its orientation by establishing two short F-Se contact distances and four F. . .H-CH 2 bonds with four neighboring TMTSF molecules located in a plane nearly perpendicular to a.…”

One-dimensional electronic properties of the Bechgaard salts

“…The first one comes from X-ray and neutron measurements which have provided accurate values for the structural parameters of these compounds [4]. Using them as inputs, quantum chemistry calculations [5] have provided values for various hopping integrals, in particular for the hopping integrals along the stacks tl and t2, for a few compounds (see table 1).…”

Distortion of Crystal Structures of Some CoIII Ammine Complexes. I. Distortion of Crystal Structure of [Co(NH3)5NO2]Cl(NO3) on Cooling

“…Some examples of such studies are known. For example, high-pressure and low-temperature compressions were shown to have similar effects on the structure of an organic superconductor di (2,3,6,7-tetramethyl-1,4,5,8-tetraselenafulvalenium) hexafluorophosphate (Gallois, Gaultier, Hauw, Lamcharfi & Filhol, 1986). However, in general, comparative studies of anisotropic strain induced in the same structure by different actions are still rare.…”