Low Temperature and High Pressure (1.7K & 7 10² MPa) Structure of the First Superconducting Organic Salt (TMTSF)₂ PF₆

Abstract

The neutron (1. =1.1750Å) high pressure (7 10²MPa) and low temperature (1.7K) structure of (TMTSF)₂PF₆ (Di(2,3,6,7-tetramethyl-1,4,5,8-tetraselenafulvalenium hexafluorophosphate; 2(C₁₀H₁₂Se₄)^1/2+, PF₆ ⁻, Mr=1041.06) is reported. The lattice is triclinic P and cell parameters:

The geometry of the Tmtsf stacks changes little under constraint exception made for a shortening in the interplanar spacing. For the titled compound at 7 10² MPa a temperature decrease from 300K to 4K is equivalent to the effect of an extra 2 10²MPa applied pressure. However temperature (rather than pressure) is responsible for a small change in the transverse shift of adjacent Tmtsf molecules in a same column. Similarly pressure has little effect on the PF₆ ⁻ anion disorder at a given temperature.

If the low temperature TMTSF packing of (TMTSF)₂PF₆ (superconducting below ≃ 1.2K and above 9.5 10² MPa) is compared to that of (Tmtsf)₂ C10₄ (superconducting below ≃1,2K at ambient pressure), the intrachain interactions are found to be very similar. On the contrary, the low temperature strength of the interchain interactions in the PF₆ ⁻ salt requires an extra high pressure constraint to reach that observed in the ClO₄ salt at ambient pressure.

Keywords:

• organic superconductors
• structure
• low dimensional solids