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

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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.

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