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

Abstract

We present a detailed low-temperature investigation of the statics and dynamics of the anions and methyl groups in the organic conductors (TMTSF) 2 PF 6 and (TMTSF) 2 AsF 6 (TMTSF: tetramethyl-tetraselenafulvalene). The 4 K neutron-scattering structure refinement of the fully deuterated (TMTSF) 2 PF 6 -D12 salt allows locating precisely the methyl groups at 4 K. This structure is compared to the one of the fully hydrogenated (TMTSF) 2 PF 6 -H12 salt previously determined at the same temperature. Surprisingly, it is found that deuteration corresponds to the application of a negative pressure of 5 × 10 2 MPa to the H12 salt. Accurate measurements of the Bragg intensity show anomalous thermal variations at low temperature both in the deuterated PF 6 and AsF 6 salts. Two different thermal behaviors have been distinguished. Small Bragg-angle measurements reflect the presence of low-frequency modes at characteristic energies θ E = 8.3 K and θ E = 6.7 K for the PF 6 -D12 and AsF 6 -D12 salts, respectively. These modes correspond to the low-temperature methyl group motion. Large Bragg-angle measurements evidence an unexpected structural change around 55 K, which probably corresponds to the linkage of the anions to the methyl groups via the formation of F. . .D-CD 2 bonds observed in the 4 K structural refinement. Finally we show that the thermal expansion coefficient of (TMTSF) 2 PF 6 is dominated by the librational motion of the PF 6 units. We quantitatively analyze the low-temperature variation of the lattice expansion via the contribution of Einstein oscillators, which allows us to determine for the first time the characteristic frequency of the PF 6 librations: θ E ≈ 50 K and θ E = 76 K for the PF 6 -D12 and PF 6 -H12 salts, respectively.

Lattice effects in the quasi-two-dimensional valence-bond-solid Mott insulator EtMe3P[Pd(dmit)2]2

“…This implies that the in-plane expansivities are dominated by anharmonic motions of the EtMe 3 P counterions. A dominant contribution of these isolated, weakly bound molecular units is consistent with recent results obtained on the metallic (TMTSF) 2 PF 6 salt [11]. There it was found that the thermal expansion coefficient is dominated by librational motions of the PF 6 units [11] which could be well described by Einstein oscillators.…”

Charge-ordering transition in (TMTTF)2X explored via dilatometry

“…This observation is complemented by a recent study 48 of the thermal dependence of the lattice expansivity of (TMTSF) 2 PF 6 below 200 K, which exhibits also high temperature negative contributions 99 , and whose analysis shows that anion rotation exhibits a huge Grüneisen parameter of Γ(PF 6 ) ≈ 28. A similar value of Γ(PF 6 ) is expected in (TMTTF) 2 PF 6 because the thermal expansion coefficients have the same magnitude in (TMTSF) 2 PF 6 and (TMTTF) 2 PF 6 .…”

“…For example, in (TMTSF) 2 ReO 4 and BF 4 the AO transition drives a MI transition stabilizing a 2k F BCDW ground state. In (TMTSF) 2 ClO 4 the kinetics of AO drives the salt to either a SDW ground state (quenched samples) or a superconducting ground state (relaxed samples), for more details see 13,27,48 .…”

Structural Aspects of the Bechgaard and Fabre Salts: An Update

“…More precisely, the thermal variation of the lattice expansion in the direction i divided by T, α i /T, or more likely the volume expansion divided by T, (Σ i α i )/T, is proportional to the entropy derivative ∂S/∂T [34]. α ┴ /T taken from the data of [24] exhibits broad maxima around 30-40 K in (TMTSF) 2 PF 6 before dropping to zero at lower temperature [34]. This finding implies that below these maxima a net decrease of the "lattice" entropy, probably due to the vanishing of the structural disorder, should occur.…”