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We present a detailed low-temperature investigation of the statics anddynamics 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 structurerefinement of the fully deuterated (TMTSF)$_2$PF$_6$-D12 salt allows locatingprecisely the methyl groups at 4 K. This structure is compared to the one ofthe fully hydrogenated (TMTSF)$_2$PF$_6$-H12 salt previously determined at thesame temperature. Surprisingly it is found that deuteration corresponds to theapplication of a negative pressure of 5 x 10$^2$ MPa to the H12 salt. Accuratemeasurements of the Bragg intensity show anomalous thermal variations at lowtemperature both in the deuterated PF$_6$ and AsF$_6$ salts. Two differentthermal behaviors have been distinguished. Low-Bragg-angle measurements reflectthe presence of low-frequency modes at characteristic energies {\theta}$_E$ =8.3 K and {\theta}$_E$ = 6.7 K for the PF$_6$-D12 and AsF$_6$-D12 salts,respectively. These modes correspond to the low-temperature methyl groupmotion. Large-Bragg-angle measurements evidence an unexpected structural changearound 55 K which probably corresponds to the linkage of the anions to themethyl groups via the formation of F...D-CD2 bonds observed in the 4 Kstructural refinement. Finally we show that the thermal expansion coefficientof (TMTSF)$_2$PF$_6$ is dominated by the librational motion of the PF$_6$units. We quantitatively analyze the low-temperature variation of the latticeexpansion via the contribution of Einstein oscillators, which allows us todetermine for the first time the characteristic frequency of the PF6librations: {\theta}$_E$ = 50 K and {\theta}$_E$ = 76 K for the PF$_6$-D12 andPF$_6$-H12 salts, respectively.

Low temperature structural effects in the (TMTSF)$_2$PF$_6$ and AsF$_6$ Bechgaard salts