Neutron-Scattering Evidence for a Spin-Peierls Ground State in (TMTTF)2PF6

~MTTF)2X, with X=PF6, exhibits, at Tsp, a spin-Peierls transition. Below Tsp elastic neutron scattering measurements performed on large protonated and deuterated samples reveal the formation of very weak superstructure reflections at the (1/2,1/2,1/2) reduced wave vector. Upon deuteration Tsp decreases by 30% while the charge ordering (CO) transition increases by 25%. In this framework we stress the importance of the anions, X,, and of their organic cavities to stabilize the CO transition of the (TMTTF):X salts. PACS numbers: 71.27+a, 71.45 Lr, 77.80 Bh.