X-RAY DIFFUSE-SCATTERING STUDY OF SUPERSTRUCTURE FORMATION IN TETRAMETHYLTETRASELENAFULVALENIUM PERRHENATE (TMTSF)2REO4 AND NITRATE (TMTSF)2NO3

X-ray diffuse scattering studies of one dimensional conductors (TMTSF)//2ReO//4 (TMTSF)//2NO//3 reveal the formation of low temperature superstructures which might involve an ordering of the counter ions ReO//4** minus and NO//3** minus . The superstructure reflections are characterized by wave vectors q equals ( one-half , one-half , one-half ) for the perrhenate and q equals ( one-half ,0,0) for the nitrate, which double the unit cell volume in both cases. In (TMTSF)//2ReO//4 this phase transition coincides with the metal insulator transition observed at 180 K, whereas in (TMTSF)//2NO//3 it corresponds to a better conducting state observed below 41 K.