Synthetic Metals, 41-43 (1991) 1879-1883 1879 STRUCTURAL INVESTIGATION OF NEW 2 : 1 SERIES OF ORGANIC CONDUCTORS Q. LIU, S. RAVY, R. MORET AND J.P. POUGET Laboratoire de Physique des Solides, associ6 au CNRS, 8&timent 510, Universit~ de Paris Sud, 91405 Orsay (France). C. COULON Centre de Recherche P. Pascal, Avenue du Dr. Schweitzer, 33600 Pessac (France). K. BECHGAARD H.C. Oersted Institute, Universitetsparken 5, 2100 Copenhagen (Denmark). ABSTRACT We report on the relation between the structural and physical properties of 3 new series of 2 : 1 salts isomorphous to the Bechgaard salts, but based on the unsymmetrical molecules BCPTTF, tTTF and TMDTDSF. In the two first series the orientation of the molecules is ordered. With octahedral anions, (BCPTTF)2X and (tTTF)2X exhibit respectively spin-Peierls and antiferromagnetic ground states at critical temperatures sizeably higher than for the (TMTTF)2X series. In (TMDTDF)2X when the anions are small the orientation of the TMDTDSF molecule is found to be disordered. This disorder has dramatic effects on the anion ordering (A.O.) transition occuring when X is non-centrosymmetric. These effects are analyzed in the light of our present understanding of the A.O. mechanism and compared with previous results obtained with the (TMTSF)2_ x (TMTTF)x CIO 4 and (TMTSF)2 (CIO4) l_x (ReO4)x solids solutions. INTRODUCTION In contrast with two-chain organic one-dimensional (1D) conductors of the TTF-TCNQ family which always present a low temperature 2k F charge-density wave (CDW) ground state, the 2 : 1 one-chain quasi-lD conductors based on the TMTSF or TMTTF molecules exhibit a rich variety of phase transitions leading either to superconductivity or to magnetic or dielectric insulating ground states. Among these latter, and for centrosymmetric anions, an antiferromagnetic SDW ground state is observed in (TMTSF)2 PF6 and (TMTTF)2 Br or SbF 6, and a non-magnetic spin-Peierls ground state in (TMTTF)2 PF6 and AsF6. In addition a rich variety of 0379-6779/91/$3.50 © Elsevier Sequoia/Printed in The Netherlands