Synthetic Metals, 41--43 (1991) 1939-1942 1939 (TMTSF)3T~a2-F-F11: SYNTHESIS, STRUCTURAL CHEMISTRY, ELECTRONIC STRUCTURE AND PHYSICAL PROPERTIES C. LENOIR, K. BOUBEKEUR, P. BATAIL, E. CANADELL, P. AUBAN, O. TRAETTEBERG and D. JEROME Laboratoire de Physique des Solides (associ6 au CNRS), Universit6 Paris Sud 91405 Orsay (France). ABSTRACT The title compound contains the discrete, monovalent anion Ta2F 11- as evidenced by resolution of the crystal structure: triclinic P~, a=7.798(2), b=10.976(4), c=13.557(4) /k; tx=90.71(3), [~=99.47(2), 7=102.37(3) o, V=I 116.5/~3, Z=I, R=0.059, Rto=0.070, esd=l.850 based on 1851 reflexions and 250 parameters. The stack is strongly trimerized and short inter-stacks contacts confer a significant 2D character to the material as substantiated by EHT-type tight-binding band structure calculations. This material is metallic at ambient pressure and displays charge localization below T=220K. The conductivity is increasing under pressure and the location of the resistivity minimum is shifted to temperatures higher than room temperature at P=I lkbar. This suggests that charge localization is increased under pressure possibly due to an enhancement of the trimerization. This situation is contrasting sha/ply with the usual behaviour of the (TMTSF-TMTSF)2X family. SYNTHESIS, STRUCTURAL CHEMISTRY AND BAND STRUCTURE When TMTSF is electrooxidized in a dichloromethane solution containing ButNTaF6 and 20% of ButNPF6, needdle-like single crystals of a 3:1 salt of the monovalent anion Ta2F 11- are harvested. Such in-situ formation of this bis-tantalum anion is unprecedent and its yet unknown structural features (fig. 1) are determined. Note that when the experiment is conducted without PF6-, the classical (TMTSF)2TaF6 phase is obtained which tends to suggest that PF 6- plays a role on the fluoride abstraction and subsequent comer-sharing dimerization of TaF6-. The stack (elongated along g, fig. 1) is strongly trimerized with the shortest inter-tdmer Se...Se separations (4.1130 ./% ) actually longer than a set of three short Se...Se inter-stacks contacts (3.957, 3.963 and 4.013 /~ in the transverse direction ) which might confer a significant 2D character to the material. This is substantiated by Extended Huckel-type fight-binding band structure calculations which indicates that, although the orientation of the molecules is more favourable for 0379-6779/91/$3.50 © Elsevier Sequoia/Printed in The Netherlands