Novel Structural Features, and their Relationship to the Electrical Properties, of the Organic Conductor (TMTSF)₂ NO₃ at 298 K and 125 K

Abstract

The crystal structures, determined at 298 K and 125 K, of the most highly conducting (ambient pressure, T > 12 K) member of the TMTSF (tetramethyltetraselenafulvalene) family, (TMTSF)₂ NO₃, are presented. Specific features in the structure of (TMTSF)₂ NO₃ at 125 K form the basis for structural relations to the observed d.c. conductivity and the likelihood of an anion-ordered state below a 41 K phase transition. Although the differences in the crystal structures of (TMTSF)₂ NO₃ at the two temperatures are minor, the magnitude and anisotropy of the cation-anion and cation-cation interactions are much better described using the low temperature data. It is observed here that the NO⁻ ₃ anion remains disordered at 125 K with an N-N′ separation of 0.6 Å. The disordered anion model presented here is not the same as that reported earlier, which described C_6h (6/m) site symmetry for the nitrate without any N-N′ separation since the N atom remained on the inversion center. In particular, the previously proposed structural ordering of the nitrate ion, which may lead to the novel 2a × b × c superstructure observed for (TMTSF)₂ NO₃ at 41 K, does not now appear likely based upon an analysis of cation-anion interactions at 125 K. The maintained disorder of the NO⁻ ₃ anion may contribute to the observed lack of superconductivity in (TMTSF)₂ NO₃.