Organic Superconductors: Structure--Property Relations and New Materials Design

Most known organic materials are electrical insulators having extremely low electrical conductivities of σ < 10¹⁰ Ω ^-1 cm^-1. A small number of organic materials are semiconductors having, for classification purposes, conductivities of σ ≈ 10¹⁰-1 Ω ^-1 cm^-1. A very small, but growing, number of organic substances are metallic in nature, i.e. having conductivities that rise with decreasing temperature (σ ≈ 1-10¹⁰ Ω ^-1 cm^-1). The latter systems comprise a class of intensely studied materials known as `organic metals' of which fewer than ten can display the complete absence of electrical resistance at low temperatures, i.e. superconductivity (σ ≈ infinity). The known organic superconductors are novel, being derived from radical-cation donors and monovalent anions, X. The donors are derived from two kinds of molecules, neither of which contain any metallic elements. These are TMTSF (tetramethyltetraselenafulvalene) and BEDT-TTF (bis-ethylenedithiotetrathiafulvalene, or `ET' in abbreviated form). Most of the (TMTSF)₂X and (ET)₂X conducting materials require applied pressure to induce superconductivity that is thus far observed at very low temperatures (T_c≈ 1-2 K). However, two materials, (TMTSF)₂ ClO₄ and (ET)₂I₃ are ambient pressure organic superconductors (T_c = 1.2 and 1.4 K, respectively). Within each class the crystal structures have many similarities, the most important being a complex `infinite sheet network' of short Se--Se interactions in (TMTSF)<sub>2</sub>X and a `corrugated sheet network' of short S--S interactions in ET₂X. In this paper we discuss structure--property relations of the (TMTSF)₂X salts, and of the (ET)₂X salts as far as is known. In addition, we attempt to provide insight and guidelines for the synthesis of new highly conducting anionic derivatives of TMTSF and ET. It appears that while highly conducting (TMTSF)₂X materials can be designed before synthesis, the onset of superconductivity depends heavily on the presence of anion order in the crystal, which is a parameter not easily controlled. For the (ET)₂X systems the structural disorder apparent at 298 and 125 K may persist to very low temperature, making it difficult to correlate structural order with superconductivity as is the case for (TMTSF)₂X systems.