The Infrared Conductivities of Semiconducting (TMTSF)₂ReO₄ and (TMTSF)₂BF₄, Compared with Several Model Calculations

Abstract

The (TMTSF)₂X family of salts, where TMTSF is tetramethyltetraselenafulvalene, contain the first organic superconductors, most of which require the application of pressure. At ambient pressure all except the Perchlorate undergo a metal-insulator transition as the temperature is lowered. For the perrhenate, (TMTSF)₂ReO₄, and the boron tetrafluoroborate (TMTSF)₂BF₄ salts, this transition occurs at 177 K and 38 K respectively. The resulting semiconductors have energy gaps of approximately 1700 cm⁻¹ and 1100 cm⁻¹ respectively. The two compounds are very similar in that, if they are slowly cooled, the energy gap is due to the periodic potential produced by the orientational ordering of the tetrahedral anions. Two orientations are possible and the ordering results in a new unit cell of size 2a × 2b × 2c, for both compounds.