Tetramethyltetrathiafulvalene [(NbOF₄)^-]_∞: One-Dimensional Charge Transfer Salt with an Infinite Anion Chain

An unusual valence one-dimensional (1D) molecular charge transfer salt (TMTTF)(NbOF₄) [TMTTF = tetramethyltetrathiafulvalene] with infinite anion chains was prepared. To understand the crystal structure and electronic states of the (TMTTF)(NbOF₄) salt, we performed synchrotron X-ray diffraction, electron spin resonance, and static magnetization measurements. There is only one independent TMTTF molecule in the unit cell of (TMTTF)(NbOF₄). The TMTTF¹⁺ cation radicals stack to form 1D columns. The effective charge of the TMTTF molecule in the crystal was estimated to be +1. The electric charge of TMTTF donors is compensated by the infinite anion chains [(NbOF₄)^-]_∞. The magnetic susceptibility of (TMTTF)(NbOF₄) is 4 × 10^-4 emu/mol at room temperature and shows weak temperature dependence above 60 K. However, some deviation appears below 60 K. The temperature dependence of the spin susceptibility shows a noticeable enhancement below 60 K. Below 5 K, the magnetization curve as a function of the magnetic field deviates from the straight line and shows a saturation tendency. The experimental results can be reproduced well with the S = 2 spin system at 2 K. The detailed analysis of the crystal structure and anomalous low-temperature magnetic state magnetic properties of (TMTTF)(NbOF₄) are discussed.