Anomalous Temperature Dependence ofg-Tensor in Organic Conductor, (TMTTF)2X (X=Br, PF6, and SbF6)

Abstract

The magnetic properties of organic conductor (TMTTF) 2 X (X=Br, PF 6 , and SbF 6 ), where TMTTF is tetramethyltetrathiafulvalene, were examined by electron spin resonance (ESR) spectroscopy, X-ray diffraction (XRD) of the single crystals, and quantum–chemical calculation of the g -tensor. In the case of salts with bulky counter anions such as the PF 6 and SbF 6 , an anomalous temperature dependence of the g -tensor was observed in the temperature range from 20 to 296 K. This anomalous behavior of the g -tensor signifies the rotation of the principal axes as well as the shift of the principal values. The g -tensor of the Br salt is, however, temperature independent. No remarkable change in the intra-molecular structure as a function of temperature was observed for all salts. On the other hand, the distance between TMTTF and counter-anion molecules obviously decreases as the temperature decreases for the PF 6 and SbF 6 salts, while thermal contraction is not remarkable for the Br salt. In order to clarify the origin of the anomalous behavior of the g -tensor, we investigated the possibility of deformation of the wave-function by the counter-anion potentials using a quantum–chemical calculation for the actual crystal structures measured at low-temperatures. In this paper, we describe the first direct observation of the deformation of the frontier orbital by the counter anion potential for organic conductors. The intra-molecular spin-distribution as a function of temperature is also discussed from the microscopic point of view.