Temperature dependence of the transfer integrals in the (TMTSF)2X and (TMTTF)2X families

The authors have evaluated the temperature dependence of the tight-binding transfer integral between neighbouring cation molecules for some representative compounds of the (TMTSF) 2 X and (TMTTF) 2 X families using their recently determined low-temperature and/or high-pressure crystallographic structures. The changes in the transverse integrals in going from the new structures to the ambient temperature and pressure structures are largest for those integrals which show an antibonding character compared to the intrastack interaction. These changes lead to a low-temperature electronic picture which differs markedly from the well known high-temperature description. The transverse bonding and antibonding interactions tend to equalise for the atmospheric pressure supraconductor (TMTSF) 2 ClO 4 . The results suggest that the structure of the most metallic TMTTF salt, (TMTTF) 2 Br, is equivalent at a temperature T 1 to the structure of (TMTSF) 2 X at T=T 1 or (TMTTF) 2 PF 6 at T=T 1 +300K with regard to the values of the transverse transfer integrals.