Electron spin resonance studies on the organic linear-chain compounds (TMTCF)(2)X (C = S,Se; X = PF6, AsF6, ClO4, Br)

We have conducted comprehensive electron spin resonance (ESR) investigation s on single crystals of the one-dimensional organic compounds (TMTTF)(2)PF6 , (TMTTF)(2)ClO4, (TMTTF)(2)Br, (TMTSF)(2)PF6, and (TMTSF)(2)AsF6 in the te mperature range from 4 to 500 K and additionally, (TMTSF)(2)ReO4 and (TMTSF )(2)ClO4 at room temperature. In contrast to the selenium analogs TMTSF whi ch are one-dimensional metals, the sulfur salts are semiconductors with loc alized spins on the TMTTF dimers. Taking into account the thermal expansion of the crystals at high temperature (T>20 K) the ESR intensity of all sulf ur compounds can be described as a spin-1/2 antiferromagnetic Heisenberg ch ain with exchange constants 420 less than or equal to J less than or equal to 500 K. Although the TMTSF compounds are one-dimensional organic metals d own to 10 K, the temperature dependence of the spin susceptibility can also be described within the framework of the Hubbard model in the limit of str ong Coulomb repulsion with J approximate to 1400 K. By modeling (TMTTF)(2)C lO4 as an alternating spin chain, the change of the alternation parameter a t the first-order phase transition (T-AO=72.5 K) indicates a tetramerizatio n of the chain. (TMTTF)(2)PF6 undergoes a spin-Peierls transition at T-SP=1 9 K which can be well described by Bulaevskii's model with a singlet-triple r gap Delta(sigma)(0)= 32.3 K. We find evidence of antiferromagnetic fluctu ations at temperatures well above the magnetic ordering in (TMTTF)2Br, (TMT SF)2PF6, and (TMTSF)(2)AsF6 which follow the critical behavior expected for three-dimensional ordering. (TMTTF)(2)PF6 and (TMTTF)(2)Br show one-dimens ional lattice fluctuations.