Structural and magnetic properties of the alternating Heisenberg spin-chain compound dichloro(2,5-dithiahexane)copper(II)

Temperature dependence and anisotropy of the electron-spin-resonance (ESR) signal at 9.5 GHz of the one-dimensional alternating Heisenberg antiferromagnet dichloro(2,5-dithiahexane)copper(II) are reported in the temperature range between 5 and 300 K. The ESR measurements reveal a structural phase transition at Tc ≅150 K. By x-ray diffraction the structure of the low-temperature phase is determined at T=120 K. Symmetry reduction from the monoclinic space group P21/n to the triclinic group P10-bar results in the formation of twin domains and the creation of two different kinds of spin chains below Tc. The influence of the structural modifications on the magnetic properties is discussed in detail. Measurements of the static magnetic susceptibility yield the exchange constants and alternation parameters J1/kB=-29.3(1) K, α1=0.76(1) and J2/kB=-27.9(1) K, α2=0.99(1) of chain 1 and 2, respectively