Unusual structure-related magnetism in the organic conductor TSF(Ni(dmit)2)3 (tetraselenafulvalenium-tri-(bis-(4,5-dimercapto-1,3-dithiole-2-thione)nickel)-ate)

TSF(Ni(dmit)2)3 crystallizes in the triclinic space group PI with unit cell dimensions a = 5.875(1), B = 11.846(1), C = 18.873(1) Å, α = 71.35(1), β = 81.63(1), γ = 78.16(1)°. Thermopower and electrical conductivity measurements show semiconducting behaviour localized in the Ni(dmit)2 network. At 65 K the unit cell doubles in the a direction and this feature is utilized in explaining an unusual cross-over from a Bonner-Fischer model to a simple singlet-triplet magnetic behaviour in the sheets of weakly interacting TSF cation radicals.