2k_F transitions in a series of (DMET)₂(X ₁X₂TCNQ) (X₁, X₂=Br, Cl, F, CH ₃, H): Subsidiary lattice effect by anion radicals

Structural and magnetic properties were investigated in a series of isomorphous 2:1 donor-acceptor-type organic charge-transfer (CT) complexes of (DMET)₂(X₁X₂TCNQ) [DMET denotes dimethyl(ethylenedithio)diselenadithiafulvalene, TCNQ denotes tetracyanoquinodimethane, X₁, X₂=Br, Cl,F,CH ₃,H]. In these complexes, donors (D's) formed quasi-one-dimensional (Q1D) molecular stacks (D = + 1/2), which were bridged with each other, by acceptor molecules whose molecular planes were parallel to the stacking axes. By the magnetic measurements, we found that the compounds could be classified into two groups in terms of the 2k_F transitions of the donor stacks: two complexes exhibited the transitions at around room temperature, while the other two at lower temperature below 10 K. We discuss the origin of the difference in the observed 2k_F transitions in terms of the subsidiary lattice effect by the acceptor molecules on the Q1D donor stacks.