The magnetic susceptibilities of the mixed radical crystals, (p-CyDOV)1 - x (p-BrDOV) x (x = 0−1), were measured to study the doping effect of magnetic impurities on the spin-Peierls (SP) transition of p-CyDOV (3-(4-cyanophenyl)-1,5-dimethyl-6-oxoverdazyl) and on the dimer state of p-BrDOV (3-(4-bromophenyl)-1,5-dimethyl-6-oxoverdazyl) radical crystals. p-BrDOV and p-CyDOV, which have van der Waals radii and unpaired spin distributions similar to each other and S = 1/2 spin, were chosen as dopants in the above SP and dimer systems, respectively. The effect of doping on the SP system is very remarkable. The SP transition temperature (T SP = 15.0 K) decreases rapidly upon p-BrDOV doping and the SP state almost disappears in the lightly doped sample with x = 0.05, although the effect of p-BrDOV doping was not observed in the powder X-ray diffraction pattern and the ESR spectrum of p-CyDOV at the low concentration region (x = 0−0.09) of p-BrDOV. The susceptibilities of mixed crystals with x = 0.05 and 0.07 can be well explained by the sum of the susceptibilities of one-dimensional (1D) Heisenberg antiferromagnet (χ1D - HAF) with intrachain exchange interaction (2J/k B) similar to that of pure p-CyDOV and Curie impurity (χCurie). Similarly, the temperature dependence of the susceptibilities for the mixed crystals with x = 0.01 and 0.03 above T SP is described by the above two contributions (χ1D - HAF and χCurie). However, the susceptibilities below T SP can be explained by considering the contribution from the SP state (χSP) in addition to the above two contributions. On the other hand, the dimer state in p-BrDOV survives in the heavily doped sample with x = 0.85. The intradimer exchange interaction 2J/k B remains constant between x = 0.85 and 1, suggesting that the substituted p-CyDOV molecules play a role as p-BrDOV ones in the mixed crystals with x = 0.85−1. This work is the first report of the effects of magnetic impurities on the organic SP transition system.

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