Magnetic properties of 1,5-dimethyl-6-oxoverdazyl radical crystals. Observation of a spin-Peierls transition in 3-(4-cyanophenyl)-1,5-dimethyl-6-oxoverdazyl radical, p-CyDOV

Magnetic susceptibilities of 5 kinds of 3-(4-R-phenyl)-1,5-dimethyl-6-oxoverdazyl radicals (PDOV (R=H), p-MDOV (R-CH3O), p-MeDOV (R=CH3), p-CyDOV (R=CN), and p-NDOV (R=NO2)) have been measured in the temperature range of 4.2-300 K. The susceptibilities of PDOV, p-MDOV, and p-MeDOV can be explained by an antiferromagnetic Heisenberg alternating chain model. The susceptibility of p-CyDOV shows a broad maximum at 54 K, and decreases abruptly at 15 K, suggesting a spin-Peierls transition, High-field magnetization data (H=0-35 T) which provide a clear evidence for a spin-Peierls transition were reported.