Magnetic Properties and Structures of the α- and δ-Phases ofp-NPNN

Abstract

We have investigated the crystal structures and magnetic properties of the α- and δ-phases of an organic radical p -NPNN ( p -nitrophenyl α-nitronyl nitroxide), which yielded the first organic bulk ferromagnet, β- p -NPNN (the β-phase). The results are compared with those of the β- and γ-phases, which show distinct magnetic ordering due to intermolecular ferromagnetic couplings. Common structural features are found in the four phases. Unlike the other phases, the α-phase exhibits a weak antiferromagnetic interaction. This is attributable to phenyl–phenyl overlapping found only in the α-phase. The crystal structure of δ- p -NPNN contains a packing motif similar to that of the γ-phase. Though the δ-phase exhibits ferromagnetic interactions, it shows no magnetic ordering down to 0.46 K. It is suggested that ferromagnetic two-leg spin ladders are formed in the δ-phase, instead of a two-dimensional network as in γ- p -NPNN. This low dimensionality explains the absence of a magnetic transition.