Honeycomb Lattice Antiferromagnets

We have studied the crystal structures of transition metal polynuclear coordination polymers M2(pymca)3(ClO4) (M=Fe, Co, Ni, and Cu, pymca=pyrimidine-2-carboxylate) by using X-ray diffraction techniques. In these compounds, each pymca ligand connects to two M2+ ions, forming a honeycomb network in the ab plane. The temperature dependences of magnetic susceptibilities in these compounds show broad maxima, indicating antiferromagnetic interactions within two-dimensional honeycomb layers. We also observed an antiferromagnetic phase transition at low temperatures by magnetic susceptibility and heat capacity measurements. From the crystal structures and magnetic properties, we conclude that these compounds are good realizations of honeycomblattice antiferromagnets.