Structural and magnetic properties of the new low-dimensional spin magnet InCu2/3V1/3O3

We report structural and magnetic data of a new transition metal oxide InCu2/3V1/3O3 containing hexagonal planes of magnetic Cu2+ (S=12) ions in a 5-fold trigonal-bipyramidal coordination of oxygen ions. Static magnetic susceptibility and electron spin resonance (ESR) data as well as numerical quantum Monte-Carlo calculations suggest that Cu spins in the layers are strongly antiferromagnetically coupled with J∼140 K and form a two-dimensional S=12-Heisenberg honeycomb lattice. Owing to the appreciable interlayer coupling the compound orders magnetically at 38K. Remarkably, ESR gives evidence that the orbital ground state of Cu2+ is not the x2-y2 orbital as in copper oxides with the usual octahedral, square-pyramidal or square-planar coordination, but it is the 3z2-r2 orbital, which is unique for the cuprates.