[La.sub.2][Co.sub.2][Se.sub.2][O.sub.3]: a quasi-two-dimensional Mott insulator with unusual cobalt spin state and possible orbital ordering

The new oxyselenide [La.sub.2][Co.sub.2][Se.sub.2][O.sub.3], containing [Co.sub.2]O square-planar layers, is prepared by using solid-state reactions under vacuum. The studies have shown that the spin and orbital degrees of freedom in the antiferromagnetic checkerboard spin-lattice are mutually coupled.