  A new compound with a quasi-two-dimensional array of FeSe3O tetrahedra and an
orthorombic structure, namely BaFe2Se2O, has been successfully fabricated.
Experimental results show that this compound is an insulator and has an
antiferromagnetic (AF) transition at 240 K. Band structure calculation reveals
the narrowing of Fe 3d bands near the Fermi energy, which leads to the
localization of magnetism and the Mott insulating behavior. The large distances
between the Fe atoms perhaps are responsible for the characters. Linear
response calculation further indicates a strong in-plane AF exchange $J$, this
can account for the enhanced magnetic susceptibility (which has a maximum at
about 450 K) above the Neel temperature.
