Ba₂IrO₄: A spin-orbit Mott insulating quasi-two-dimensional antiferromagnet

A layered iridate Ba₂IrO₄ was synthesized using a high-pressure synthesis technique. Its electronic state was studied through electrical resistivity, magnetic susceptibility, and μSR experiments. It was found that Ba₂IrO₄ crystallizes in a K₂NiF ₄-type structure (space group I4/mmm) with lattice parameters a =4.030(1) and c =13.333(4). The structure includes flat IrO₂ square planar lattices with straight Ir-O-Ir bonds. Ba₂IrO₄ is a Mott insulator (activation energy ΔE_a ∼ 0.07 eV) driven by a spin-orbit interaction. The magnetic susceptibility and μSR studies revealed that the magnetic ground state is antiferromagnetic long-range order (T_N ∼ 240 K) in which the magnetic moment (∼0. 34μ_B/Ir atom) is significantly reduced by a low-dimensional quantum spin fluctuation with a large intraplane correlation |J|. The behavior is similar to that in parent materials of high-T_C cuprate superconductors such as La₂CuO₄.