Structural investigation of BaIrO _3 by neutron diffraction

We report a temperature-dependent neutron diffraction (ND) study on polycrystalline monoclinic BaIrO _3 which is famous for charge density wave (CDW) and weak ferromagnetic phase transitions at T _C ∼ 180 K simultaneously. A Rietveld analysis on the ND patterns reveals that even though there is no symmetry breaking in crystal structure, a noticeable change in the four kinds of IrO _6 octahedra is isolated as the temperature approaches to T _C . Based on the structure analysis results, we calculated the d -orbital energy level splittings by crystal electric field for each type of the IrO _6 octahedra. By taking into account the strong spin-orbit coupling in Ir 5 d orbitals and the lattice distortions obtained from the ND analysis, we propose an electronic configuration model to understand the phase transition of the system, where an effective J_eff, 1/2 Mott insulating phase and a charge gap phase induced by bonding states between the J_eff, 1/2 states compete each other.