Structural investigation of BaIrO$$_3$$ by neutron diffraction

Abstract

We report a temperature-dependent neutron diffraction (ND) study on polycrystalline monoclinic BaIrO3 which is famous for charge density wave (CDW) and weak ferromagnetic phase transitions at TC ∼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 IrO6 octahedra is isolated as the temperature approaches to TC . Based on the structure analysis results, we calculated the d-orbital energy level splittings by crystal electric field for each type of the IrO6 octahedra. By taking into account the strong spin-orbit coupling in Ir 5d 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.