Broken local symmetry and phase transitions in BaIrO₃ and Sr₂IrO₄

The Ir-based 5d transition metal oxides (TMOs) exhibit rich electric and magnetic phenomena because of the extended range of Ir 5d orbitals and strong spin-orbit coupling effects. Typical examples are BaIrO₃ and Sr₂IrO₄. Quazi-one-dimensional BaIrO₃ is known to exhibit weak ferromagnetism and non-conventional charge ordering below Tc = 180 K leading to a resistivity jump, but its ground state is not well understood. Similarly, layered Sr₂IrO₄ exhibits weak ferromagnetism and markedly increased resistivity below T_c = 240 K. In addition, both systems exhibit little discussed anomalies in the magnetic properties at 80 K and 100 K, respectively. The unusual transitions into an ferromagnetic insulator state and its temperature evolution are associated with changes in the distortions and rotations of the constituent Ir-oxygen octahedra that, according to traditional crystallographic studies, do not break the crystal symmetry.

We conducted variable temperature total x-ray scattering studies coupled to non-traditional atomic pair distribution function analysis. In the talk, we will present results from the study which show that, locally, the crystallographic symmetry is broken in the low-temperature insulating state. The broken symmetry assists the phase transitions and contributes to its unusual properties. The results help understand better the structure-properties relationship in 5d TMOs.