Thin-film growth and structural characterization of a novel layered iridate Ba ₇ Ir ₃ O _13+δ

Iridates have attracted immense interest since their strong spin-orbit coupling (SOC) can lead to rich exotic phenomena such as a J _eff = 1/2 Mott insulating state. Here we report a novel iridate discovered in our efforts which aimed to synthesize Ba ₂ IrO ₄ thin films. Through systematic transmission and scanning transmission electron microscopy studies, we have shown this new compound possesses a layered orthorhombic structure with the composition of Ba ₇ Ir ₃ O _13+δ (BIO). This material is an insulator with an optical band gap of ∼1.3 eV. Furthermore, we found that the thin films of this material can be grown on differently orientated perovskite substrates or MgO substrates. Although all these films maintain an identical crystallographic orientation, i.e. its c-axis perpendicular to the substrate surface, they form various domain structures dependent on the substrate. When (001)-oriented LaAlO ₃ and (111) oriented SrTiO ₃ perovskites are used as substrates, the domains show 12 fold and 6 fold symmetry respectively, and the domain orientations are highly coherent and the domain-walls are atomically sharp. However, the films on the (110) oriented MgO substrates feature much less coherent domain walls and thread dislocations occur at the domain boundaries. These findings not only reveal a new playground for the study of the novel SOC physics of iridates, but also provide a route to tailor the domain wall structure via epitaxial lattice mismatch in films.