Enhanced insulating behavior in the Ir-vacant Sr₂Ir_1–xO₄ system dominated by the local structure distortion

Sr₂IrO₄, known as the J_eff = 1/2 Mott insulator, was predicted to be an unconventional superconductor upon doping since it highly resembles the high-temperature cuprates. However, recent work pointed out an enhanced insulating behavior in the Ir-vacant Sr₂Ir_1–xO₄ system. In this contribution, to investigate the microscopic mechanism of its enhanced insulating behavior, X-ray absorption spectroscopy was applied to study the electronic structure and local structure distortion of Sr₂Ir_1–xO₄. Due to the presence of Ir⁵⁺ ions, the preconceived holes are barely doped in the Ir-vacant system. Nevertheless, Ir vacancies finely modulate the local atomic structure, i.e. the topology of IrO₆ octahedra and the in-plane Ir—O1—Ir bond angle. Combined with theoretical calculations, it is demonstrated that both the more distorted IrO₆ octahedra and decreased Ir—O1—Ir angle contribute to the increment of the band gap, and then result in the enhanced insulating state for Sr₂Ir_1–xO₄.