The origin of different iridium oxidation states of Sr₂IrO₄

The results of<i>ab initio</i>calculations performed for Sr₂IrO₄using the density functional theory with the spin-orbit coupling and the onsite Hubbard-U correction applied to the Ir-d states are reported in order to provide more insights into known experimental data. The results demonstrate that in the stoichiometric oxide all Ir ions are in the +4 oxidation state. In the presence of an O vacancy some Ir ions turn into +3 and +2 states that is contrary to experimental observations where only the +3 state has been observed. These transitions are accompanied by the change in the magnetic moments from 0.10<i>μ</i>_Bfor Ir⁴⁺to 0.26<i>μ</i>_Band 0.31<i>μ</i>_Bfor Ir³⁺and Ir²⁺, respectively. Depending on the site where the O vacancy is formed, the change in the oxidation state leads to either the appearance of empty defect states in the gap or filled localized Ir d-states. The reasons for the absence of the Ir²⁺state in experimental data are also discussed. The formation of a Sr vacancy does not cause alternation in ion oxidation states providing instead a remarkable change in the band structure of the oxide resulting in the appearance of Dirac cones at the Fermi level. No experimentally determined Ir⁵⁺state could be detected in the presence of an Ir vacancy or an additional interstitial oxygen indicating a more complex nature of defects to provide the +5 state.