Doped Sr₂FeIrO₆-Phase Separation and a J_eff ≠ 0 State for Ir⁵

High-resolution synchrotron X-ray and neutron powder diffraction data demonstrate that, in contrast to recent reports, Sr₂FeIrO₆ adopts an I1̅ symmetry double perovskite structure with an a^-b^-c^- tilting distortion. This distorted structure does not tolerate cation substitution, with low levels of A-site (Ca, Ba, La) or Fe-site (Ga) substitution leading to separation into two phases: a stoichiometric I1̅ phase and a cation-substituted, P2₁/ n symmetry, a^-a^-c⁺ distorted double perovskite phase. Magnetization, neutron diffraction, and ⁵⁷Fe Mössbauer data show that, in common with Sr₂FeIrO₆, the cation substituted Sr_{2- x}A _xFe_{1- y}Ga _yIrO₆ phases undergo transitions to type-II antiferromagnetically ordered states at T_N ∼ 120 K. However, in contrast to stoichiometric Sr₂FeIrO₆, cation substituted samples exhibit a further magnetic transition at T_A ∼ 220 K, which corresponds to the ordering of J_eff ≠ 0 Ir⁵⁺ centers in the cation-substituted, P2₁/ n symmetry, double perovskite phases.