Covalent Stabilization of the Iridium-Containing Oxyhydrides Sr₂Mn_0.5Ir_0.5O_3.25H_0.75 and Sr₂Mn_0.5Ir_0.5O_2.66H_1.33

Reaction between Sr₂Mn_0.5Ir_0.5O₄ and CaH₂ or LiH yields the iridium-containing oxyhydride phases Sr₂Mn_0.5Ir_0.5O_3.25H_0.75 or Sr₂Mn_0.5Ir_0.5O_2.66H_1.33, respectively. Analysis of Mn K-edge XANES data indicate the presence of Ir³⁺ centers in these oxyhydride phases, whose low-spin d⁶ configuration is consistent with the “covalent stabilization” of the metastable oxyhydride phases, as seen previously in analogous ruthenium and rhodium containing materials. Neutron powder diffraction data indicate the hydride ions are located exclusively within the “equatorial” anion sites of Sr₂Mn_0.5Ir_0.5O_3.25H_0.75. In contrast, hydride ions are observed on both the equatorial and axial anion sites of Sr₂Mn_0.5Ir_0.5O_2.66H_1.33. This highly unusual anion distribution is attributed to a combination of the strong trans-influence of Ir–H σ-bonds and the stabilization of fac-IrO₃H₃ centers by spin–orbit coupling effects. Magnetization data indicate that Sr₂Mn_0.5Ir_0.5O₄ and Sr₂Mn_0.5Ir_0.5O_3.25H_0.75 adopt spin glass states at low temperature, behavior which is attributable to the cation disorder in Sr₂Mn_0.5Ir_0.5O₄ and the cation and anion disorder in Sr₂Mn_0.5Ir_0.5O_3.25H_0.75. In contrast, magnetization data collected from Sr₂Mn_0.5Ir_0.5O_2.66H_1.33 show no evidence of any magnetic phase transition down to 5 K, consistent with the dilution of the magnetic network by the introduction of diamagnetic Ir³⁺ on the formation of the oxyhydride phase.