Ba₃Fe_1.56Ir_1.44O₉: A Polar Semiconducting Triple Perovskite with Near Room Temperature Magnetic Ordering

The crystal chemistry and magnetic properties for two triple perovskites, Ba₃Fe_1.56Ir_1.44O₉ and Ba₃NiIr₂O₉, grown as large, highly faceted single crystals from a molten strontium carbonate flux, are reported. Unlike the idealized A₃MM₂′O₉ hexagonal symmetry characteristic of most triple perovskites, including Ba₃NiIr₂O_9, Ba₃Fe_1.56Ir_1.44O₉ possesses significant site-disorder, resulting in a noncentrosymmetric polar structure with trigonal symmetry. The valence of iron and iridium in the heavily distorted Fe/Ir sites was determined to be Fe­(III) and Ir­(V) by X-ray absorption near edge spectroscopy (XANES). Density functional theory calculations were conducted to understand the effect of the trigonal distortion on the local Fe­(III)­O₆ electronic structure, and the spin state of iron was determined to be S = 5/2 by Mössbauer spectroscopy. Conductivity measurements indicate thermally activated semiconducting behavior in the trigonal perovskite. Magnetic properties were measured and near room temperature magnetic ordering (T_N = 270 K) was observed for Ba₃Fe_1.56Ir_1.44O₉.