Sr₂FeIrO₄: Square-Planar Ir(II) in an Extended Oxide

Topochemical reduction of the double-perovskite oxide Sr₂FeIrO₆ under dilute hydrogen leads to the formation of Sr₂FeIrO₄. This phase consists of ordered infinite sheets of apex-linked Fe²⁺O₄ and Ir²⁺O₄ squares stacked with Sr²⁺ cations and is the first report of Ir²⁺ in an extended oxide phase. Plane-wave density functional theory calculations indicate high-spin Fe²⁺ (d⁶, S = 2) and low-spin Ir²⁺ (d⁷, S = ¹/₂) configurations for the metals and confirm that both ions have a doubly occupied d _z orbital, a configuration that is emerging as a consistent feature of all layered oxide phases of this type. The stability and double occupation of d _z in the Ir²⁺ ions invites a somewhat unexpected analogy to the extensively studied Ir⁴⁺ ion as both ions share a common near-degenerate (d _{xy/ xz/ yz})⁵ valence configuration. On cooling below 115 K, Sr₂FeIrO₄ enters a magnetically ordered state in which the Ir and Fe sublattices adopt type II antiferromagnetically coupled networks which interpenetrate each other, leading to frustration in the nearest-neighbor Fe-O-Ir couplings, half of which are ferromagnetic and half antiferromagnetic. The spin frustration drives a symmetry-lowering structural distortion in which the four equivalent Ir-O and Fe-O distances of the tetragonal I4/ mmm lattice split into two mutually trans pairs in a lattice with monoclinic I112/ m symmetry. This strong magneto-lattice coupling arises from the novel local electronic configurations of the Fe²⁺ and Ir²⁺ cations and their cation-ordered arrangement in a distorted perovskite lattice.