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

Topochemical reduction of the double-perovskite oxide SrFeIrO under dilute hydrogen leads to the formation of SrFeIrO. This phase consists of ordered infinite sheets of apex-linked FeO and IrO squares stacked with Sr cations and is the first report of Ir in an extended oxide phase. Plane-wave density functional theory calculations indicate high-spin Fe (d, S = 2) and low-spin Ir (d, S = /) configurations for the metals and confirm that both ions have a doubly occupied d 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 in the Ir ions invites a somewhat unexpected analogy to the extensively studied Ir ion as both ions share a common near-degenerate (d ) valence configuration. On cooling below 115 K, SrFeIrO 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 Fe and Ir cations and their cation-ordered arrangement in a distorted perovskite lattice.