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

• Published in: Inorganic chemistry 2018-11, Vol.57 (21), p.13577-13585
• Main Authors: Page, Jacob E, Morgan, Harry W. T, Zeng, Dihao, Manuel, Pascal, McGrady, John E, Hayward, Michael A
• Format: Article
• Language: English
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/acs.inorgchem.8b02198

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 2 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 2 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.