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

Abstract

Topochemical reduction of the double-perovskite oxide Sr 2 FeIrO 6 under dilute hydrogen leads to the formation of Sr 2 FeIrO 4 . This phase consists of ordered infinite sheets of apex-linked Fe 2+ O 4 and Ir 2+ O 4 squares, stacked with Sr 2+ cations, and is the first report of Ir 2+ in an extended oxide phase. Plane-wave DFT calculations indicate high-spin Fe 2+ (d 6 , S = 2) and low-spin Ir 2+ (d 7 , S = ½) configurations for the metals and confirm that both ions have a doubly occupied orbital, a configuration that is emerging as a consistent feature of all layered oxide phases of this type. The stability and double occupation of in the Ir 2+ ions invites a somewhat unexpected analogy to the extensively studied Ir 4+ ion as both ions share a common near-degenerate ( / / ) 5 valence configuration. On cooling below 115 K Sr 2 FeIrO 4 enters a magnetically ordered state in which the Ir and Fe sub-lattices 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 anti-ferromagnetic. 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 2+ and Ir 2+ cations and their cation-ordered arrangement in a distorted perovskite lattice.

Structure and Magnetism of (La/Sr)2M0.5IrV0.5O4 and Topochemically Reduced (La/Sr)2M0.5IrII0.5O3 (M = Fe, Co) Complex Oxides

“…12 This spin-glass behavior indicates that both Co/Ir phases exhibit a combination of crystallographic disorder (consistent with the diffraction data) and a frustration between the various Co-Co, Co-Ir and Ir-Ir magnetic couplings present. 10 In contrast to the cobalt phases, Sr 2 Fe 0.5 Ir 0.5 O 4 does not exhibit spin-glass behavior at low temperature and instead there is evidence for a short-range magnetically ordered state. Given that Sr 2 Fe 0.5 Ir 0.5 O 3 (which shares a common cation lattice with Sr 2 Fe 0.5 Ir 0.5 O 4 ) does exhibit spin-glass behavior we can conclude that both iron phases fulfil the crystallographic disorder criterion for spin-glass behavior, but in the case of Sr 2 Fe 0.5 Ir 0.5 O 4 the Fe-Fe, Fe-Ir and Ir-Ir magnetic couplings are not frustrated.…”

“…Figure 12 shows the d-orbital fillings expected for Ir 2+ , Fe 2+ , Co 2+ and Co 1+ in square-planar coordination sites, by analogy to previously reported systems. 10,[23][24] This shows that Fe 2+ , Co 2+ and Co 1+ cations have a single electron in both the and orbital, while Ir 2+ has only a single electron in with being empty. As a result we can expect strong intrachain couplings in all three A 2 M 0.5 Ir 0.5 O 3 phases.…”

“…The observation of spin-glass behavior for Sr 2 Fe 0.5 Ir 0.5 O 3 is not surprising given the Fe/Ir cation disorder present in the phase and the complex competition between Fe-Ir and Ir-Ir couplings in the analogous reduced perovskite Sr 2 FeIrO 4 . 10 What is perhaps a little surprising is that Sr 2 Co 0.5 Ir 0.5 O 3 is not a spin-glass at low temperature, given that the couplings in Sr 2 Fe 0.5 Ir 0.5 O 3 and Sr 2 Co 0.5 Ir 0.5 O 3 are expected to be similar.…”

“…Thus we can anticipate that the Fe 2+ /Ir 2+ oxidation state combination present in Sr 2 FeIrO 4 is also present in Sr 2 Fe 0.5 Ir 0.5 O 3 -an idea which is supported by noting that the average Ir/Fe-O bond length of the (Fe/Ir)O 4 coordination site of Sr 2 Fe 0.5 Ir 0.5 O 3 (2.003 Å, Table 2) is almost identical to the average of the Fe-O and Ir-O bond lengths in Sr 2 FeIrO 4 (1.996 Å). 10 The analogous reductions of the cobalt double perovskite phases Sr 2 CoIrO 6 and LaSrCoIrO 6 have not yet been achieved, despite multiple attempts, so the hypothetical reduced phases cannot provide guidance as to the oxidation state combinations present in Sr 2 Co 0.5 Ir 0.5 O 3 and La 0.5 Sr 1.5 Co 0.5 Ir 0.5 O 3 . However a comparison of the average (Co/Ir)-O bond length in Sr 2 Co 0.5 Ir 0.5 O 3 (2.012 Å, Table 2) with the Ir-O bond length in the Ir 2+ O 4 units in Sr 2 FeIrO 6 (1.982 Å) 10 and the Co-O bond length in the Co 2+ O 4 units in Sr 3 Co 2 O 4 Cl 2 (2.008 Å) 22 indicate a Co 2+ /Ir 2+ combination for Sr 2 Co 0.5 Ir 0.5 O 3 , with a Co 1+ /Ir 2+ combination being deduced for La 0.5 Sr 1.5 Co 0.5 Ir 0.5 O 3 by a similar method.…”

“…Support for this analogy is also provided by observing the density of states close to the Fermi level of Sr 2 FeIrO 4 and La 2 ZnIrO 6 are very similar -the only exception being the Sr 2 FeIrO 4 has a filled Ir band below the Fermi level while the corresponding band in La 2 ZnIrO 6 is empty and lies well above the Fermi level. [10][11] The analogy between square-planar Ir 2+ and octahedral Ir 4+ has motivated us to investigate further Ir 2+ -containing oxides. To that end we have prepared the Ir 5+ -containing Ruddlesden-Popper phases Sr 2 Fe 0.5 Ir 0.5 O 4 , Sr 2 Co 0.5 Ir 0.5 O 4…”