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.

High-resolution synchrotron X-ray and neutron powder diffraction data demonstrate that, in contrast to recent reports, SrFeIrO adopts an I1̅ symmetry double perovskite structure with an abc tilting distortion. This distorted structure does not tolerate cation substitution, with low levels of A-site (Ca, Ba, La) or Fe-site (Ga) substitution leading to separation into two phases: a stoichiometric I1̅ phase and a cation-substituted, P2/ n symmetry, aac distorted double perovskite phase. Magnetization, neutron diffraction, and Fe Mössbauer data show that, in common with SrFeIrO, the cation substituted SrA FeGa IrO phases undergo transitions to type-II antiferromagnetically ordered states at T ∼ 120 K. However, in contrast to stoichiometric SrFeIrO, cation substituted samples exhibit a further magnetic transition at T ∼ 220 K, which corresponds to the ordering of J ≠ 0 Ir centers in the cation-substituted, P2/ n symmetry, double perovskite phases.

Reaction between CaMn0.5Ir0.5O3 and NaH, either through solid-solid contact or via a gas mediated reaction process, yields the topochemically reduced phase CaMn0.5­Ir0.5O2.5 in which Mn3+ and Ir3+ cations are located within a partially anion-vacancy disordered lattice. Magnetization data from CaMn0.5­Ir0.5O2.5 can be fit by the Curie–Weiss law to yield C = 1.586 cm3 K mol–1 and θ = −86.9 K, consistent with a combination of S = 2, Mn3+ and S = 0, Ir3+. On cooling below T ∼ 110 K, the system undergoes a transition to a spin-glass state, consistent with the observed Mn/Ir cation disorder and frustration between Mn-O-Mn and Mn-O-Ir-O-Mn magnetic couplings. The degree of reduction and the observed anion-vacancy disorder are discussed on the basis of the d-orbital filling of the transition-metal cations.

No abstract

all adopt undistorted, n = 1 Ruddlesden-Popper structures in which the Ir 5+ and Fe 3+ /Co 3+ /Co 2+ cations are statistically disordered over all the octahedral coordination sites. Magnetization data indicate the two cobalt phases are spin-glasses at low temperature while Sr 2 Fe 0.5 Ir 0.5 O 4 appears to adopt an antiferromagnetic state with very small magnetically ordered domains. Topochemical reduction with a Zr getter converts the tetragonal A 2 M 0.5 Ir 0.5 O 4 phases to the corresponding orthorhombic A 2 M 0.5 Ir 0.5 O 3 phases in which the Ir 2+ and Fe 2+ /Co 2+ /Co 1+ cations are located in approximately square-planar coordination sites. Magnetization data indicate Sr 2 Fe 0.5 Ir 0.5 O 3 is a spin-glass below T G~3 0 K, while Sr 2 Co 0.5 Ir 0.5 O 3 appears to be antiferromagnetic below T N~2 5 K and La 0.5 Sr 1.5 Co 0.5 Ir 0.5 O 3 shows no sign of magnetic order for T > 5 K. The magnetic behavior of both the A 2 M 0.5 Ir 0.5 O 4 and A 2 M 0.5 Ir 0.5 O 3 phases is discussed on the basis of metal d-electron count and structural features. A 2 M 0.5 Ir 0.5 O 3 Ir 2+ compounds to study the effect of strong SOC on the magnetic behavior both the Ir 5+ and Ir 2+ systems. Experimental Synthesis of A 2 M 0.5 Ir 0.5 O 4. Samples of Sr 2 Fe 0.5 Ir 0.5 O 4 , Sr 2 Co 0.5 Ir 0.5 O 4 and La 0.5 Sr 1.5 Co 0.5 Ir 0.5 O 4 were prepared using a citrate gel method. 5 Appropriate quantities of SrCO 3 (99.994%), La 2 O 3 (99.999%, dried at 900°C) and Fe (99.99%) or Co (99.996%) were dissolved in a 1:1 mixture of concentrated nitric acid and distilled water, then the required amount of IrO 2 (99.99%, dried at 700°C for 2 hours) was added. Citric acid and analar ethylene glycol were added and the solutions were heated whilst being stirred. The gels thus formed were allowed to combust in air and the subsequent products were ground into fine powders, placed in alumina crucibles and then heated in air, at a rate of 1 o C min-1 to 1000°C, to remove the remaining organic components from the samples. The samples were then pressed into pellets and Sr 2 Fe 0.5 Ir 0.5 O 4 and Sr 2 Co 0.5 Ir 0.5 O 4 were heated in air for two periods of 2d at 1000 ºC with the Sr 2 Fe 0.5 Ir 0.5 O 4 samples heated for a further 2d at 1100 ºC. Samples of La 0.5 Sr 1.5 Co 0.5 Ir 0.5 O 4 were heated at 1300°C for two periods of 2d. Topochemical reduction of A 2 M 0.5 Ir 0.5 O 4 phases. Samples of Sr 2 Fe 0.5 Ir 0.5 O 4 , Sr 2 Co 0.5 Ir 0.5 O 4 and La 0.5 Sr 1.5 Co 0.5 Ir 0.5 O 4 were reduced using a zirconium getter. Samples to be reduced were sealed in an evacuated silica ampoule along with a glass 'thimble' containing powdered zirconium such that the two powders shared an atmosphere but were not in physical contact. This apparatus was then heated for multiple periods with the sample being reground, and the Zr powder being replaced with unreacted material between heating periods. Samples of Sr 2 Fe 0.5 Ir 0.5 O 4-x were prepared by heating for 3 periods of 7d at 410°C; Samples of Sr 2 Co 0.5 Ir 0.5 O 4-x were prepared by heating for 5 periods of 7d at 320°C;...