Structure and Magnetism of (La/Sr)₂M_0.5Ir^V_0.5O₄ and Topochemically Reduced (La/Sr)₂M_0.5Ir^II_0.5O₃ (M = Fe, Co) Complex Oxides

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;...